Explain calloc and realloc with example

There are two additional memory allocation functions, calloc() and realloc(). Their prototypes are given below:
void *calloc(size_t num_elements, size_t element_size};

void *realloc( void *ptr, size_t new_size);
malloc does not initialise memory (to zero) in any way. If you wish to initialise memory then use calloc. calloc there is slightly more computationally expensive but, occasionally, more convenient than malloc. Also note the different syntax between calloc and malloc in that calloc takes the number of desired elements, num_elements, and element_size, element_size, as two individual arguments.
Thus to assign 100 integer elements that are all initially zero you would do:

    int *ip;
         ip = (int *) calloc(100, sizeof(int));
realloc is a function which attempts to change the size of a previous allocated block of memory. The new size can be larger or smaller. If the block is made larger then the old contents remain unchanged and memory is added to the end of the block. If the size is made smaller then the remaining contents are unchanged.
If the original block size cannot be resized then realloc will attempt to assign a new block of memory and will copy the old block contents. Note a new pointer (of different value) will consequently be returned. You must use this new value. If new memory cannot be reallocated then realloc returns NULL.
Thus to change the size of memory allocated to the *ip pointer above to an array block of 50 integers instead of 100, simply do:
   ip = (int *) calloc( ip, 50);