Pointers.pdf

Introduction to Computers and
Programming in C
Module V
Pointers
1

Pointers
Pointer declaration:
A pointer is a variable that contains the memory location of
another variable. We start by specifying the type of data stored
in the location identified by the pointer. The asterisk tells the
compiler that we are creating a pointer variable. Finally we
give the name of the variable.
The syntax is as shown below.
type *variable_name
Example:
int *ptr;
float *ptr1;
2

Address Operator
Once we declare a pointer variable, we must point it to something
we can do this by assigning to the pointer the address of the
variable you want to point as in the following example:
ptr=&num;
Another Example
int i=3;
• Reserve space in memory to hold the integer value
• Associate the name i with this memory location.
• Store the value 3 at this location
3

Pointers
4
Source: Let Us C, Yashvant Kanetkar, BPB Publications

Pointers
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Indirection Operator
6
Value at address pointer (*)-Returns the
value stored at a particular address
(indirection operator).
Printing the value of *(&i) is same as printing
the value of i.

Pointers
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Pointers
8
• Pointers are variables that contain
addresses and since addresses are
always whole numbers, pointers always
contain whole numbers.

Pointers
9
❖float *s
• The declaration float *s does not mean
that s is going to contain a floating-point
value. It means that s is going to contain
the address of a floating-point value.

Pointers
10

Pointers
11

Jargon of Pointers
12
int a=35;
int *b;
b=&a;
• b contains address of an int
• Value at address contained in b is an int
• b is an int pointer
• b points to an int
• b is a pointer which points in direction of
an int.

/* Program to display the contents of the variable their address using pointer
variable*/
#include<stdio.h>
void main()
{
int num, *intptr;
float x, *floptr;
char ch, *cptr;
num=123;
x=12.34;
ch='a';
intptr=&num;
cptr=&ch;
floptr=&x;
printf("Num %d stored at address %un",*intptr,intptr);
printf("Value %f stored at address %un",*floptr,floptr);
printf("Character %c stored at address %un",*cptr,cptr);
}

char,int and float pointer
main()
{
char c,*cc;
int i, *ii;
float a,*aa;
c=‘A’ //ASCII value stored in c
i=54;
a=3.14;
cc=&c;ii=&i;aa=&a;
Printf(“address contained in cc = %u", cc);
Printf(“address contained in ii = %u", ii);
Print(“address contained in aa = %u", aa);
printf(“value of c=%c”, *cc);
} 14

char,int and float pointer
15

Like other variables pointer variables can be used in expressions.
For example if p1 and p2 are properly declared and initialized pointers, then
the following statements are valid.
y = *p1 * *p2;
sum = sum + *p1;
z = 5 - *p2/*p1;
*p2 = *p2 + 10;
we can also compare pointers by using relational operators the expressions such
as
p1 >p2 , p1==p2 and p1!=p2 are allowed.
Pointer expressions &
pointer arithmetic

/*Program to illustrate the pointer expression and pointer arithmetic*/
#include<stdio.h>
#include<conio.h>
void main()
{
int *ptr1,*ptr2;
int a,b,x,y,z;
clrscr();
a=30;b=6;
ptr1=&a;
ptr2=&b;
x=*ptr1+ *ptr2-6; /* 30+6-6 =30 */
y=6* (- *ptr1) / *ptr2 + 30; /* 6*(-30)/6+30=0 */
printf("nAddress of a %u",ptr1);
printf("nAddress of b %u",ptr2);
printf("na=%d, b=%d",a,b);
printf("nx=%d,y=%d",x,y);
ptr1=ptr1 + 70;
printf("na=%d, b=%d",a,b);
printf("nAddress of a %u",ptr1);
getch();
}

Passing address to functions
Arguments are generally passed to functions
in one of the two ways
• Sending the value of the arguments
• Sending the addresses of the arguments
18

Sending the values of the
arguments
• ‘value’ of each actual argument in the
calling function is copied into
corresponding formal argument of the
called function.
• Changes made to the formal arguments in
the called function have no effect on the
values of actual arguments in the calling
function.
19

Call by value
main()
{
int a =10, b=20;
Swap(a,b);
Printf(“na=%d”,a);
Printf(“n b=%d”,b);
}
Swap(int x, int y)
{
int t;
t=x;
x=y;y=t;
printf(“nx=%d“, x);
printf (“ny = %d",y );
} 20

Call by reference
• The address of actual arguments in the
calling function are copied into formal
arguments of the called function.
• Using formal arguments in the called
function changes can be made in the
actual arguments of the calling function.
21

