Strings
A string is a group of characters, usually letters of the alphabet. In
order to format your printout in such a way that it looks nice, has
meaningful titles and names, and is aesthetically pleasing to you and the
people using the output of your program, you need the ability to output
text data. We have used strings extensively already, without actually
defining them. A complete definition of a string is ‘a sequence of char
type data terminated by a NULL character,’.
When C is going to use a string of data in some way, either to compare
it with another, output it, copy it to another string, or whatever, the
functions are set up to do what they are called to do until a NULL
character (which is usually a character with a zero ASCII code number)
is detected. You should also recall (from Module 813: Fundamental
Programming Structures in C) that the char type is really a special form
of integer – one that stores the ASCII code numbers which represent
characters and symbols.
An array (as we shall discover shortly) is a series of homogeneous
pieces of data that are all identical in type. The data type can be quite
complex as we will see when we get to the section of this module
discussing structures. A string is simply a special case of an array, an
array of char type data. The best way to see these principles is by use of
an example [CHRSTRG.C].
#include "stdio.h"
void main( )
{
char name[5]; /* define a string of characters */
name[0] = 'D';
name[1] = 'a';
name[2] = 'v';
name[3] = 'e';
name[4] = 0; /* Null character - end of text */
printf("The name is %s\n",name);
printf("One letter is %c\n",name[2]);
printf("Part of the name is %s\n",&name[1]);
}
The data declaration for the string appears on line 4. We have used this
declaration in previous examples but have not indicated its meaning.
The data declaration defines a string called name which has, at most, 5
characters in it. Not only does it define the length of the string, but it
also states, through implication, that the characters of the string will be
numbered from 0 to 4. In the C language, all subscripts start at 0 and
increase by 1 each step up to the maximum which in this case is 4. We
have therefore named 5 char type variables: name[0], name[1],
name[2], name[3], and name[4]. You must keep in mind that in C the
subscripts actually go from 0 to one less than the number defined in the
definition statement. This is a property of the original definition of C
and the base limit of the string (or array), i.e. that it always starts at
zero, cannot be changed or redefined by the programmer.
Using strings
The variable name is therefore a string which can hold up to 5
characters, but since we need room for the NULL terminating
character, there are actually only four useful characters. To load
something useful into the string, we have 5 statements, each of which
assigns one alphabetical character to one of the string characters.
Finally, the last place in the string is filled with the numeral 0 as the end
indicator and the string is complete. (A #define statement which sets
NULL equal to zero would allow us to use NULL instead of an actual
zero, and this would add greatly to the clarity of the program. It would
be very obvious that this was a NULL and not simply a zero for some
other purpose.) Now that we have the string, we will simply print it out
with some other string data in the output statement.
You will, by now, be familiar with the %s is the output definition to
output a string and the system will output characters starting with the
first one in name until it comes to the NULL character; it will then quit.
Notice that in the printf statement only the variable name needs to be
given, with no subscript, since we are interested in starting at the
beginning. (There is actually another reason that only the variable name
is given without brackets. The discussion of that topic will be found in
the next section.)
Outputting part of a string
The next printf illustrates that we can output any single character of the
string by using the %c and naming the particular character of name we
want by including the subscript. The last printf illustrates how we can
output part of the string by stating the starting point by using a
subscript. The & specifies the address of name[1].
This example may make you feel that strings are rather cumbersome to
use since you have to set up each character one at a time. That is an
incorrect conclusion because strings are very easy to use as we will see
in the next example program.
Some string subroutines
The next example [STRINGS.C] illustrates a few of the more common
string handling functions. These functions are found in the C standard
library and are defined in the header file string.h .
Page 813-5
#include "stdio.h"
#include "string.h"
void main( )
{
char name1[12], name2[12], mixed[25];
char title[20];
strcpy(name1, "Rosalinda");
strcpy(name2, "Zeke");
strcpy(title,"This is the title.");
printf(" %s\n\n" ,title);
printf("Name 1 is %s\n", name1);
printf("Name 2 is %s\n", name2);
if(strcmp(name1,name2)>0) /* returns 1 if name1 > name2 */
strcpy(mixed,name1);
else
strcpy(mixed,name2);
printf("The biggest name alpabetically is %s\n" ,mixed);
strcpy(mixed, name1);
strcat(mixed," ");
strcat(mixed, name2);
printf("Both names are %s\n", mixed);
}
First, four stringsare defined. Next, a new function that is commonly
found in C programs, the strcpy function, or string copy function is
used. It copies from one string to another until it comes to the NULL
character. Remember that the NULL is actually a 0 and is added to the
character string by the system. It is easy to remember which one gets
copied to which if you think of the function as an assignment statement.
Thus if you were to say, for example, ‘x = 23;’, the data is copied from
the right entity to the left one. In the strcpy function the data are also
copied from the right entity to the left, so that after execution of the
first statement, name1 will contain the string Rosalinda, but without the
double quotes. The quotes define a literal string and are an indication
to the compiler that the programmer is defining a string.
Similarly, Zeke is copied into name2 by the second statement, then the
title is copied. Finally, the title and both names are printed out. Note
that it is not necessary for the defined string to be exactly the same size
as the string it will be called upon to store, only that it is at least as
long as the string plus one more character for the NULL.
Alphabetical sorting of strings
The next function to be considered is the strcmp or the string compare
function. It will return a 1 if the first string is lexicographically larger
than the second, zero if they are the two strings are identical, and -1 if
the first string is lexicographically smaller than the second. A
lexicographical comparison uses the ASCII codes of the characters as
the basis for comparison. Therefore, ‘A’ will be “smaller” than ‘Z’
because the ASCII code for ‘A’ is 65 whilst the code for ‘Z’ is 90.
Sometimes however, strange results occur. A string that begins with the
letter ‘a’ is larger than a string that begins with the letter ‘Z’ since the
ASCII code for ‘a’ is 97 whilst the code for ‘Z’ is 90.
Page 813-6
One of the strings, depending on the result of the compare, is copied
into the variable mixed, and therefore the largest name alphabetically is
printed out. It should come as no surprise to you that Zeke wins
because it is alphabetically larger: length doesn’t matter, only the ASCII
code values of the characters.
Combining strings
The last four statements in the [STRINGS.C] example have another
new feature, the strcat , or string concatenation function. This function
simply adds the characters from one string onto the end of another
string taking care to adjust the NULL so a single string is produced. In
this case, name1 is copied into mixed, then two blanks are concatenated
to mixed, and finally name2 is concatenated to the combination. The
result is printed out which shows both names stored in the one variable
called mixed.
Exercise 1
Write a program with three short strings, about 6 characters each, and
use strcpy to copy one, two, and three into them. Concatenate the three
strings into one string and print the result out 10 times.
Write a program thatwill output the characters of a string backwards.
For example, given the string “computer”, the program will produce
Page 813-7
Objective 2 After working through this module you should be able to declare and
manipulate single and multi-dimensional arrays of the C data types.
ARRAYS
The last objective discussed the structure of strings which are really
special cases of an array. Arrays are a data type that are used to
represent a large number of homogeneous values, that is values that are
all of the one data type. The data type could be of type char, in which
case we have a string. The data type could just as easily be of type int,
float or even another array.
An array of integers
The next program [INTARRAY.C] is a short example of using an array
of integers.
#include "stdio.h"
void main( )
{
int values[12];
int index;
for (index = 0;index <>
values[index] = 2 * (index + 4);
for (index = 0;index <>
printf("The value at index = %2d is %3d\n", index, values[index]);
}
Notice that the array is defined in much the same way we defined an
array of char in order to do the string manipulations in the last section.
We have 12 integer variables to work with not counting the one named
index. The names of the variables are values[0], values[1], ... , and
values[11]. Next we have a loop to assign nonsense, but well defined,
data to each of the 12 variables, then print all 12 out. Note carefully
that each element of the array is simply an int type variable capable of
storing an integer. The only difference between the variables index and
values[2], for example, is in the way that you address them. You should
have no trouble following this program, but be sure you understand it.
Compile and run it to see if it does what you expect it to do.
An array of floating point data
Now for an example of a program [BIGARRAY.C] with an array of
float type data. This program also has an extra feature to illustrate how
strings can be initialised.
#include "stdio.h"
#include "string.h"
char name1[ ] = "First Program Title";
void main( )
{
int index;
int stuff[12];
Page 813-8
float weird[12];
static char name2[] = "Second Program Title";
for (index = 0; index <>
stuff[index] = index + 10;
weird[index] = 12.0 * (index + 7);
}
printf("%s\n", name1);
printf("%s\n\n", name2);
for (index = 0; index <>
printf("%5d %5d %10.3f\n", index, stuff[index], weird[index]);
}
The first line of the program illustrates how to initialise a string of
characters. Notice that the square brackets are empty, leaving it up to
the compiler to count the characters and allocate enough space for our
string including the terminating NULL. Another string is initialised in
the body of the program but it must be declared static here. This
prevents it from being allocated as an automatic variable and allows it
to retain the string once the program is started. You can think of a staic
declaration as a local constant.
