Pointer is a variable which holds the memory address of another variable. Pointers are represented by '*'. It is a derive data type in C. Pointer returns the value of stored address.
Reference operator(&)
If var is a variable then, &var is the address in memory.
Example program :
Output
In above source code, value 5 is stored in the memory location 2686778. var is just the name given to that location.
You, have already used reference operator in C program while using scanf() function.
scanf("%d",&var);
Reference operator(*) and Pointer variables
Pointers variables are used for taking addresses as values, i.e., a variable that holds address value is called a pointer variable or simply a pointer.
Syntax:
<data_type>*pointer_name;
Example To Demonstrate Working of Pointers
Output
Explanation of program and figure
Arrays are closely related to pointers in C programming. Arrays and pointers are synonymous in terms of how they use to access memory. But, the important difference between them is that, a pointer variable can take different addresses as value whereas, in case of array it is fixed. This can be demonstrated by an example:
Output
Relation between Arrays and Pointers
Consider and array:
int arr[4];
In arrays of C programming, name of the array always points to the first element of an array. Here, address of first element of an array is &arr[0]. Also, arr represents the address of the pointer where it is pointing. Hence, &arr[0] is equivalent to arr.
Also, value inside the address &arr[0] and address arr are equal. Value in address &arr[0] is arr[0] and value in address arr is *arr. Hence, arr[0] is equivalent to *arr.
Similarly,
&a[1] is equivalent to (a+1) AND, a[1] is equivalent to *(a+1).
&a[2] is equivalent to (a+2) AND, a[2] is equivalent to *(a+2).
&a[3] is equivalent to (a+1) AND, a[3] is equivalent to *(a+3).
.
.
&a[i] is equivalent to (a+i) AND, a[i] is equivalent to *(a+i).
In C, you can declare an array and can use pointer to alter the data of an array.
Program to find the sum of six numbers with arrays and pointers
Output
Pointers and Functions - Call by Reference
When, argument is passed using pointer, address of the memory location is passed instead of value.
Example of Pointer And Functions
C Program to swap two numbers using pointers and function.
Output
Explanation
The address of memory location num1 and num2 are passed to function and the pointers *a and *b accept those values. So, the pointer a and b points to address of num1 and num2 respectively. When, the value of pointer are changed, the value in memory location also changed correspondingly. Hence, change made to *a and *b was reflected in num1 and num2 in main function.
This technique is known as call by reference in C programming.
Dynamic Memory Allocation
The exact size of array is unknown until the compile time,i.e., time when a compiler compiles code written in a programming language into a executable form. The size of array you have declared initially can be sometimes insufficient and sometimes more than required. Dynamic memory allocation allows a program to obtain more memory space, while running or to release space when no space is required.
Although, C language inherently does not has any technique to allocated memory dynamically, there are 4 library functions under "stdlib.h" for dynamic memory allocation.
malloc()
The name malloc stands for "memory allocation". The function malloc() reserves a block of memory of specified size and return a pointer of type void which can be casted into pointer of any form.
Syntax of malloc()
ptr=(cast-type*)malloc(byte-size)
Here, ptr is pointer of cast-type. The malloc() function returns a pointer to an area of memory with size of byte size. If the space is insufficient, allocation fails and returns NULL pointer.
ptr=(int*)malloc(100*sizeof(int));
This statement will allocate either 200 or 400 according to size of int 2 or 4 bytes respectively and the pointer points to the address of first byte of memory.
calloc()
The name calloc stands for "contiguous allocation". The only difference between malloc() and calloc() is that, malloc() allocates single block of memory whereas calloc() allocates multiple blocks of memory each of same size and sets all bytes to zero.
Syntax of calloc()
ptr=(cast-type*)calloc(n,element-size);
This statement will allocate contiguous space in memory for an array of n elements. For example:
ptr=(float*)calloc(25,sizeof(float));
This statement allocates contiguous space in memory for an array of 25 elements each of size of float, i.e, 4 bytes.
free()
Dynamically allocated memory with either calloc() or malloc() does not get return on its own. The programmer must use free() explicitly to release space.
syntax of free()
free(ptr);
This statement cause the space in memory pointer by ptr to be deallocated.
Examples of calloc() and malloc()
Write a C program to find sum of n elements entered by user. To perform this program, allocate memory dynamically using malloc() function.
Write a C program to find sum of n elements entered by user. To perform this program, allocate memory dynamically using calloc() function.
realloc()
If the previously allocated memory is insufficient or more than sufficient. Then, you can change memory size previously allocated using realloc().