Call by Reference
22

Call by reference
• Using call by reference function can return
more than one value at a time.
main()
{int radius; float area, perimeter;
Printf(“Enter the radius of a circle”);
scanf(“%d”,&radius);
Areaperi(radius,&area,&perimeter);
printf(%f %f,area,perimeter);
}
Areaperi (int r,float *a,float *p)
{
*a= 3.14*r*r;
*p= 2*3.14*r;
}
23

Functions Returning Pointers
main()
{
Int *p;
Int *fun(); // prototype declaration
p=fun();
printf(“n%u”,p);
printf(“n%d”,*p);
}
Int *fun() //function definition
{
int i=20;
return(&i);
}
24

*ptr++ and ++*ptr
• *ptr++ increments the pointer not the value
pointed by it.
• ++*ptr increments the value being pointed
to by ptr.
25

Void pointer
main()
{
int a=10, *j;
void *k;
j=k=&a;
j++;k++;
printf(“n %u %u”,j,k);
}
26

Passing Array Elements
• Array elements can be passed to a
function by calling the function
(a)By value
(b)By reference
27

Passing Array ELements
Call by value Call by reference
28

Pointers and Arrays
main()
{
int i=3, *x;
float j=1.5, *y;
char k= ‘c’,*z;
printf(“%d%f%c”,i,j,k);
x=&i,y=&j,z=&k;
printf(%u%u%u”,x,y,z);
x++;y++;z++;
printf(%u%u%u”,x,y,z);
}
29

Pointers and Arrays
• Do not attempt the following operations on
Pointers
Addition of two pointers
Multiplying pointer with a number
Dividing a pointer with a number
int i=4,*j,*k;
j=&i;
j=j+1,j=j-5; //Allowed
30

Accessing Array Elements
main()
{
int num[]={24,34,12,44,56,17};
int i=0;
while(i<=5)
{
printf(“n address is %u”,&num[i]);
printf(“element=%d”,num[i]);
i++;
}
}
31

Accessing Array Elements-
using pointer
main()
{
int num[]={24,34,12,44,56,17};
int i=0, *j;
j=&num[0];
while(i<=5)
{
printf(“n address is %u”,&num[i]);
printf(“element=%d”,*j);
i++;
j++; // increment pointer to the next location
}
}
32

Passing an Entire Array to a
function
main()
{
int num[]={24,34,12,44,56,17};
display(&num[0],6); // or display(num,6);
}
display( int *j, int n)
{ int i=1;
While(i<=n)
{
printf(“element=%d”,*j);
i++;
j++; // increment pointer to point the next location
}
33

Accessing array element in
different ways
34

2-D Array using Pointer
36

Pointer to an array
• As we have pointer to int,float and char we
can have pointer to an array
• For example int (*q)[4] means that q is a
pointer to an array of 4 integers
37

Pointer to an array
38

Passing 2-D array to a function
39

Passing 2-D array to a function
40

Returning Array from Function
• A pointer to an integer
• A pointer to the zeroth 1-D array
• A pointer to the 2-D array
41

Array of Pointers
• A pointer variable always contains an
address, an array of pointers would be
nothing but collection of addresses.
Main()
{
Static int a[]={0,1,2,34};
Static int *p[]={a,a+1,a+2,a+3,a+4};
printf(“n%u%u%d”,p,*p,*(*p));
}
42

Dynamic Menory Allocation
• If we want to allocate memory at the time of
excecution then it can be done using standard library
functions calloc() and malloc() and are known as
dynamic memory allocation functions.
• In C, malloc is a memory allocation function that
dynamically allocates a large block of memory with a
specified size. It's useful when you don't know the
amount of memory needed during compile time.
• The calloc() function in C is used to allocate multiple
blocks of memory that are the same size. It's a
dynamic memory allocation function that sets each
block to zero. calloc() stands for "contiguous
allocation". 43

Example
int main()
{
int n,avg,i,*p,sum=0;
printf("nEnter the number of students");
scanf("%d",&n);
p=(int *)malloc(n*2);
if(p==NULL)
{
printf("Memory allocation unsuccessful");
exit(0);
}
for(i=0;i<n;i++)
{
scanf("%d",(p+i));
}
for(i=0;i<n;i++)
{
sum=sum+*(p+i);
}
avg=sum/n;
printf("Avarage marks=%d",avg);
return 0;
} 44

Example problem
45

References
• Programming in ANSI C, E. Balagurusamy. McGrawHill
• Let Us C, Yashvant Kanetkar, BPB Publications
• Programming in C, Reema Thareja, Oxford University Press
• https://www.dotnettricks.com
46

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