There is nothing else new here, the variables are assigned nonsense data
and the results of all the nonsense are printed out along with a header.
This program should also be easy for you to follow, so study it until
you are sure of what it is doing before going on to the next topic.
Getting data back from a function
The next program [PASSBACK.C] illustrates how a function can
manipulate an array as a variable parameter.
#include "stdio.h"
void main( )
{
int index;
int matrix[20];
/* generate data */
for (index = 0 ;index <>
matrix[index] = index + 1;
/* print original data */
for (index = 0; index <>
printf("Start matrix[%d] = %d\n", index, matrix[index]);
/* go to a function & modify matrix */
dosome(matrix);
/* print modified matrix */
for (index = 0; index <>
printf("Back matrix[%d] = %d\n", index, matrix[index]);
}
dosome(list) /* This will illustrate returning data */
int list[ ];
{
int i;
/* print original matrix */
for (i = 0;i <>
printf("Before matrix[%d] = %d\n", i list[i]);
/* add 10 to all values */
for (i = 0; i <>
list[i] += 10;
/* print modified matrix */
for (i = 0; i <>
printf("After matrix[%d] = %d\n", i, list[i]);
Page 813-9
}
An array of 20 variables named matrix is defined, some nonsense data is
assigned to the variables, and the first five values are printed out. Then
we call the function dosome taking along the entire array as a
parameter.
The function dosome has a name in its parentheses also but it uses the
local name ‘list’ for the array. The function needs to be told that it is
really getting an array passed to it and that the array is of type int. Line
20 does that by defining list as an integer type variable and including the
square brackets to indicate an array. It is not necessary to tell the
function how many elements are in the array, but you could if you so
desired. Generally a function works with an array until some end-ofdata
marker is found, such as a NULL for a string, or some other
previously defined data or pattern. Many times, another piece of data is
passed to the function with a count of how many elements to work
with. In our present illustration, we will use a fixed number of elements
to keep it simple.
So far nothing is different from the previous functions we have called
except that we have passed more data points to the function this time
than we ever have before, having passed 20 integer values. We print out
the first 5 again to see if they did indeed get passed here. Then we add
ten to each of the elements and print out the new values. Finally we
return to the main program and print out the same 5 data points. We
find that we have indeed modified the data in the function, and when we
returned to the main program, we brought the changes back. Compile
and run this program to verify this conclusion.
Arrays pass data both ways
It was stated during our study of functions that when data was passed
to a function, the system made a copy to use in the function which was
thrown away when we returned. This is not the case with arrays. The
actual array is passed to the function and the function can modify it any
way it wishes to. The result of the modifications will be available back
in the calling program. This may seem strange (that arrays are handled
differently from single point data) but it is correct. The reason is that
when an array is passed to a function, the address of (or a pointer to)
the function is the piece of data that is used. The is analagous to the
Pascal construct of placing a VAR in front of a parameter in a
procedure definition.
Multiple-dimensional arrays
Arrays need not be one-dimensional as the last three examples have
shown, but can have any number of dimensions. Most arrays usually
have either one or two dimensions. Higher dimesions have applications
Page 813-10
in mathematics but are rarely used in data processing environments. The
use of doubly-dimensioned arrays is illustrated in the next program
[MULTIARAY.C].
#include "stdio.h"
void main( )
{
int i, j;
int big[8][8], large[25][12];
/* Create big as a a multiplication table */
for (i = 0; i <>
for (j = 0; j <>
big[i][j] = i * j;
/* Create large as an addition table */
for (i = 0; i <>
for (j = 0; j <>
large[i][j] = i + j;
big[2][6] = large[24][10] * 22;
big[2][2] = 5;
big[big[2][2]][big[2][2]] = 177;
/* this is big[5][5] = 177; */
for (i = 0; i <>
for (j = 0; j <>
printf("%5d ", big[i][j]);
printf("\n"); /* newline for each increase in i */
}
}
The variable big is an 8 by 8 array that contains 8 times 8 or 64
elements in total. The first element is big[0][0], and the last is big[7][7].
Another array named large is also defined which is not square to
illustrate that the array need not be square! Both are filled up with data,
one representing a multiplication table and the other being formed into
an addition table.
To illustrate that individual elements can be modified at will, one of the
elements of big is assigned the value from one of the elements of large
after being multiplied by 22. Next, big[2][2] is assigned the arbitrary
value of 5, and this value is used for the subscripts of the next
assignment statement. The third assignment statement is in reality
big[5][5] = 177 because each of the subscripts contain the value 5. This
is only done to demonstrate that any valid expression can be used for a
subscript. It must only meet two conditions: it must be an integer
(although a char will work just as well), and it must be within the range
of the subscript for which it is being used.
The entire matrix variable big is printed out in a square form so you can
check the values to see if they were set the way you expected them to
be. You will find many opportunities to use arrays, so do not
underestimate the importance of the material in this section.
Page 813-11
Exercise 2
Define two integer arrays, each 10 elements long, called array1 and
array2. Using a loop, put some kind of nonsense data in each and add
them term for term into another 10 element array named arrays. Finally,
print all results in a table with an index number, for example
1 2 + 10 = 12
2 4 + 20 = 24
3 6 + 30 = 36 etc.
Hint: the print statement will be similar to this:
printf("%4d %4d + %4d = %4d\n", index, array1[index],
array2[index], arrays[index]);
Page 813-12
Objective 3 After working through this module you should be able to create
manipulate and manage C pointers to data elements.
Pointers
Simply stated, a pointer is an address. Instead of being a variable, it is a
pointer to a variable stored somewhere in the address space of the
program. It is always best to use an example, so examine the next
program [POINTER.C] which has some pointers in it.
#include "stdio.h"
/* illustration of pointer use */
void main( )
{
int index, *pt1, *pt2;
index = 39; /* any numerical value */
pt1 = &index; /* the address of index */
pt2 = pt1;
printf("The value is %d %d %d\n", index, *pt1, *pt2);
*pt1 = 13; /* this changes the value of index */
printf("The value is %d %d %d\n", index, *pt1, *pt2);
}
Ignore for the moment the data declaration statement where we define
index and two other fields beginning with an star. (It is properly called
an asterisk , but for reasons we will see later, let’s agree to call it a star.)
If you observe the first statement, it should be clear that we assign the
value of 39 to the variable index; this is no surprise. The next statement,
however, says to assign to pt1 a strange looking value, namely the
variable index with an ampersand (&) in front of it. In this example, pt1
and pt2 are pointers, and the variable index is a simple variable.
Now we have a problem. We need to learn how to use pointers in a
program, but to do so requires that first we define the means of using
the pointers in the program!
Two very important rules
The following two rules are very important when using pointers and
must be thoroughly understood. They may be somewhat confusing to
you at first but we need to state the definitions before we can use them.
Take your time, and the whole thing will clear up very quickly.
1. A variable name with an ampersand (&) in front of it defines the
address of the variable and therefore points to the variable. You
can therefore read line 7 as ‘pt1 is assigned the value of the address
2. A pointer with a star in front of it refers to the value of the variable
pointed to by the pointer. Line 10 of the program can be read as
‘The stored (starred) value to which the pointer pt1 points is
Page 813-13
assigned the value 13’. Now you can see why it is perhaps
convenient to think of the asterisk as a star,: it sort of sounds like
the word ‘store’!
Memory aids
1. Think of & as an address.
2. Think of * as a star referring to stored.
Assume for the moment that pt1 and pt2 are pointers (we will see how
to define them shortly). As pointers they do not contain a variable value
but an address of a variable and can be used to point to a variable. Line
7 of the program assigns the pointer pt1 to point to the variable we
have already defined as index because we have assigned the address of
index to pt1. Since we have a pointer to index, we can manipulate the
value of index by using either the variable name itself, or the pointer.
Line 10 modifies the value by using the pointer. Since the pointer pt1
points to the variable index, then putting a star in front of the pointer
name refers to the memory location to which it is pointing. Line 10
therefore assigns to index the value of 13. Anywhere in the program
where it is permissible to use the variable name index, it is also
permissible to use the name *pt1 since they are identical in meaning
until the pointer is reassigned to some other variable.
Just to add a little intrigue to the system, we have another pointer
defined in this program, pt2. Since pt2 has not been assigned a value
prior to line 8, it doesn’t point to anything, it contains garbage. Of
course, that is also true of any variable until a value is assigned to it.