Syntax of realloc()
ptr=realloc(ptr,newsize);
Here, ptr is reallocated with size of newsize.
Pointer Examples C program to find average of n numbers using pointers
Write a C program to multiply matrix using pointers
C program to swap numbers in cyclic order using call by reference.
Important questions:
1.What is the difference between const char* p and char const* p?
In const char* p, the character pointed by ‘p’ is constant, so u cant
change the value of character pointed by p but u can make ‘p’ refer to
some other location.
in char const* p, the ptr ‘p’ is constant not the character referenced by
it, so u cant make ‘p’ to reference to any other location but u can
change the value of the char pointed by ‘p’.
2.What is a null pointer?
A null pointer does not point to any object.
NULL and 0 are interchangeable in pointer contexts.Usage of NULL should be considered a gentle reminder that a pointer is involved.
It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required
3.What is generic pointer in C?
In C void* acts as a generic pointer. When other pointer types are assigned to generic pointer, conversions are applied automatically (implicit conversion).
Reference operator(&)
If var is a variable then, &var is the address in memory.
Example program :
/* Example to demonstrate use of reference
operator in C programming. */
#include <stdio.h>
int main(){
int var=5;
printf("Value: %d\n",var);
printf("Address: %d",&var); //Notice, the ampersand(&) before var.
return 0;
}
Output
Value: 5 Address: 2686778
In above source code, value 5 is stored in the memory location 2686778. var is just the name given to that location.
You, have already used reference operator in C program while using scanf() function.
scanf("%d",&var);
Reference operator(*) and Pointer variables
Pointers variables are used for taking addresses as values, i.e., a variable that holds address value is called a pointer variable or simply a pointer.
Syntax:
<data_type>*pointer_name;
Example To Demonstrate Working of Pointers
/* Source code to demonstrate, handling of pointers in C program */
#include <stdio.h>
int main(){
int *pc,c;
c=22;
printf("Address of c:%d\n",&c);
printf("Value of c:%d\n\n",c);
pc=&c;
printf("Address of pointer pc:%d\n",pc);
printf("Content of pointer pc:%d\n\n",*pc);
c=11;
printf("Address of pointer pc:%d\n",pc);
printf("Content of pointer pc:%d\n\n",*pc);
*pc=2;
printf("Address of c:%d\n",&c);
printf("Value of c:%d\n\n",c);
return 0;
}
Output
Address of c: 2686784 Value of c: 22 Address of pointer pc: 2686784 Value of pc: 22 Address of c: 2686784 Value of c: 11 Address of pointer pc: 2686784 Value of pc: 2
Explanation of program and figure
- Code int *pc, p; creates a pointer pc and a variable c. Pointer pc points to some address and that address has garbage value. Similarly, variable c also has garbage value at this point.
- Code c=22; makes the value of c equal to 22, i.e.,22 is stored in the memory location of variable c.
- Code pc=&c; makes pointer, point to address of c. Note that, &c is the address of variable c (because c is normal variable) and pc is the address of pc (because pc is the pointer variable). Since the address of pc and address of c is same, *pc (value of pointer pc) will be equal to the value of c.
- Code c=11; makes the value of c, 11. Since, pointer pc is pointing to address of c. Value of *pc will also be 11.
- Code *pc=2; change the address pointed by pointer pc to change to 2. Since, address of pointer pc is same as address of c, value of c also changes to 2.
Arrays are closely related to pointers in C programming. Arrays and pointers are synonymous in terms of how they use to access memory. But, the important difference between them is that, a pointer variable can take different addresses as value whereas, in case of array it is fixed. This can be demonstrated by an example:
#include <stdio.h>
int main(){
char c[4];
int i;
for(i=0;i<4;++i){
printf("Address of c[%d]=%x\n",i,&c[i]);
}
return 0;
}
Output
Address of c[0]=28ff44 Address of c[1]=28ff45 Address of c[2]=28ff46 Address of c[3]=28ff47
Relation between Arrays and Pointers
Consider and array:
int arr[4];
In arrays of C programming, name of the array always points to the first element of an array. Here, address of first element of an array is &arr[0]. Also, arr represents the address of the pointer where it is pointing. Hence, &arr[0] is equivalent to arr.
Also, value inside the address &arr[0] and address arr are equal. Value in address &arr[0] is arr[0] and value in address arr is *arr. Hence, arr[0] is equivalent to *arr.
Similarly,
&a[1] is equivalent to (a+1) AND, a[1] is equivalent to *(a+1).
&a[2] is equivalent to (a+2) AND, a[2] is equivalent to *(a+2).
&a[3] is equivalent to (a+1) AND, a[3] is equivalent to *(a+3).
