Chapter 13.1.8

17
POINTERS

17.1 INTRODUCTION

As you know by now, variables are stored in memory. Each memory
location has a numeric address, in much the same way that each
element of an array has its own subscript. Variable names in C and
other high-level languages enable the programmer to refer to memory
locations by name, but the compiler must translate these names
into addresses. A pointer is a variable that represents the location of
a data item, such as a variable or an array element. Pointers are
used frequently in C. Pointers can be used to pass information back
and forth between a function and its reference point. In particular,
pointers provide a way to return multiple data items from a function
via function arguments. Pointers also permit references to other
functions to be specified as arguments to a given function. This has
the effect of passing functions as arguments to the given function.

17.2 OBJECTIVES

After going through this lesson you will be in a position to

explain address operator
define pointer declarations

Pointers :: 265

describe pointers and one-dimensional arrays
define pointers and multidimensional arrays
explain arrays of pointers

17.3 THE ADDRESS OPERATOR

Within a computer’s memory, every stored data item occupies one
or more contiguous memory cells. The number of memory cells re-
quired to store a data item depends on the type of data item. For
example, a single character will typically be stored in 1 byte of
memory, an integer requires two contiguous bytes. A floating point
number may require four contigueous bytes, and a double precision
quantity may require eight contiguous bytes.

Suppose 'a' is a variable that represents some particular data item.
The compiler will automatically assign memory cells for this data
item. The data item can be accessed if we know the location of the
first memory cell. The address of a’s memory location can be deter-
mined by the expression &v, where & is a unary operator called the
address operator, that evaluates the address of its operand. Now let
us assign the address of 'a' to another variable,

Thus

pa= &a;

This new variable is called a pointer to 'a', since it “points” to the
locations where 'a' is stored in memory. However, pa represents a’s
address, not its value. Thus, pa is referred to as a pointer variable.

Address of a → value of a
pa a

The data item represented by 'a' ( i.e the data item stored in a’s
memory cells) can be accessed by the expression *pa, where * is a
unary operator, called the indirection operator, that operates only
on a pointer variable. Therefore, *pa and 'a' both represent the
same data item (ie the contents of the same memory cells). Further
more if we write pa= &a and b = *pa, then 'a' and 'b' will both
represents the same value i.e the value of a will indirectly be as-
signed to 'b'

266 :: Computer Applications

Let us consider an example :

# include <stdio.h>
main ( )
{
int a=3;
int b;
int *pa; /* to an integer */
int *pb;/*pointer to an integer */
pa=&a;/* assign address of a to pa */
b=*pa;/* assign value of a to b*/
pb=&b;/* assign address of b to pb */
printf (“n% d %d % d %d”, a, &a,pa, *pa);
printf (“n% d %d % d %d”, b, &b, pb, *pb);
}

The output is : 3 F8E F8E 3
3 F8C F8C 3

The unary operators & and * are members of the same precedence
group as the other unary operators i.e. -, ++, —, ! , size of and(type).
The address operator (&) must act upon operands associated with
unique addresses, such as ordinary variables or single array ele-
ments. Thus, the address operator cannot act upon arithmetic ex-
pression, such as 2*(a+b). The indirection operator(*) can only act
upon operands that are pointers.

Let us consider an example

# include <stdio.h>
main()
{
int i=3,* j;k; /*j is a pointer to integer */
j=&i; /*j points to the location of i*/
k=*j ; /*assign to k the value pointed to by j*/
*j=4 /assign 4 to the location pointed to by j*/
printf(“i=%d, *j=%d, k=%dn,” i, *j, k);
}
The output is as follows:

Pointers :: 267

i=4,*j=4, k=3

17.4 POINTER DECLARATION

Pointer variables, like all other variables, must be declared before
they may be used in a C program. The interpretation of a pointer
declaration is somewhat different than the interpretation of other
variable declarations. When a pointer variable is declared, the variable
name must be preceded by an asterisk(*). This identifies the fact
that the variable is a pointer. The data type that appears in the
declaration refers to the object of the pointer, i.e the data item that
is stored in the address represented by the pointer, rather than the
pointer itself.

Thus, a pointer declaration may be written in general terms as
datatype *ptr;

Where ptr is the name of the pointer variable and data-type refers to
the data type of the pointer’s object.

For example,

float a,b;
float *pb;

The first line declares a and b to be floating-point variables. The
second line declares pb to be a pointer variable whose object is a
floating-point quantity i.e. 'pb' point to a floating point quantity. 'pb'
represents an address, not a floating-point quantity.

Within a variable declaration, a pointer variable can be initialized
by assigning it the address of another variable.