Line 8 assigns pt2 the same address as pt1, so that now pt2 also points
to the variable index. So to continue the definition from the last
paragraph, anywhere in the program where it is permissible to use the
variable index, it is also permissible to use the name *pt2 because they
are identical in meaning. This fact is illustrated in the first printf
statement since this statement uses the three means of identifying the
same variable to print out the same variable three times.
Note carefully that, even though it appears that there are three
variables, there is really only one variable. The two pointers point to the
single variable. This is illustrated in the next statement which assigns the
value of 13 to the variable index, because that is where the pointer pt1
is pointing. The next printf statement causes the new value of 13 to be
printed out three times. Keep in mind that there is really only one
variable to be changed, not three.
This is admittedly a difficult concept, but since it is used extensively in
all but the most trivial C programs, it is well worth your time to stay
with this material until you understand it thoroughly.
Page 813-14
Declaring a pointer
Refer to the fifth line of the program and you will see the familiar way
of defining the variable index, followed by two more definitions. The
second definition can be read as the storage location to which pt1
points will be an int type variable. Therefore, pt1 is a pointer to an int
type variable. Similarly, pt2 is another pointer to an int type variable.
A pointer must be defined to point to some type of variable. Following
a proper definition, it cannot be used to point to any other type of
variable or it will result in a type incompatibility error. In the same
manner that a float type of variable cannot be added to an int type
variable, a pointer to a float variable cannot be used to point to an
integer variable.
Compile and run this program and observe that there is only one
variable and the single statement in line 10 changes the one variable
which is displayed three times.
A second pointer program
Consider the next program [POINTER2.C], which illustrates some of
the more complex operations that are used in C programs:
#include "stdio.h"
void main( )
{
char strg[40],*there,one,two;
int *pt,list[100],index;
strcpy(strg,"This is a character string.");
one = strg[0]; /* one and two are identical */
two = *strg;
printf("The first output is %c %c\n", one, two);
one = strg[8]; /* one and two are indentical */
two = *(strg+8);
printf("the second output is %c %c\n", one, two);
there = strg+10; /* strg+10 is identical to strg[10] */
printf("The third output is %c\n", strg[10]);
printf("The fourth output is %c\n", *there);
for (index = 0; index <>
list[index] = index + 100;
pt = list + 27;
printf("The fifth output is %d\n", list[27]);
printf("The sixth output is %d\n", *pt);
}
In this program we have defined several variables and two pointers. The
first pointer named there is a pointer to a char type variable and the
second named pt points to an int type variable. Notice also that we have
defined two array variables named strg and list . We will use them to
show the correspondence between pointers and array names.
Page 813-15
String variables as pointers
In C a string variable is defined to be simply a pointer to the beginning
of a string – this will take some explaining. You will notice that first we
assign a string constant to the string variable named strg so we will
have some data to work with. Next, we assign the value of the first
element to the variable one , a simple char variable. Next, since the
string name is a pointer by definition, we can assign the same value to
two by using the star and the string name. The result of the two
assignments are such that one now has the same value as two , and both
contain the character ‘T’, the first character in the string. Note that it
would be incorrect to write the ninth line as two = *strg[0]; because the
star takes the place of the square brackets.
For all practical purposes, strg is a pointer. It does, however, have one
restriction that a true pointer does not have. It cannot be changed like a
variable, but must always contain the initial value and therefore always
points to its string. It could be thought of as a pointer constant, and in
some applications you may desire a pointer that cannot be corrupted in
any way. Even though it cannot be changed, it can be used to refer to
other values than the one it is defined to point to, as we will see in the
next section of the program.
Moving ahead to line 9, the variable one is assigned the value of the
ninth variable (since the indexing starts at zero) and two is assigned the
same value because we are allowed to index a pointer to get to values
farther ahead in the string. Both variables now contain the character ‘a’.
The C compiler takes care of indexing for us by automatically adjusting
the indexing for the type of variable the pointer is pointing to.(This is
why the data type of a variable must be declared before the variable is
used.) In this case, the index of 8 is simply added to the pointer value
before looking up the desired result because a char type variable is one
byte long. If we were using a pointer to an int type variable, the index
would be doubled and added to the pointer before looking up the value
because an int type variable uses two bytes per value stored. When we
get to the section on structures, we will see that a variable can have
many, even into the hundreds or thousands, of bytes per variable, but
the indexing will be handled automatically for us by the system.
Since there is already a pointer, it can be assigned the address of the
eleventh element of strg by the statement in line 12 of the program.
Remember that since there is a true pointer, it can be assigned any value
as long as that value represents a char type of address. It should be
clear that the pointers must be typed in order to allow the pointer
arithmetic described in the last paragraph to be done properly. The third
and fourth outputs will be the same, namely the letter ‘c’.
Page 813-16
Pointer arithmetic
Not all forms of arithmetic are permissible on a pointer: only those
things that make sense. Considering that a pointer is an address
somewhere in the computer, it would make sense to add a constant to
an address, thereby moving it ahead in memory that number of places.
Similarly, subtraction is permissible, moving it back some number of
locations. Adding two pointers together would not make sense because
absolute memory addresses are not additive. Pointer multiplication is
also not allowed, as that would be a ‘funny’ number. If you think about
what you are actually doing, it will make sense to you what is allowed,
and what is not.
Integer pointers
The array named list is assigned a series of values from 100 to 199 in
order to have some data to work with. Next we assign the pointer pt
the address of the 28th element of the list and print out the same value
both ways to illustrate that the system truly will adjust the index for the
int type variable. You should spend some time in this program until you
feel you fairly well understand these lessons on pointers.
Function data return with a pointer
You may recall that back in the objective dealing with functions it was
mentioned that a function could use variable data if the parameter was
declared as an array. This works because an array is really a pointer to
the array elements. Functons can manipulate variable data if that data is
passed to the function as a pointer. The following program
[TWOWAY.C] illustrates the general approach used to manipulate
variable data in a function.
#include "stdio.h"
void fixup(nuts,fruit); /* prototype the function */
void main( )
{
int pecans,apples;
pecans = 100;
apples = 101;
printf("The starting values are %d %d\n",pecans,apples);
fixup(pecans,&apples);
/* when we call "fixup" we send the value */
/* of pecans and the address of apples */
printf("The ending values are %d %d\n", pecans, apples);
}
fixup(nuts,fruit) /* nuts is an integer value */
int nuts,*fruit; /* fruit points to an integer */
{
printf("The values are %d %d\n", nuts, *fruit);
nuts = 135;
*fruit = 172;
printf("The values are %d %d\n", nuts ,*fruit);
}
Page 813-17
There are two variables defined in the main program: pecans and
apples ; notice that neither of these is defined as a pointer. We assign
values to both of these and print them out, then call the function fixup
taking with us both of these values. The variable pecans is simply sent
to the function, but the address of the variable apples is sent to the
function. Now we have a problem. The two arguments are not the
same, the second is a pointer to a variable. We must somehow alert the
function to the fact that it is supposed to receive an integer variable and
a pointer to an integer variable. This turns out to be very simple. Notice
that the parameter definitions in the function define nuts as an integer,
and fruit as a pointer to an integer. The call in the main program
therefore is now in agreement with the function heading and the
program interface will work just fine.
In the body of the function, we print the two values sent to the function,
then modify them and print the new values out. The surprise occurs
when we return to the main program and print out the two values again.
We will find that the value of pecans will be restored to its value before
the function call because the C language made a copy of the variable
pecans and takes the copy to the called function, leaving the original
intact. In the case of the variable apples , we made a copy of a pointer
to the variable and took the copy of the pointer to the function. Since
we had a pointer to the original variable, even though the pointer was a
copy, we had access to the original variable and could change it in the
function. When we returned to the main program, we found a changed
value in apples when we printed it out.
By using a pointer in a function call, we can have access to the data in
the function and change it in such a way that when we return to the
calling program, we have a changed value for the data. It must be
pointed out however, that if you modify the value of the pointer itself in
the function, you will have a restored pointer when you return because
the pointer you use in the function is a copy of the original. In this
example, there was no pointer in the main program because we simply
sent the address to the function, but in many programs you will use
pointers in function calls.
Compile and run the program and observe the output.
Programming Exercises
Define a character array and use strcpy to copy a string into it. Print the
string out by using a loop with a pointer to print out one character at a
time. Initialise the pointer to the first element and use the double plus
sign to increment the pointer. Use a separate integer variable to count
the characters to print.
Page 813-18
Modify the program to print out the string backwards by pointing to the
end and using a decrementing pointer.
Page 813-19
Objective 4 After working through this module you should be able to create and
manage complex data types in C.
