Assign an array in a struct in c language

Question

I have the following code:

#include<stdio.h>

struct student {
    int* grades; 
    int num_grades;
};

double* get_means(struct student* arr, int n); // Function Signature

// grade range is: 0 - 100
// assume max number of grades to one student is up to 100
double* get_means(struct student* arr, int n)
{
    arr->grades = 90;
    printf("%d", arr->grades);
}

int main()
{
    struct student s, *p;
    s.grades = malloc(s.num_grades * (sizeof(int)));


    p->grades[0] = 1;
        printf("%d", p->grades[0]);
}

and I'm having an issue assigning a value (it doesn't matter what the value is, it can be: 0, 7, 50).

The compiler gives me the error:

Error   C4700   uninitialized local variable 's' used
Error   C4700   uninitialized local variable 'p' used

What can I do to fix this?

In other words: how do I assign a value to an array in a struct?

s.num_grades - this is definitely not initialized. And you need to include stdlib.h — Eugene Sh.
– Eugene Sh., Commented Jun 23, 2020 at 15:58
(1) You're using s.num_grades in the argument to malloc, but you never gave it a value. It is undefined. Either don't use it, or else assign something to it first. (2) You're assigning to p->grades[0] without setting p first. So set p to some valid pointer value first. If you don't understand what pointers are, then take a step back and find a C tutorial to learn about them. — Tom Karzes
– Tom Karzes, Commented Jun 23, 2020 at 16:00
arr->grades = 90; is wrong. grades is a pointer to an array of grades, not a single grade. — Barmar
– Barmar, Commented Jun 23, 2020 at 16:46

ValeriF21 · Accepted Answer · 2020-06-23 17:11:41Z

When you try to do that p->grades[0] = 1 first you need to assign the address for s to p and only than trying to access s from the pointer p. Second issue here is you trying to access array only through is name (in the function get means), As you know array name its only the base address of the array its not the value in it. Third issue its the function does not return nothing but in the prototype it returns pointer to double, I suggest you to change it to avoid any unnecessary warnings or errors. And another suggestion is including <stdlib.h> when using malloc and always check malloc does not returns you NULL.

The fixed code attached:

  struct student {
  int* grades; 
  int num_grades;
  };

void get_means(struct student* arr, int n); 

void get_means(struct student* arr, int n)
{
   arr->grades[0] = 90;
   printf("%d", arr->grades[0]);
}

int main()
{
   struct student s, *p;
   s.grades = malloc(s.num_grades * (sizeof(int)));

   /* Option 1 
   s.grades[0] = 1;
   printf("%d", s.grades[0]);
   */
    
   /* Option 2 */
   p = &s;
   p->grades[0] = 1;
   printf("%d", p->grades[0]);
 }

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