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Pointers

1. Define Pointer ?​

Definition: A pointer is a variable which stores the address of another variable.

2. Explain about pointer Notation ?​

Consider the declaration,

int i = 3 ;

This declaration tells the C compiler to: (a) Reserve space in memory to hold the integer value. (b) Associate the name i with this memory location. (c) Store the value 3 at this location.

Example:

#include <stdio.h>
int main( )
{
 int i = 3 ;
 
 printf ( "\nAddress of i = %u", &i ) ;
 printf ( "\nValue of i = %d", i ) ;
 printf ( "\nValue of i = %d", *(&i) ) ;

 return 0;
}

* OUTPUT: *
Address of i = 4652484
Value of i = 3
Value of i = 3

We may represent i’s location in memory by the following memory map.

3. What is the difference between Address of (&) and Value at address (*) operators in pointers?​

  • Address of (&) operator : * Address of -> & The &variable which is &i in the above program returns the address of variable i. so the output shows Address of i = 4652484

  • Value at address () operator Value at address -> * The ‘value at address’ operator is also called as ‘indirection’ operator.*(&variable) which is *(&i) in the above program returns the value stored at a particular address. Value of i = 3

3. Write a program to return more than one value from a function ?​

void Calc(int a,int b,int *add, int*sub);

void main()
{
  int add, sub;
  Calc(4,3, &add, &sub);
  cout<<"4+3="<<add<<endl;
  cout<<"4-3="<<sub;
  getchar();
}

void Calc(int a,int b,int *add, int*sub)
{
 *add = a+b;
 *sub = a-b;
}

* OUTPUT: *
4+3=7
4-3=1

4. What is the difference between *p++ and ++*p expressions ?​

Both expressional are not one and the same. *p++ increments the pointer and not the value pointed by it. Where as ++*p increments the value being pointed to by p.

5. What would be the equivalent pointer expression for referring the same element arr[i][j][k][l] ?​

((((arr+i)+j)+k)+l)

6. Where we can use pointers ?​

a. Accessing array or string elements ? b. Call by refertence c. Dynamic memory allocation d. Implementing linked lists, trees, graphs and many other data structures

7. Declare an array of three function pointers where each function receives two ints and returns a float ?​

float (*arr[3])(int, int);

8. Explain about Dynamic Memory allocation ?​

If we need a variable amount of memory that can only be determined during runtime in that case we need some user input to determine the necessary amount of memory space.

9. What are dynamic memory management functions in C?​

malloc, calloc, free, realloc

10. Explain about malloc ?​

malloc allocates a block of size of memory, returning a pointer to the beginning of the block. Example: char arr; int i; arr = (char) malloc(i+1);

Since malloc() returns a void pointer it is necessary to explicitly typecast it into an appropriate type of pointer.

Example: int p1 = (int)malloc(sizeof(int));

11. Explain about calloc ?​

Allocate space for array in memory. calloc allocates a block of memory for an array of num elements each of them size bytes long and initializes all its bits to zero. The effective result of an zero-initialized memory block of (num * size) bytes.

void* calloc(size_t num, size_t size);

num - No. of elements to be allocated size - size of elements

NOTE: In calloc and malloc if the function failed to allocate the requested block of memory a NULL pointer is returned.

12. Explain about realloc?​

voidrealloc(void ptr, size_t size); ptr - pointer to a memory block previously allocated with malloc, calloc or realloc to be reallocated. If this is NULL a new block is allocated and a pointer to it is returned by the function.

13. Explain about free?​

free deallocate space in memory. void free(void* ptr); ptr – pointer to a memory block previously allocated with malloc,calloc or realloc to be deal located.