STRUCTURES
A structure is a user-defined data type. You have the ability to define a
new type of data considerably more complex than the types we have
been using. A structure is a collection of one or more variables, possibly
of different types, grouped together under a single name for convenient
handling. Structures are called “records” in some languages, notably
Pascal. Structures help organise complicated data, particularly in large
programs, because they permit a group of related variables to be treated
as a unit instead of as separate entities.
The best way to understand a structure is to look at an example
[STRUCT1.C].
#include "stdio.h"
void main( )
{
struct {
char initial; /* last name initial */
int age; /* childs age */
int grade; /* childs grade in school */
} boy, girl;
boy.initial = 'R';
boy.age = 15;
boy.grade = 75;
girl.age = boy.age - 1; /* she is one year younger */
girl.grade = 82;
girl.initial = 'H';
printf("%c is %d years old and got a grade of %d\n",
girl.initial, girl.age, girl.grade);
printf("%c is %d years old and got a grade of %d\n",
boy.initial, boy.age, boy.grade);
}
The program begins with a structure definition. The key word struct is
followed by some simple variables between the braces, which are the
components of the structure. After the closing brace, you will find two
variables listed, namely boy , and girl . According to the definition of a
structure, boy is now a variable composed of three elements: initial ,
age , and grade . Each of the three fields are associated with boy , and
each can store a variable of its respective type. The variable girl is also
a variable containing three fields with the same names as those of boy
but are actually different variables. We have therefore defined 6 simple
variables.
A single compound variable
Let’s examine the variable boy more closely. As stated above, each of
the three elements of boy are simple variables and can be used anywhere
in a C program where a variable of their type can be used. For example,
Page 813-20
the age element is an integer variable and can therefore be used
anywhere in a C program where it is legal to use an integer variable: in
calculations, as a counter, in I/O operations, etc. The only problem we
have is defining how to use the simple variable age which is a part of
the compound variable boy . We use both names with a decimal point
between them with the major name first. Thus boy.age is the complete
variable name for the age field of boy . This construct can be used
anywhere in a C program that it is desired to refer to this field. In fact,
it is illegal to use the name boy or age alone because they are only
partial definitions of the complete field. Alone, the names refer to
nothing.
Assigning values to the variables
Using the above definition, we can assign a value to each of the three
fields of boy and each of the three fields of girl . Note carefully that
boy.initial is actually a char type variable, because it was assigned that
in the structure, so it must be assigned a character of data. Notice that
boy.initial is assigned the character R in agreement with the above
rules. The remaining two fields of boy are assigned values in accordance
with their respective types. Finally the three fields of girl are assigned
values but in a different order to illustrate that the order of assignment
is not critical.
Using structure data
Now that we have assigned values to the six simple variables, we can
do anything we desire with them. In order to keep this first example
simple, we will simply print out the values to see if they really do exist
as assigned. If you carefully inspect the printf statements, you will see
that there is nothing special about them. The compound name of each
variable is specified because that is the only valid name by which we can
refer to these variables.
Structures are a very useful method of grouping data together in order
to make a program easier to write and understand. This first example is
too simple to give you even a hint of the value of using structures, but
continue on through these lessons and eventually you will see the value
of using structures.
An array of structures
The next program [STRUCT2.C] contains the same structure definition
as before but this time we define an array of 12 variables named kids .
This program therefore contains 12 times 3 = 36 simple variables, each
of which can store one item of data provided that it is of the correct
type. We also define a simple variable named index for use in the for
loops.
Page 813-21
#include "stdio.h"
void main( )
{
struct {
char initial;
int age;
int grade;
} kids[12];
int index;
for (index = 0; index <>
kids[index].initial = 'A' + index;
kids[index].age = 16;
kids[index].grade = 84;
}
kids[3].age = kids[5].age = 17;
kids[2].grade = kids[6].grade = 92;
kids[4].grade = 57;
kids[10] = kids[4]; /* Structure assignment */
for (index = 0; index <>
printf("%c is %d years old and got a grade of %d\n",
kids[index].initial, kids[index].age,
kids[index].grade);
}
To assign each of the fields a value we use a for loop and each pass
through the loop results in assigning a value to three of the fields. One
pass through the loop assigns all of the values for one of the kids . This
would not be a very useful way to assign data in a real situation, but a
loop could read the data in from a file and store it in the correct fields.
You might consider this the crude beginning of a data base – which, of
course, it is.
In the next few instructions of the program we assign new values to
some of the fields to illustrate the method used to accomplish this. It
should be self explanatory, so no additional comments will be given.
Copying structures
C allows you to copy an entire structure with one statement. Line 17 is
an example of using a structure assignment. In this statement, all 3
fields of kids[4] are copied into their respective fields of kids[10] .
The last few statements contain a for loop in which all of the generated
values are displayed in a formatted list. Compile and run the program to
see if it does what you expect it to do.
Using pointers and structures together
The next program [STRUCT3.C] is an example of using pointers with
structures; it is identical to the last program except that it uses pointers
for some of the operations.
#include "stdio.h"
void main( )
{
struct {
char initial;
int age;
Page 813-22
int grade;
} kids[12], *point, extra;
int index;
for (index = 0; index <>
point = kids + index;
point->initial = 'A' + index;
point->age = 16;
point->grade = 84;
}
kids[3].age = kids[5].age = 17;
kids[2].grade = kids[6].grade = 92;
kids[4].grade = 57;
for (index = 0; index <>
point = kids + index;
printf("%c is %d years old and got a grade of %d\n",
(*point).initial, kids[index].age, point->grade);
}
extra = kids[2]; /* Structure assignment */
extra = *point; /* Structure assignment */
}
The first difference shows up in the definition of variables following the
structure definition. In this program we define a pointer named point
which is defined as a pointer that points to the structure. It would be
illegal to try to use this pointer to point to any other variable type.
There is a very definite reason for this restriction in C as we have
alluded to earlier and will review in the next few paragraphs.
The next difference is in the for loop where we use the pointer for
accessing the data fields. Since kids is a pointer variable that points to
the structure, we can define point in terms of kids . The variable kids is a
constant so it cannot be changed in value, but point is a pointer variable
and can be assigned any value consistent with its being required to point
to the structure. If we assign the value of kids to point then it should be
clear that it will point to the first element of the array, a structure
containing three fields.
Pointer arithmetic
Adding 1 to point will now cause it to point to the second field of the
array because of the way pointers are handled in C. The system knows
that the structure contains three variables and it knows how many
memory elements are required to store the complete structure.
Therefore if we tell it to add one to the pointer, it will actually add the
number of memory elements required to get to the next element of the
array. If, for example, we were to add 4 to the pointer, it would
advance the value of the pointer 4 times the size of the structure,
resulting in it pointing 4 elements farther along the array. This is the
reason a pointer cannot be used to point to any data type other than the
one for which it was defined.
Now return to the program. It should be clear from the previous
discussion that as we go through the loop, the pointer will point to the
beginning of one of the array elements each time. We can therefore use
Page 813-23
the pointer to reference the various elements of the structure. Referring
to the elements of a structure with a pointer occurs so often in C that a
special method of doing that was devised. Using point->initial is the
same as using (*point).initial which is really the way we did it in the
last two programs. Remember that *point is the stored data to which
the pointer points and the construct should be clear. The -> is made up
of the minus sign and the greater than sign.
Since the pointer points to the structure, we must once again define
which of the elements we wish to refer to each time we use one of the
elements of the structure. There are, as we have seen, several different
methods of referring to the members of the structure, and in the for
loop used for output at the end of the program, we use three different
methods. This would be considered very poor programming practice,
but is done this way here to illustrate to you that they all lead to the
same result. This program will probably require some study on your
part to fully understand, but it will be worth your time and effort to
grasp these principles.
Nested and named structures
The structures we have seen so far have been useful, but very simple. It
is possible to define structures containing dozens and even hundreds or
thousands of elements but it would be to the programmer’s advantage
not to define all of the elements at one pass but rather to use a
hierarchical structure of definition. This will be illustrated with the next
program [NESTED.C], which shows a nested structure.
#include "stdio.h"
void main( )
{
struct person {
char name[25];
int age;
char status; /* M = married, S = single */
} ;
struct alldat {
int grade;
struct person descrip;
char lunch[25];
} student[53];
struct alldat teacher,sub;
teacher.grade = 94;
teacher.descrip.age = 34;
teacher.descrip.status = 'M';
strcpy(teacher.descrip.name,"Mary Smith");
strcpy(teacher.lunch,"Baloney sandwich");
sub.descrip.age = 87;
sub.descrip.status = 'M';
strcpy(sub.descrip.name,"Old Lady Brown");
sub.grade = 73;
strcpy(sub.lunch,"Yogurt and toast");
student[1].descrip.age = 15;
student[1].descrip.status = 'S';
strcpy(student[1].descrip.name,"Billy Boston");
strcpy(student[1].lunch,"Peanut Butter");
student[1].grade = 77;
Page 813-24
student[7].descrip.age = 14;
student[12].grade = 87;
}
The first structure contains three elements but is followed by no
variable name. We therefore have not defined any variables only a
structure, but since we have included a name at the beginning of the
structure, the structure is named person . The name person can be used
to refer to the structure but not to any variable of this structure type. It
is therefore a new type that we have defined, and we can use the new
type in nearly the same way we use int, char, or any other types that
exist in C. The only restriction is that this new name must always be
associated with the reserved word struct.