.
.
&a[i] is equivalent to (a+i) AND, a[i] is equivalent to *(a+i).
In C, you can declare an array and can use pointer to alter the data of an array.
Program to find the sum of six numbers with arrays and pointers
#include <stdio.h>
int main(){
int i,class[6],sum=0;
printf("Enter 6 numbers:\n");
for(i=0;i<6;++i){
scanf("%d",(class+i)); // (class+i) is equivalent to &class[i]
sum += *(class+i); // *(class+i) is equivalent to class[i]
}
printf("Sum=%d",sum);
return 0;
}
Output
Enter 6 numbers: 2 3 4 5 3 4 Sum=21
Pointers and Functions - Call by Reference
When, argument is passed using pointer, address of the memory location is passed instead of value.
Example of Pointer And Functions
C Program to swap two numbers using pointers and function.
#include <stdio.h>
void swap(int *a,int *b);
int main(){
int num1=5,num2=10;
/* address of num1 and num2 is passed to swap function */
swap(&num1,&num2);
printf("Number1 = %d\n",num1);
printf("Number2 = %d",num2);
return 0;
}
/* pointer a and b points to address of num1 and num2 respectively */
void swap(int *a,int *b){
int temp;
temp=*a;
*a=*b;
*b=temp;
}
Output
Number1 = 10 Number2 = 5
Explanation
The address of memory location num1 and num2 are passed to function and the pointers *a and *b accept those values. So, the pointer a and b points to address of num1 and num2 respectively. When, the value of pointer are changed, the value in memory location also changed correspondingly. Hence, change made to *a and *b was reflected in num1 and num2 in main function.
This technique is known as call by reference in C programming.
Dynamic Memory Allocation
The exact size of array is unknown until the compile time,i.e., time when a compiler compiles code written in a programming language into a executable form. The size of array you have declared initially can be sometimes insufficient and sometimes more than required. Dynamic memory allocation allows a program to obtain more memory space, while running or to release space when no space is required.
Although, C language inherently does not has any technique to allocated memory dynamically, there are 4 library functions under "stdlib.h" for dynamic memory allocation.
| Function | Use of Function |
| malloc() | Allocates requested size of bytes and returns a pointer first byte of allocated space |
| calloc() | Allocates space for an array elements, initializes to zero and then returns a pointer to memory |
| free() | dellocate the previously allocated space |
| realloc() | Change the size of previously allocated space |
malloc()
The name malloc stands for "memory allocation". The function malloc() reserves a block of memory of specified size and return a pointer of type void which can be casted into pointer of any form.
Syntax of malloc()
ptr=(cast-type*)malloc(byte-size)
Here, ptr is pointer of cast-type. The malloc() function returns a pointer to an area of memory with size of byte size. If the space is insufficient, allocation fails and returns NULL pointer.
ptr=(int*)malloc(100*sizeof(int));
This statement will allocate either 200 or 400 according to size of int 2 or 4 bytes respectively and the pointer points to the address of first byte of memory.
calloc()
The name calloc stands for "contiguous allocation". The only difference between malloc() and calloc() is that, malloc() allocates single block of memory whereas calloc() allocates multiple blocks of memory each of same size and sets all bytes to zero.
Syntax of calloc()
ptr=(cast-type*)calloc(n,element-size);
This statement will allocate contiguous space in memory for an array of n elements. For example:
ptr=(float*)calloc(25,sizeof(float));
This statement allocates contiguous space in memory for an array of 25 elements each of size of float, i.e, 4 bytes.
free()
Dynamically allocated memory with either calloc() or malloc() does not get return on its own. The programmer must use free() explicitly to release space.
syntax of free()
free(ptr);
This statement cause the space in memory pointer by ptr to be deallocated.
Examples of calloc() and malloc()
Write a C program to find sum of n elements entered by user. To perform this program, allocate memory dynamically using malloc() function.
#include <stdio.h>
#include <stdlib.h>
int main(){
int n,i,*ptr,sum=0;
printf("Enter number of elements: ");
scanf("%d",&n);
ptr=(int*)malloc(n*sizeof(int)); //memory allocated using malloc
if(ptr==NULL)
{
printf("Error! memory not allocated.");
exit(0);
}
printf("Enter elements of array: ");
for(i=0;i<n;++i)
{
scanf("%d",ptr+i);
sum+=*(ptr+i);
}
printf("Sum=%d",sum);
free(ptr);
return 0;
}
Write a C program to find sum of n elements entered by user. To perform this program, allocate memory dynamically using calloc() function.