17.5 POINTERS AS ARGUMENTS

Pointers are often passed to a function as arguments. This allows
data items within the calling portion of the program to be accessed
by the function, altered within the function, and then returned to
the calling portion of the program in altered form.

When an argument is passed by value, the data item is copied to the
function. Thus any alteration made to the data item within the func-
tion is not carried over into the calling routine when an argument
is passed by reference, however the address of a data item is passed
to the function. The contents of that address can be accessed freely,


either within the function or within the calling routine. Moreover,
any change that is made to the data item will be recognized in both
the function and the calling routine. Thus, the use of a pointer as a
function argument permits the corresponding data item to be al-
tered globally from within the function.

If formal arguments are pointers, then each must be preceded by an
asterisk. Also, if a function declaration is included in the calling
portion of the program, the data type of each argument that corre-
sponds to a pointer must be followed by an asterisk.

Let us consider an example of passing by reference

#include <stdio.h>
main ()
{
int i,j;
i=2;
j=5;
printf(“i=%d and j=%dn”, i,j);
/* Now exchange them */
swap(&i,&j);
printf(“nnow i=%d and j=%dn”, i,j);
}
swap(i,j)
int *i,*j;
{
int temp=*i; /* create temp and store into it the value pointed
to by “i”*/
*i=*j; /*The value pointed to by j is stored in the location pointed
to by i*/
*j=temp;/* Assign temp to the location pointed to by j*/
}
The output is as follows:
i=2 and j=5
Now i=5 and j=2

If the formal parameters i and j of the swap function were declared
merely as integers, and the main function passed only i and j rather

Pointers :: 269

than their addresses, the exchange made in swap would have no
effect on the variables i and j in main. The variable temp in swap
function is of type int, not int*. The values being exchanged are not
the pointers, but the integers being pointed to by them. Also temp
is initialized immediately upon declaration to the value pointed to
by i.

Let us consider it again with the help of another example:-

The following program counts the no. of vowels and consonants in a
line of text with the help of pointers.

# include <stdio.h>
main()
{
char str[80];
int vowels=0;
int consonants=0;
void scan_line(char str[ ], int *v, int *c);
printf(“Enter a line of textn”);
scanf(“%[ “n]” , str);
scan_line(str, &vowels, &consonants);
printf(“No. of vowels %d”,vowels);
printf(“No. of consonants %d”, consonants);
}
void scan_line(char str[ ], int * v, int *c)
{
char c;
int count=0;
while ((c=toupper(str[count]))!=’0’)
{
if(c = = A !! c= =E !! c= =I !! c= =O !! c= =U)
++*v;
else if(c>=’A’&& c<=’Z’)
++*c;
++ count;
}


return;
}

It is possible to pass a portion of an array, rather than an entire
array, to a function. To do so, the address of the first array element
to be passed must be specified as an argument. The remainder of
the array, starting with the specified array element, will then be
passed to the function.

INTEXT QUESTIONS

1. What numeric value is associated with every memory location ?
2. How are variable names translated to their corresponding ad-
dresses?
3. What must be done to a pointer variable before it can be put to
use?
4. What is the role played by the & operator in a call to the scanf
function ?
5. Can addresses be stored ? If so, where ?

17.6 POINTERS AND ONE MIDEMNSIONAL ARRAYS

Array name is really a pointer to the first element in that array.
Therefore, if x is a one-dimensional array, then the address of the
first array element can be expressed as either &x[0] or x. Similarly,
the address of the second array element can be written as either & x
[1] or as (x+1) and so on. Here we should keep it in mind that the
expression (x+1) is a symbolic representation for an address specifi-
cation rather than an arithmetic expression. When writing the
address of an array element in the form(x+i), there is no need to
concern with the number of memory cells associated with each type
of array element, the C compiler adjusts for this automatically. The
programmer must specify only the address of the first array element
and the number of array elements beyond the first.

Since &x[i] and (x+i) both represent the address of the ith element of
x then x [i] and *(x+i) both represent the contents of that address.
The two terms are interchangeble.

When assigning a value to an array element such as x[i] the left side
of the assignment statement may be written as either x[i]or *(x+i).

Pointers :: 271

Thus a value may be assigned directly to an array element, or it
may be assigned to the memory area whose address is that of the
array element . Thus expression such as x,(x+i) and &x [i] cannot
appear on the left side of an assignment statement.

Numerical array elements cannot be assigned initial values if the
array is defined as a pointer variable. Therefore a conventional array
definition is required if initial values will be assigned to the ele-
ments of a numerical array.