The next structure definition contains three fields with the middle field
being the previously defined structure which we named person . The
variable which has the type of person is named descrip. So the new
structure contains two simple variables, grade and a string named
lunch[25] , and the structure named descrip . Since descrip contains
three variables, the new structure actually contains 5 variables. This
structure is also given a name, alldat , which is another type definition.
Finally we define an array of 53 variables each with the structure
defined by alldat , and each with the name student . If that is clear, you
will see that we have defined a total of 53 times 5 variables, each of
which is capable of storing a value.
Since we have a new type definition we can use it to define two more
variables. The variables teacher and sub are defined in line 13 to be
variables of the type alldat , so that each of these two variables contain
5 fields which can store data.
Using fields
In the next five lines of the program we assign values to each of the
fields of teacher . The first field is the grade field and is handled just like
the other structures we have studied because it is not part of the nested
structure. Next we wish to assign a value to her age which is part of the
nested structure. To address this field we start with the variable name
teacher to which we append the name of the group descrip , and then
we must define which field of the nested structure we are interested in,
so we append the name age . The teacher’s status field is handled in
exactly the same manner as her age , but the last two fields are assigned
strings using the string copy function strcpy , which must be used for
string assignment. Notice that the variable names in the strcpy function
are still variable names even though they are each made up of several
parts.
The variable sub is assigned nonsense values in much the same way, but
in a different order since they do not have to occur in any required
Page 813-25
order. Finally, a few of the student variables are assigned values for
illustrative purposes and the program ends. None of the values are
printed for illustration since several were printed in the last examples.
It is possible to continue nesting structures until you get totally
confused. If you define them properly, the computer will not get
confused because there is no stated limit as to how many levels of
nesting are allowed. There is probably a practical limit of three beyond
which you will get confused, but the language has no limit. In addition
to nesting, you can include as many structures as you desire in any level
of structures, such as defining another structure prior to alldat and
using it in alldat in addition to using person . The structure named
person could be included in alldat two or more times if desired.
Structures can contain arrays of other structures which in turn can
contain arrays of simple types or other structures. It can go on and on
until you lose all reason to continue. Be conservative at first, and get
bolder as you gain experience.
Exercise 4
Define a named structure containing a string field for a name, an integer
for feet, and another for arms. Use the new type to define an array of
about 6 items. Fill the fields with data and print them out as follows:
A human being has 2 legs and 2 arms.
A dog has 4 legs and 0 arms.
A television set has 4 legs and 0 arms.
A chair has 4 legs and 2 arms.
etc.
2. Rewrite the previous exercise using a pointer to print the data out.
Page 813-26
Objective 5 After working through this module you should be able to use unions to
define alternate data sets for use in C programs.
Unions
A union is a variable that may hold (at different times) data of different
types and sizes. For example, a programmer may wish to define a
structure that will record the citation details about a book, and another
that records details about a journal. Since a library item can be neither a
book or a journal simultaneously, the programmer would declare a
library item to be a union of book and journal structures. Thus, on one
occasion item might be used to manipulate book details, and on another
occassion, item might be used to manipulate journal details. It is up to
the programmer, of course, to remember the type of item with which
they are dealing.
Examine the next program [UNION1.C] for examples.
void main( )
{
union {
int value; /* This is the first part of the union
*/
struct {
char first; /* These two values are the second part
*/
char second;
} half;
} number;
long index;
for (index = 12; index <>
number.value = index;
printf("%8x %6x %6x\n", number.value, number.half.first,
number.half.second);
}
}
In this example we have two elements to the union, the first part being
the integer named value , which is stored as a two byte variable
somewhere in the computer’s memory. The second element is made up
of two character variables named first and second . These two variables
are stored in the same storage locations that value is stored in, because
that is what a union does. A union allows you to store different types of
data in the same physical storage locations. In this case, you could put
an integer number in value , then retrieve it in its two halves by getting
each half using the two names first and second . This technique is often
used to pack data bytes together when you are, for example, combining
bytes to be used in the registers of the microprocessor.
Accessing the fields of the union is very similar to accessing the fields of
a structure and will be left to you to determine by studying the example.
One additional note must be given here about the program. When it is
run using the C compiler the data will be displayed with two leading f’s,
Page 813-27
due to the hexadecimal output promoting the char type variables to int
and extending the sign bit to the left. Converting the char type data
fields to int type fields prior to display should remove the leading f’s
from your display. This will involve defining two new int type variables
and assigning the char type variables to them. This will be left as an
exercise for you. Note that the same problem will also come up in a few
of the later examples.
Compile and run this program and observe that the data is read out as
an int and as two char variables. The char variables are reversed in
order because of the way an int variable is stored internally in your
computer. Don’t worry about this. It is not a problem but it can be a
very interesting area of study if you are so inclined.
The next program [UNION2.C] contains another example of a union,
one which is much more common. Suppose you wished to build a large
database including information on many types of vehicles. It would be
silly to include the number of propellers on a car, or the number of tires
on a boat. In order to keep all pertinent data, however, you would need
those data points for their proper types of vehicles. In order to build an
efficient data base, you would need several different types of data for
each vehicle, some of which would be common, and some of which
would be different. That is exactly what we are doing in the example
program, in which we define a complete structure, then decide which of
the various types can go into it.
#include "stdio.h"
#define AUTO 1
#define BOAT 2
#define PLANE 3
#define SHIP 4
void main( )
{
struct automobile { /* structure for an automobile */
int tires;
int fenders;
int doors;
} ;
typedef struct { /* structure for a boat or ship */
int displacement;
char length;
} BOATDEF;
struct {
char vehicle; /* what type of vehicle? */
int weight; /* gross weight of vehicle */
union { /* type-dependent data */
struct automobile car; /* part 1 of the union */
BOATDEF boat; /* part 2 of the union */
struct {
char engines;
int wingspan;
} airplane; /* part 3 of the union */
BOATDEF ship; /* part 4 of the union */
} vehicle_type;
int value; /* value of vehicle in dollars */
char owner[32]; /* owners name */
} ford, sun_fish, piper_cub; /* three variable structures */
Page 813-28
/* define a few of the fields as an illustration */
ford.vehicle = AUTO;
ford.weight = 2742; /* with a full petrol tank */
ford.vehicle_type.car.tires = 5; /* including the spare */
ford.vehicle_type.car.doors = 2;
sun_fish.value = 3742; /* trailer not included */
sun_fish.vehicle_type.boat.length = 20;
piper_cub.vehicle = PLANE;
piper_cub.vehicle_type.airplane.wingspan = 27;
if (ford.vehicle == AUTO) /* which it is in this case */
printf("The ford has %d tires.\n", ford.vehicle_type.car.tires);
if (piper_cub.vehicle == AUTO) /* which it is not */
printf("The plane has %d tires.\n",
piper_cub.vehicle_type.car.tires);
}
First, we define a few constants with the #defines, and begin the
program itself. We define a structure named automobile containing
several fields which you should have no trouble recognising, but we
define no variables at this time.
The typedef
Next we define a new type of data with a typedef. This defines a
complete new type that can be used in the same way that int or char can
be used. Notice that the structure has no name, but at the end where
there would normally be a variable name there is the name BOATDEF .
We now have a new type, BOATDEF , that can be used to define a
structure anywhere we would like to. Notice that this does not define
any variables, only a new type definition. Capitalising the name is a
common preference used by programmers and is not a C standard; it
makes the typedef look different from a variable name.
We finally come to the big structure that defines our data using the
building blocks already defined above. The structure is composed of 5
parts, two simple variables named vehicle and weight , followed by the
union, and finally the last two simple variables named value and owner .
Of course the union is what we need to look at carefully here, so focus
on it for the moment. You will notice that it is composed of four parts,
the first part being the variable car which is a structure that we defined
previously. The second part is a variable named boat which is a
structure of the type BOATDEF previously defined. The third part of
the union is the variable airplane which is a structure defined in place in
the union. Finally we come to the last part of the union, the variable
named ship which is another structure of the type BOATDEF .
It should be stated that all four could have been defined in any of the
three ways shown, but the three different methods were used to show
you that any could be used. In practice, the clearest definition would
probably have occurred by using the typedef for each of the parts.