#include <stdio.h>
#include <stdlib.h>
int main(){
int n,i,*ptr,sum=0;
printf("Enter number of elements: ");
scanf("%d",&n);
ptr=(int*)calloc(n,sizeof(int));
if(ptr==NULL)
{
printf("Error! memory not allocated.");
exit(0);
}
printf("Enter elements of array: ");
for(i=0;i<n;++i)
{
scanf("%d",ptr+i);
sum+=*(ptr+i);
}
printf("Sum=%d",sum);
free(ptr);
return 0;
}
realloc()
If the previously allocated memory is insufficient or more than sufficient. Then, you can change memory size previously allocated using realloc().
Syntax of realloc()
ptr=realloc(ptr,newsize);
Here, ptr is reallocated with size of newsize.
#include <stdio.h>
#include <stdlib.h>
int main(){
int *ptr,i,n1,n2;
printf("Enter size of array: ");
scanf("%d",&n1);
ptr=(int*)malloc(n1*sizeof(int));
printf("Address of previously allocated memory: ");
for(i=0;i<n1;++i)
printf("%u\t",ptr+i);
printf("\nEnter new size of array: ");
scanf("%d",&n2);
ptr=realloc(ptr,n2);
for(i=0;i<n2;++i)
printf("%u\t",ptr+i);
return 0;
}
Pointer Examples C program to find average of n numbers using pointers
#include <stdio.h>
#include <stdio.h>
int main(){
float n[100],sum=0.0;
int i,num;
printf("Enter the total number: ");
scanf("%d",&num);
for(i=0;i<num;++i){
printf("Number %d: ",i+1);
scanf("%f",(n+i));
sum+=*(n+i);
}
printf("Average=%.2f",sum/num);
return 0;
}
Write a C program to multiply matrix using pointers
#include <stdio.h>
void multiply(int a[][50],int b[][50],int c[][50],int r1,int c2,int c1);
int main(){
int i,j,k,a[50][50],b[50][50],c[50][50],r1,c1,r2,c2;
printf("For first matrix: Enter rows and column respectively.");
scanf("%d%d",&r1,&c1);
printf("For second matrix: Enter rows and column respectively.");
scanf("%d%d",&r2,&c2);
if(c1==r2)
{
for(i=0;i<r1;++i)
for(j=0;j<c1;++j)
{
printf("Enter element a%d%d: ",i+1,j+1);
scanf("%d",(*(a+i)+j));
}
for(i=0;i<r1;++i)
for(j=0;j<c1;++j)
{
printf("Enter element b%d%d: ",i+1,j+1);
scanf("%d",(*(b+i)+j));
}
for(i=0;i<r1;++i)
for(j=0;j<c2;++j)
*(*(c+i)+j)=0;
for(i=0;i<r1;++i)
for(j=0;j<c2;++j)
for(k=0;k<c1;++k)
*(*(c+i)+j)+=(*(*(a+i)+k))*(*(*(b+k)+j));
for(i=0;i<r1;++i)
for(j=0;j<c2;++j)
{
printf("%d ",*(*(c+i)+j));
if(j==c2-1)
printf("\n");
}
}
else
printf("Error! column of 1st matrix not equal to row of 2nd");
return 0;
}
C program to swap numbers in cyclic order using call by reference.
#include <stdio.h>
void Cycle(int *a,int *b,int *c);
int main(){
int a,b,c;
printf("Enter value of a, b and c respectively: ");
scanf("%d%d%d",&a,&b,&c);
printf("Value before swamping:\n");
printf("a=%d\nb=%d\nc=%d\n",a,b,c);
Cycle(&a,&b,&c);
printf("Value after swaping numbers in cycle:\n");
printf("a=%d\nb=%d\nc=%d\n",a,b,c);
return 0;
}
void Cycle(int *a,int *b,int *c){
int temp;
temp=*b;
*b=*a;
*a=*c;
*c=temp;
}
Important questions:
1.What is the difference between const char* p and char const* p?
In const char* p, the character pointed by ‘p’ is constant, so u cant
change the value of character pointed by p but u can make ‘p’ refer to
some other location.
in char const* p, the ptr ‘p’ is constant not the character referenced by
it, so u cant make ‘p’ to reference to any other location but u can
change the value of the char pointed by ‘p’.
2.What is a null pointer?
A null pointer does not point to any object.
NULL and 0 are interchangeable in pointer contexts.Usage of NULL should be considered a gentle reminder that a pointer is involved.
It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required
3.What is generic pointer in C?
In C void* acts as a generic pointer. When other pointer types are assigned to generic pointer, conversions are applied automatically (implicit conversion).
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