Let us assume that x is to be defined as a one-adimentional, 10
element array of integers. It is possible to define x as a pointer
variable rather than as an array. Thus we can write int *x; instead of
int x[10];

However x is not automatically assigned a memory block when it is
defined as a pointer variable, To assign sufficient memory for x ,we
can make use of the library function malloc as follows :

x= malloc (10*size of (int));

This function reserves a block of memory whose size is equivalent to
the size of an integer quantity. The function return a pointer to a
character. If the declaration is to include the assignment of initial
values, then x must be defined as an array rather then as a pointer
variable for example

int x[10]= {1,2,3,4,5,6,7,8,9,10};
or int x[]= {1,2,3,4,5,6,7,8,9,10};

For character arrays you can write them in the form of pointer as
char*x="this is string";

17.7 ARITHMETICS ON POINTERS

An integer value can be added to or subtracted from a pointer vari-
able. For example, px is a pointer variable representing the address
of some variable x. We can write expressions such as a ++ px ,- - px
( px +3 ),( px+i ) and (px – i ) where i is an integer variable. Each
expression will represent an address located some distance from
the original address represented by px. The exact distance will be
the product of the integer quantity and the number. of bytes or
words associated with the data item to which px points. One pointer
variable can be subtracted from another provided both variables
points to elements of the same array. The resulting value indicates
the number of words or bytes separating the corresponding array
elements.


Pointer variables can be compared provided both variables point to
objects of the same data type. Such comparisons can be useful when
both pointer variables point to elements of the same array. The fol-
lowing points must be kept in mind about operations on pointers:-

– A pointer variable can be assigned the address of an ordinary
variable.

– A pointer variable can be assigned the value of another pointer
variable provided both pointers point to objects of the same data
type.

– An integer quantity can be added to or subtracted from a pointer
variable.

– A pointer variable can be assigned a null (zero) value.

– One pointer variable can be subtracted from another provided
both pointers point to elements of the same array.

– Two pointer variables can be compared provided both pointers
point to objects of the same data type.

17.8 POINTERS AND MULTIDIMENSIONAL ARRAYS

A two dimensional array, is actually a collection of one dimensional
arrays. Therefore, we can define a two dimensional array as a pointer
to a group of contiguous one-dimensional arrays. A two dimensional
array declaration can be written as

data type (*ptr) [expression2];

Notice the parentheses that surround the array name and the pre-
ceding asterisk in the pointer version of each declaration. These
parentheses must be present. Without them we would be defining
an array of pointers rather than a pointer to a group of arrays, since
these particulars symbols (i.e. The square brackets and asterisk)
would normally be evaluated right-to-by.

For example, if x is a two dimensional integer array having 10 rows
and 20 columns, then we can declare x as

int(*x)[20];
rather than int x(10) (20);

Pointers :: 273

Similarly, three dimmensional integer array can be written as
int (*x) (20) (30)

An individual array element within a multi-dimensional array can
be accessed by repeatedly using the indirection operator. If n is a
2D array having 10 rows and 20 columns, then item in row 2, column
5 can be accessed by writing either

x[2][5]
or
*(*(x+2)+5)

Here, (x+2) is a pointer to row 2. Therefore, the object of this pointer
*(x+2), refers to the entire row. Since row 2 is a one-dimensional
array, *(x+2) is actually a pointer to the first element in row 2. We
now add 5 to this pointer. Hence,(*(x+2)x5) is a pointer to element 5
in row 2. The object of this pointer, (*(x+2)+5) therefore refers to the
item in column 5 of row2 which is x[2][5].

INTEXT QUESTIONS

6. What is meant by passing by reference ?

7. Why is only the address of an array’s first element necessary in
order to access the entire array ?

8. If an array is declared as

char x[10][12];

What is referred to by x[5] ?

17.9 ARRAYS OF POINTERS

A multi-dimensional array can be expressed in terms of an array of
pointers rather than as a pointer to a group of contiguous arrays. In
such cases the newly defined array will have one less dimension
than the original multi-dimensional array. Each pointer will indi-
cate the beginning of a separate (n-1) dimensional array.

In general terms, a 2D array can be defined as a one dimensional
array of pointers by writing.


data type *array [expression 1];

In this type of declaration, array name and its preceding asterisk
are not enclosed in parentheses. Thus, a right-to-left rule first asso-
ciates the pairs of square brackets with array, defining the named
object as an array. The preceding asterisk then establishes that the
array will contain pointers.

For example x is a 2D integer array having 10 rows and 20 columns.
We can define x as a one-dimensional array of pointers by writing

int *x[10];

An individual array element, such as x[2][5], can be accessed by
writing *(x[2]+5)

In this expression, x[2] is a pointer to the first element in row 2, so
that (x[2] +5) points to element 5 within row 2. The object of this
pointer, *(x[2]+5), therefore refers to x[2][5].