Page 813-29
We now have a structure that can be used to store any of four different
kinds of data structures. The size of every record will be the size of that
record containing the largest union. In this case part 1 is the largest
union because it is composed of three integers, the others being
composed of an integer and a character each. The first member of this
union would therefore determine the size of all structures of this type.
The resulting structure can be used to store any of the four types of
data, but it is up to the programmer to keep track of what is stored in
each variable of this type. The variable vehicle was designed into this
structure to keep track of the type of vehicle stored here. The four
defines at the top of the page were designed to be used as indicators to
be stored in the variable vehicle. A few examples of how to use the
resulting structure are given in the next few lines of the program. Some
of the variables are defined and a few of them are printed out for
illustrative purposes.
The union is not used frequently, and almost never by novice
programmers. You will encounter it occasionally so it is worth your
effort to at least know what it is.
Bitfields
The bitfield is a relatively new addition to the C programming language.
In the next program [BITFIELD.C] we have a union made up of a
single int type variable in line 5 and the structure defined in lines 6 to
10. The structure is composed of three bitfields named x , y , and z . The
variable named x is only one bit wide, the variable y is two bits wide and
adjacent to the variable x , and the variable z is two bits wide and
adjacent to y . Moreover, because the union causes the bits to be stored
in the same memory location as the variable index , the variable x is the
least significant bit of the variable index , y forms the next two bits, and
z forms the two high-order bits.
#include "stdio.h"
void main( )
{
union {
int index;
struct {
unsigned int x : 1;
unsigned int y : 2;
unsigned int z : 2;
} bits;
} number;
for (number.index = 0; number.index <>
printf("index = %3d, bits = %3d%3d%3d\n", number.index,
number.bits.z, number.bits.y, number.bits.x);
}
}
Compile and run the program and you will see that as the variable index
is incremented by 1 each time through the loop, you will see the
bitfields of the union counting due to their respective locations within
the int definition. One thing must be pointed out: the bitfields must be
Page 813-30
defined as parts of an unsigned int or your compiler will issue an error
message.
The bitfield is very useful if you have a lot of data to separate into
separate bits or groups of bits. Many systems use some sort of a packed
format to get lots of data stored in a few bytes. Your imagination is
your only limitation to effective use of this feature of C.
Exercise 5
Page 813-31
Objective 6 After working through this module you should be able to allocate
memory to variables dynamically.
DYNAMIC ALLOCATION
Dynamic allocation is very intimidating the first time you come across
it, but that need not be. All of the programs up to this point have used
static variables as far as we are concerned. (Actually, some of them
have been automatic and were dynamically allocated for you by the
system, but it was transparent to you.) This section discusses how C
uses dynamically allocated variables. They are variables that do not
exist when the program is loaded, but are created dynamically as they
are needed. It is possible, using these techniques, to create as many
variables as needed, use them, and deallocate their space so that it can
be used by other dynamic variables. As usual, the concept is best
presented by an example [DYNLIST.C].
#include "stdio.h"
#include "stdlib.h"
void main( )
{
struct animal {
char name[25];
char breed[25];
int age;
} *pet1, *pet2, *pet3;
pet1 = (struct animal *) malloc (sizeof(struct animal));
strcpy(pet1->name,"General");
strcpy(pet1->breed,"Mixed Breed");
pet1->age = 1;
pet2 = pet1;
/* pet2 now points to the above data structure */
pet1 = (struct animal *) malloc (sizeof(struct animal));
strcpy(pet1->name,"Frank");
strcpy(pet1->breed,"Labrador Retriever");
pet1->age = 3;
pet3 = (struct animal *) malloc (sizeof(struct animal));
strcpy(pet3->name,"Krystal");
strcpy(pet3->breed,"German Shepherd");
pet3->age = 4;
/* now print out the data described above */
printf("%s is a %s, and is %d years old.\n", pet1->name,
pet1->breed, pet1->age);
printf("%s is a %s, and is %d years old.\n", pet2->name,
pet2->breed, pet2->age);
printf("%s is a %s, and is %d years old.\n", pet3->name,
pet3->breed, pet3->age);
pet1 = pet3;
/* pet1 now points to the same structure that pet3 points to */
free(pet3); /* this frees up one structure */
free(pet2); /* this frees up one more structure */
/* free(pet1); this cannot be done, see explanation in text */
}
We begin by defining a named structure – animal – with a few fields
pertaining to dogs. We do not define any variables of this type, only
three pointers. If you search through the remainder of the program, you
will find no variables defined so we have nothing to store data in. All
Page 813-32
we have to work with are three pointers, each of which point to the
defined structure. In order to do anything we need some variables, so
we will create some dynamically.
Dynamic variable creation
The first program statement is line 11, which assigns something to the
pointer pet1 ; it will create a dynamic structure containing three
variables. The heart of the statement is the malloc function buried in the
middle. This is a memory allocate function that needs the other things
to completely define it. The malloc function, by default, will allocate a
piece of memory on a heap that is n characters in length and will be of
type character. The n must be specified as the only argument to the
function. We will discuss n shortly, but first we need to define a heap.
The heap
Every compiler has a set of limitations on it that define how big the
executable file can be, how many variables can be used, how long the
source file can be, etc. One limitation placed on users by the C compiler
is a limit of 64K for the executable code if you happen to be in the small
memory model. This is because the IBM-PC uses a microprocessor
with a 64K segment size, and it requires special calls to use data outside
of a single segment. In order to keep the program small and efficient,
these calls are not used, and the memory space is limited but still
adequate for most programs.
In this model C defines two heaps, the first being called a heap, and the
second being called the far heap. The heap is an area within the 64K
boundary that can store dynamically allocated data and the far heap is
an area outside of this 64K boundary which can be accessed by the
program to store data and variables.
The data and variables are put on the heap by the system as calls to
malloc are made. The system keeps track of where the data is stored.
Data and variables can be deallocated as desired leading to holes in the
heap. The system knows where the holes are and will use them for
additional data storage as more malloc calls are made. The structure of
the heap is therefore a very dynamic entity, changing constantly.
The data and variables are put on the far heap by utilising calls to
farmalloc , farcalloc , etc. and removed through use of the function
farfree . Study your C compiler’s Reference Guide for details of how to
use these features.
Page 813-33
Segment
C compilers give the user a choice of memory models to use. The user
has a choice of using a model with a 64K limitation for either program
or data leading to a small fast program or selecting a 640K limitation
and requiring longer address calls leading to less efficient addressing.
Using the larger address space requires inter-segment addressing,
resulting in the slightly slower running time. The time is probably
insignificant in most programs, but there are other considerations.
If a program uses no more than 64K bytes for the total of its code and
memory and if it doesn’t use a stack, it can be made into a .COM file.
With C this is only possible by using the tiny memory model. Since a
.COM file is already in a memory image format, it can be loaded very
quickly whereas a file in an .EXE format must have its addresses
relocated as it is loaded. Therefore a tiny memory model can generate a
program that loads faster than one generated with a larger memory
model. Don’t let this worry you, it is a fine point that few programmers
worry about.
Even more important than the need to stay within the small memory
model is the need to stay within the computer. If you had a program
that used several large data storage areas, but not at the same time, you
could load one block storing it dynamically, then get rid of it and reuse
the space for the next large block of data. Dynamically storing each
block of data in succession, and using the same storage for each block
may allow you to run your entire program in the computer without
breaking it up into smaller programs.
The malloc function
Hopefully the above description of the heap and the overall plan for
dynamic allocation helped you to understand what we are doing with
the malloc function. The malloc function forms part of the standard
library. Its prototype is defined in the stdlib. h file, hence this file is
included using the statement on line 2. malloc simply asks the system
for a block of memory of the size specified, and gets the block with the
pointer pointing to the first element of the block. The only argument in
the parentheses is the size of the block desired and in our present case,
we desire a block that will hold one of the structures we defined at the
beginning of the program. The sizeof is a new function that returns the
size in bytes of the argument within its parentheses. It therefore returns
the size of the structure named animal , in bytes, and that number is sent
to the system with the malloc call. At the completion of that call we
have a block on the heap allocated to us, with pet1 pointing to the
block of data.
Page 813-34
Casting
We still have a funny looking construct at the beginning of the malloc
function call that is called a cast . The malloc function returns a block
with the pointer pointing to it being a pointer of type char by default.
Many (perhaps most) times you do not want a pointer to a char type
variable, but to some other type. You can define the pointer type with
the construct given on the example line. In this case we want the
pointer to point to a structure of type animal , so we tell the compiler
with this strange looking construct. Even if you omit the cast, most
compilers will return a pointer correctly, give you a warning, and go on
to produce a working program. It is better programming practice to
provide the compiler with the cast to prevent getting the warning
message.
