Question

As per suggestions I have modified the code,
but how can I initialize single element in the structure ??

#include<stdio.h>

typedef struct student
{
    int roll_id[10];
    int name_id[10];
} student;

int main()
{
    student p = { {0} };  // if i want to initialize single element ''FIX HERE, PLs'' 
    student *pptr=&p;
    pptr->roll_id[9]={0}; // here is the error pointed 

    printf (" %d\n", pptr->roll_id[7]);

    return 0;
}

Answer 1

{0} is valid only as an aggregate (array or struct) initializer.

int roll_id[10] = {0}; /* OK */
roll_id[0] = 5; /* OK */

int roll_id[10] = 5; /* error */
roll_id[0] = {0}; /* error */

What you seem to want is to initialize p of type struct student. That is done with a nested initializer.

student p = { {0} }; /* initialize the array inside the struct */