Pointer arrays offer a particularly convenient method for storing
strings. In this situation, each array element is a character-type
pointer that indicates the beginning of a separate string. Each indi-
vidual string can be accessed by referring to its corresponding
pointer.

Let us consider an example of array of pointers: (To sort list of strings)

#include<stdio.h>
#include<stdlib.h>
main()
{
int i, n=0;
char *x[10];
int reorder(int n, char *x[]);
printf(“Enter each string on a separate line,)
"Type'END' to finish”);
do
{
x[n]=malloc(12*size of (char));
printf(“nstring %d”, n+1);
scanf(“%s”, x[n]);

Pointers :: 275

}
while ( stringcmp(x[n++], “END”));
reorder(- - n,x);
printf (or nn Recordered List:n");
for(i=0; i<n; ++i)
printf(“n string %d:%s”, i+1, x[i]);
}
reorder(int n, char * x[])
{
char *temp;
int i, item;
for(item=0; item <n-1; ++ item)
for(i=item +1; i<n; ++i)
if (stringcmp(x[item], x[i]>0)
{
temp=x[item];
x[item]=x[i];
x[i]=temp;
}
return;
}

An advantage to use arrays of pointers is that a fixed block of memory
need not be reserved in advance, as is done when initializing a con-
ventional array.

If some of the strings are particularly long, there is no need to worry
about the possibility of exceeding some maximum specified string
length. Arrays of this type are oftern referred to as ragged arrays.

17.10 PASSING FUNCTIONS TO OTHER FUNCTIONS

When a function declartion appears within another function, the
name of the function being declared becomes a pointer to that
function. Such pointer can be passed to other functions as
arguments. This has the effect of passing one function to another ,
as though the first function was a variable. The first functiion can
then be accessed within the second function. Successive calls to the
second function can pas different pointers to the second funtion.


When a function accepts another function’s name as an argument a
formal argument declaration must identify that argument as a pointer
to another function. A formal argument that is a pointer to a function
can be declared as

data-type (*function name)();

Where data-type refers to the data type of the quantity returned by
the function.

This function can then be accessed by means of the indirection
operator. To do so the indirection operator must precede the func-
tion name. Both the indirection operator and the function name
must be enclosed in parentheses that is

(*function-name)(argument 1, argument 2…, argument n);

In the last, we mention that pointer declaration can become
complicated and some care is required for its interpretation. This is
particularly take in case of declarations that involve functions or
arrays.

One difficulty is the dusl use of parentheses. In particular,
parentheses are used to indicate functions and they are used for
nesting purposes within more complicated declarations. Thus the
declaration

int *p(int a);

Indicates a function that accepts an integer argument, and returns
a pointer to an integer. On the other hand, the declaration.

int(*p)(int a);

indicates a pointer to a function that accepts an integer argument
and returns an integer. In this last declaration, the first pair of pa-
rentheses is used for nesting and the second pair is used to indicate
a function.

17.11 WHAT YOU HAVE LEARNT

In this lesson you have learnt about pointers. You can now declare
pointers very effciently. Now you can pass pointers to a function.
You can do operations on pointer. Now you are familiar with arrays
of pointers. You can use pointers with one as well as multi dimen-
sional arrays.

Pointers :: 277

17.12 TERMINAL QUESTIONS

1. What is meant by the address of a memory cell?
2. What kind of information does a pointer variable represent ?
3. How is a pointer variable declared ?
4. How can a function return a pointer to its calling routine ?
5. How is a multidimensional array defined in terms of a pointer to
a collection of contiguous arrays of lower dimensioanlity ?
6. A ‘C’ program contains the following declaration :

Static int x[8]= {10,20,30,40,50,60,70,80};

(a) What is the meaning of x?

(b) What is the meaning of (x+2) ?

(c) What is the value of *x?

(d) What is the value of (*x+2) ?

(e) What is the value of *(x+2) ?

7. Write a program to find the length of a string using the concept
of pointers

8. Write a program to copy a string using pointers.

9. Write a program to concatenate two strings using pointers.

10. Write a program to compare two strings using pointers.

17.13 KEY TO IN-TEXT QUESTIONS

1. its address
2. with the & operator
3. After it has been declared, it must be initialized
4. It passes the address of the variable being input, making it pos-
sible for the contents of that location to be changed.


5. Addresses can be stored in pointer variables.

6. Passing by reference is the term used to refer to the passing of a
variable’s address to a function.

7. Since all the elements of an array are of the same type, and all
the elements are stored continguously in memory, the location
of the first element can be used to find the second element, the
third element, and so forth.

8. x[5] is a pointer to the sixth row (row #5) of the array.

Chapter 13.1.8

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Chapter 13.1.8