Using the dynamically allocated memory block
If you remember our studies of structures and pointers, you will recall
that if we have a structure with a pointer pointing to it, we can access
any of the variables within the structure. In the next three lines of the
program we assign some silly data to the structure for illustration. It
should come as no surprise to you that these assignment statements
look just like assignments to statically defined variables.
In the next statement, we assign the value of pet1 to pet2 also. This
creates no new data; we simply have two pointers to the same object.
Since pet2 is pointing to the structure we created above, pet1 can be
reused to get another dynamically allocated structure which is just what
we do next. Keep in mind that pet2 could have just as easily been used
for the new allocation. The new structure is filled with silly data for
illustration.
Finally, we allocate another block on the heap using the pointer pet3 ,
and fill its block with illustrative data. Printing the data out should pose
no problem to you since there is nothing new in the three print
statements. It is left for you to study.
Even though it is not illustrated in this example, you can dynamically
allocate and use simple variables such as a single char type variable.
This should be used wisely however, since it sometimes is very
inefficient. It is only mentioned to point out that there is nothing magic
about a data structure that would allow it to be dynamically allocated
while simple types could not.
Getting rid of dynamically allocated data
Another new function is used to get rid of the data and free up the
space on the heap for reuse. This function is called free. To use it, you
Page 813-35
simply call it with the pointer to the block as the only argument, and the
block is deallocated.
In order to illustrate another aspect of the dynamic allocation and
deallocation of data, an additional step is included in the program on
your monitor. The pointer pet1 is assigned the value of pet3 in line 31.
In doing this, the block that pet1 was pointing to is effectively lost since
there is no pointer that is now pointing to that block. It can therefore
never again be referred to, changed, or disposed of. That memory,
which is a block on the heap, is wasted from this point on. This is not
something that you would ever purposely do in a program. It is only
done here for illustration.
The first free function call removes the block of data that pet1 and pet3
were pointing to, and the second free call removes the block of data
that pet2 was pointing to. We therefore have lost access to all of our
data generated earlier. There is still one block of data that is on the heap
but there is no pointer to it since we lost the address to it. Trying to free
the data pointed to by pet1 would result in an error because it has
already been freed by the use of pet3 . There is no need to worry, when
we return to DOS, the entire heap will be disposed of with no regard to
what we have put on it. The point does need to made that, if you lose a
pointer to a block on the heap, it forever removes that block of data
storage from our use and we may need that storage later. Compile and
run the program to see if it does what you think it should do based on
this discussion.
Our discussion of the last program has taken a lot of time – but it was
time well spent. It should be somewhat exciting to you to know that
there is nothing else to learn about dynamic allocation, the last few
pages covered it all. Of course, there is a lot to learn about the
technique of using dynamic allocation, and for that reason, there are
two more files to study. But the fact remains, there is nothing more to
learn about dynamic allocation than what was given so far in this
section.
An array of pointers
Our next example [BIGDYNL.C] is very similar to the last, since we
use the same structure, but this time we define an array of pointers to
illustrate the means by which you could build a large database using an
array of pointers rather than a single pointer to each element. To keep it
simple we define 12 elements in the array and another working pointer
named point .
#include "stdio.h"
#include "stdlib.h"
void main( )
{
Page 813-36
struct animal {
char name[25];
char breed[25];
int age;
} *pet[12], *point; /* this defines 13 pointers, no variables
*/
int index;
/* first, fill the dynamic structures with nonsense */
for (index = 0; index <>
pet[index] = (struct animal *) malloc (sizeof(struct animal));
strcpy(pet[index]->name, "General");
strcpy(pet[index]->breed, "Mixed Breed");
pet[index]->age = 4;
}
pet[4]->age = 12; /* these lines are simply to
*/
pet[5]->age = 15; /* put some nonsense data into
*/
pet[6]->age = 10; /* a few of the fields.
*/
/* now print out the data described above */
for (index = 0; index <12>
point = pet[index];
printf("%s is a %s, and is %d years old.\n", point->name,
point->breed, point->age);
}
/* good programming practice dictates that we free up the
*/
/* dynamically allocated space before we quit.
*/
for (index = 0; index <>
free(pet[index]);
}
The *pet[12] might seem strange to you so a few words of explanation
are in order. What we have defined is an array of 12 pointers, the first
being pet[0] , and the last pet[11] . Actually, since an array is itself a
pointer, the name pet by itself is a pointer to a pointer. This is valid in
C, and in fact you can go farther if needed but you will get quickly
confused. A definition such as int ****pt is legal as a pointer to a
pointer to a pointer to a pointer to an integer type variable.
Twelve pointers have now been defined which can be used like any
other pointer, it is a simple matter to write a loop to allocate a data
block dynamically for each and to fill the respective fields with any data
desirable. In this case, the fields are filled with simple data for
illustrative purposes, but we could be reading in a database, reading
from some test equipment, or any other source of data.
A few fields are randomly picked to receive other data to illustrate that
simple assignments can be used, and the data is printed out to the
monitor. The pointer point is used in the printout loop only to serve as
an illustration, the data could have been easily printed using the pet[n]
notation. Finally, all 12 blocks of data are release with the free function
before terminating the program.
Compile and run this program to aid in understanding this technique. As
stated earlier, there was nothing new here about dynamic allocation,
only about an array of pointers.
Page 813-37
A linked list
The final example is what some programmers find the most intimidating
of all techniques: the dynamically allocated linked list. Examine the next
program [DYNLINK.C] to start our examination of lists.
#include "stdio.h"
#include "stdlib.h"
#define RECORDS 6
void main( )
{
struct animal {
char name[25]; /* The animal's name */
char breed[25]; /* The type of animal */
int age; /* The animal's age */
struct animal *next;
/* a pointer to another record of this type */
} *point, *start, *prior;
/* this defines 3 pointers, no variables */
int index;
/* the first record is always a special case */
start = (struct animal *) malloc (sizeof(struct animal));
strcpy(start->name, "General");
strcpy(start->breed, "Mixed Breed");
start->age = 4;
start->next = NULL;
prior = start;
/* a loop can be used to fill in the rest once it is started */
for (index = 0; index <>
point = (struct animal *) malloc (sizeof(struct animal));
strcpy(point->name, "Frank");
strcpy(point->breed, "Labrador Retriever");
point->age = 3;
prior->next = point;
/* point last "next" to this record */
point->next = NULL;
/* point this "next" to NULL */
prior = point;
/* this is now the prior record */
}
/* now print out the data described above */
point = start;
do {
prior = point->next;
printf("%s is a %s, and is %d years old.\n", point->name,
point->breed, point->age);
point = point->next;
} while (prior != NULL);
/* good programming practice dictates that we free up the */
/* dynamically allocated space before we quit. */
point = start; /* first block of group */
do {
prior = point->next; /* next block of data */
free(point); /* free present block */
point = prior; /* point to next */
} while (prior != NULL); /* quit when next is NULL
*/
}
The program starts in a similar manner to the previous two, with the
addition of the definition of a constant to be used later. The structure is
nearly the same as that used in the last two programs except for the
addition of another field within the structure in line 10, the pointer. This
pointer is a pointer to another structure of this same type and will be
Page 813-38
used to point to the next structure in order. To continue the above
analogy, this pointer will point to the next note, which in turn will
contain a pointer to the next note after that.
We define three pointers to this structure for use in the program, and
one integer to be used as a counter, and we are ready to begin using the
defined structure for whatever purpose we desire. In this case, we will
once again generate nonsense data for illustrative purposes.
Using the malloc function, we request a block of storage on the heap
and fill it with data. The additional field in this example, the pointer, is
assigned the value of NULL, which is only used to indicate that this is
the end of the list. We will leave the pointer start at this structure, so
that it will always point to the first structure of the list. We also assign
prior the value of start for reasons we will see soon. Keep in mind that
the end points of a linked list will always have to be handled differently
than those in the middle of a list. We have a single element of our list
now and it is filled with representative data.
Filling additional structures
The next group of assignments and control statements are included
within a for loop so we can build our list fast once it is defined. We will
go through the loop a number of times equal to the constant
RECORDS defined at the beginning of our program. Each time
through, we allocate memory, fill the first three fields with nonsense,
and fill the pointers. The pointer in the last record is given the address
of this new record because the prior pointer is pointing to the prior
record. Thus prior->next is given the address of the new record we
have just filled. The pointer in the new record is assigned the value
NULL, and the pointer prior is given the address of this new record
because the next time we create a record, this one will be the prior one
at that time. That may sound confusing but it really does make sense if
you spend some time studying it.
When we have gone through the for loop 6 times, we will have a list of
7 structures including the one we generated prior to the loop. The list
will have the following characteristics:
1. start points to the first structure in the list.
2. Each structure contains a pointer to the next structure.
3. The last structure has a pointer that points to NULL and can be
used to detect the end of the list.
The following diagram may help you to understand the structure of the
data at this point.
Page 813-39
start ® struct1
namef
breed
age
point ® struct2
name
breed
age
point ® struct3
name
breed
age
point ® … … struct7
name
breed
age
point ® NULL
It should be clear (if you understand the above structure) that it is not
possible to simply jump into the middle of the structure and change a
few values. The only way to get to the third structure is by starting at
the beginning and working your way down through the structure one
record at a time. Although this may seem like a large price to pay for
the convenience of putting so much data outside of the program area, it
is actually a very good way to store some kinds of data.
A word processor would be a good application for this type of data
structure because you would never need to have random access to the
data. In actual practice, this is the basic type of storage used for the text
in a word processor with one line of text per record. Actually, a
program with any degree of sophistication would use a doubly linked
list. This would be a list with two pointers per record, one pointing
down to the next record, and the other pointing up to the record just
prior to the one in question. Using this kind of a record structure would
allow traversing the data in either direction.
Printing the data out
A method similar to the data generation process is used to print the data
out. The pointers are initialised and are then used to go from record to
record reading and displaying each record one at a time. Printing is
terminated when the NULL on the last record is found, so the program
doesn’t even need to know how many records are in the list. Finally, the
entire list is deleted to make room in memory for any additional data
that may be needed, in this case, none. Care must be taken to ensure
that the last record is not deleted before the NULL is checked; once the
data are gone, it is impossible to know if you are finished yet.
Page 813-40
It is not difficult, but it is not trivial, to add elements into the middle of
a linked lists. It is necessary to create the new record, fill it with data,
and point its pointer to the record it is desired to precede. If the new
record is to be installed between the 3rd and 4th, for example, it is
necessary for the new record to point to the 4th record, and the pointer
in the 3rd record must point to the new one. Adding a new record to
the beginning or end of a list are each special cases. Consider what must
be done to add a new record in a doubly linked list.
Entire books are written describing different types of linked lists and
how to use them, so no further detail will be given. The amount of
detail given should be sufficient for a beginning understanding of C and
its capabilities.
Calloc
One more function, the calloc function, must be mentioned before
closing. This function allocates a block of memory and clears it to all
zeros – which may be useful in some circumstances. It is similar to
malloc in many ways. We will leave you to read about calloc as an
exercise, and use it if you desire.
Exercise 6
Rewrite STRUCT1.C to dynamically allocate the two structures.
Rewrite STRUCT2.C to dynamically allocate the 12 structures.
Page 813-41
Objective 7 After working through this module you should be able to manipulate
characters and bits.
Upper and lower case
The first example program [UPLOW.C] in this section does character
manipulation. More specifically, it changes the case of alphabetic
characters. It illustrates the use of four functions that have to do with
case. Each of these functions is part of the standard library. The
functions are prototyped in the ctype.h file, hence its inclusion in line 2
of the program.
#include "stdio.h"
#include "ctype.h"
void mix_up_the_chars(line);
void main( )
{
char line[80];
char *c;
do {
/* keep getting lines of text until an empty line is found */
c = gets(line); /* get a line of text */
if (c != NULL) {
mix_up_the_chars(line);
}
} while (c != NULL);
}
void mix_up_the_chars(line)
/* this function turns all upper case characters into lower */
/* case, and all lower case to upper case. It ignores all */
/* other characters. */
char line[ ];
{
int index;
for (index = 0;line[index] != 0;index++) {
if (isupper(line[index])) /* 1 if upper case */
line[index] = tolower(line[index]);
else {
if (islower(line[index])) /* 1 if lower case */
line[index] = toupper(line[index]);
}
}
printf("%s",line);
}
It should be no problem for you to study this program on your own and
understand how it works. The four functions on display in this program
are all within the user written function, mix_up_the_chars. Compile and
run the program with data of your choice. The four functions are:
isupper( ); Is the character upper case?
islower( ); Is the character lower case?
toupper( ); Make the character upper case.
tolower( ); Make the character lower case.
Many more classification and conversion routines are listed in your C
compiler’s Reference Guide.
Page 813-42
Classification of characters
We have repeatedly used the backslash n (\n) character for representing
a new line. Such indicators are called escape sequences, and some of
the more commonly used are defined in the following table:
\n Newline
\t Tab
\b Backspace
\ Double quote
\\ Backslash
\0 NULL (zero)
A complete list of escape sequences available with your C compiler are
listed in your C Reference Guide.
By preceding each of the above characters with the backslash character,
the character can be included in a line of text for display, or printing. In
the same way that it is perfectly all right to use the letter n in a line of
text as a part of someone's name, and as an end-of-line, the other
characters can be used as parts of text or for their particular functions.
The next program [CHARCLAS.C] uses the functions that can
determine the class of a character, and counts the characters in each
class.
#include "stdio.h"
#include "ctype.h"
void count_the_data(line)
void main( )
{
char line[80]
char *c;
do {
c = gets(line); /* get a line of text */
if (c != NULL) {
count_the_data(line);
}
} while (c != NULL);
}
void count_the_data(line)
char line[];
{
int whites, chars, digits;
int index;
whites = chars = digits = 0;
for (index = 0; line[index] != 0; index++) {
if (isalpha(line[index])) /* 1 if line[ is alphabetic */
chars++;
if (isdigit(line[index])) /* 1 if line[ ] is a digit */
digits++;
if (isspace(line[index]))
Page 813-43
/* 1 if line[ ] is blank, tab, or newline */
whites++;
} /* end of counting loop */
printf("%3d%3d%3d %s", whites, chars, digits, line);
}
The number of each class is displayed along with the line itself. The
three functions are as follows:
isalpha( ); Is the character alphabetic?
isdigit( ); Is the character a numeral?
isspace( ); Is the character any of \n, \t, or blank?
This program should be simple for you to find your way through so no
explanation will be given. It was necessary to give an example with
these functions used. Compile and run this program with suitable input
data.
Logical functions
The functions in this group are used to do bitwise operations, meaning
that the operations are performed on the bits as though they were
individual bits. No carry from bit to bit is performed as would be done
with a binary addition. Even though the operations are performed on a
single bit basis, an entire byte or integer variable can be operated on in
one instruction. The operators and the operations they perform are
given in the following table:
& Logical AND, if both bits are 1, the result is 1.
| Logical OR, if either bit is one, the result is 1.
^ Logical XOR, (exclusive OR), if one and only one
bit is 1, the result is 1.
~ Logical invert, if the bit is 1, the result is 0, and if
the bit is 0, the result is 1.
The following example program [BITOPS.C] uses several fields that
are combined in each of the ways given above.
#include "stdio.h"
void main( )
{
char mask;
char number[6];
char and,or,xor,inv,index;
number[0] = 0X00;
number[1] = 0X11;
number[2] = 0X22;
number[3] = 0X44;
number[4] = 0X88;
number[5] = 0XFF;
printf(" nmbr mask and or xor inv\n");
mask = 0X0F;
for (index = 0; index <= 5; index++) {
Page 813-44
and = mask & number[index];
or = mask | number[index];
xor = mask ^ number[index];
inv = ~number[index];
printf("%5x %5x %5x %5x %5x %5x\n", number[index],
mask, and, or, xor, inv);
}
printf("\n");
mask = 0X22;
for (index = 0; index <= 5; index++) {
and = mask & number[index];
or = mask | number[index];
xor = mask ^ number[index];
inv = ~number[index];
printf("%5x %5x %5x %5x %5x %5x\n", number[index],
mask, and, or, xor inv);
}
}
The data are in hexadecimal format (it is assumed that you already
know hexadecimal format if you need to use these operations). Run the
program and observe the output.
Shift instructions
The last two operations to be covered in this section are the left shift
and the right shift instructions; their use is illustrated in the next
example program [SHIFTER.C].
#include "stdio.h"
void main( )
{
int small, big, index, count;
printf(" shift left shift right\n\n");
small = 1;
big = 0x4000;
for(index = 0; index <>
printf("%8d %8x %8d %8x\n", small, small, big, big);
small = small <<>
big = big >> 1;
}
printf("\n");
count = 2;
small = 1;
big = 0x4000;
for(index = 0; index <>
printf("%8d %8x %8d %8x\n", small, small, big, big);
small = small <<>
big = big >> count;
}
}
The two operations use the following operators:
<<>
>> n Right shift n places.
Once again the operations are carried out and displayed using the
hexadecimal format. The program should be simple for you to
understand on your own as there is no tricky or novel code.
Exercise 7
Using the reference manual of your C compiler, describe any operations
on characters and bits that it provides that are not described in this
section, particularly those that are described in the ctype.h header file.
