passing array of struct to function

Hi,
I'm trying to make array of struct and I don't get where is my mistake.
can you help me please?

This is the code:

Code:

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typedef struct{
    int i_index;
    int j_index;
    int value;
}index_s;

int index_sum(int** arr, int rows, int cols, index_s** ans_arr){
    int i;
    int j;
    int ans = 0;
    int arr_i = 0;

    for(i=0; i<rows; i++){
        for(j=0; j<cols; j++){
            if ((i+j) == arr[i][j]){
                ans++;
            }
        }
    }
   
    *ans_arr = (index_s*)malloc(ans*sizeof(index_s));

    for(i=0; i<rows; i++){
        for(j=0; j<cols; j++){
            if((i+j) == arr[i][j]){
                ans_arr[arr_i]->i_index = i;
                ans_arr[arr_i]->j_index = j;
                ans_arr[arr_i]->value = arr[i][j];
                arr_i++;
            }
        }
    }

    return ans;
}

void third_main(){
    int** arr;
    index_s* ans_arr;
    int rows, cols;
    int num_of_sums;
   

    printf("Enter number of rows: ");
    scanf("%d",&rows);
    printf("Enter number of cols: ");
    scanf("%d",&cols);
   
    arr = alloc_matrix(rows, cols);

    num_of_sums = index_sum(arr, rows, cols, &ans_arr);
    printf("i=%d\t j=%d\t value=%d\n", ans_arr->i_index, ans_arr->j_index, ans_arr->value);
}

---

Thank you :)

Your code is missing necessary includes:

Code:

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#include <stdio.h>
#include <stdlib.h>

---

"third_main" should presumably be called main and it should return an int.

You're missing the "alloc_matrix" function.

You should remove (index_s*) from in front of malloc. If your compiler complains, then you are compiling as C++ instead of C.

And you probably want to access it like this:

Code:

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(*ans_arr)[arr_i].i_index

---

Thanks!

I know that I am missing includes... This is just part of the code...
The "alloc_matrix" is working fine, this is the code of it:

Code:

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int** alloc_matrix(int rows, int cols){
    int** c;
    int i;
    int j;

    c = (int**)calloc(rows,sizeof(int*));
    if(!c)
        return NULL;
    for(i=0; i<rows; i++){
        c[i] = (int*)calloc(cols,sizeof(int));
        if(!c[i]){
            free_matrix(c,i);
            return NULL;
        }
        for(j=0; j<cols; j++){
            printf("arr[%d][%d]= ",i,j);
            scanf("%d",&c[i][j]);
        }
    }
    return c;
}

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Why I need to remove (index_s*) from the malloc? I don't need to tell what type I want?

But you helped me because the problem was the access to the struct and now all works well.
thank you :)

Quote:

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Originally Posted by DoK

Why I need to remove (index_s*) from the malloc? I don't need to tell what type I want?

---

Yes. Exactly. You don't need to tell it what type you want. C automatically converts void* to whatever type you need. If your compiler complains then you are compiling as C++ instead of C, which is a mistake. Try removing it.

Your alloc_matrix seems overly-complicated. Normally we wouldn't malloc (or calloc) each row separately. That's a waste of time and space. The normal practice is to allocate the data as a single block and then set the row pointers.

Code:

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int** alloc_matrix(int rows, int cols) {
    // allocate row pointers
    int **a = malloc(rows * sizeof *a);
    if (!a) return NULL;

    // allocate a block of memory to hold the data
    a[0] = calloc(rows * cols, sizeof **a);
    if (!a[0]) { free(a); return NULL; }

    // set the row pointers to the beginning of each row
    for (int i = 1; i < rows; i++)
        a[i] = a[i - 1] + cols;

    return a;
}

void free_matrix(int **a) {
    free(a[0]);
    free(a);
}

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You should have a separate function to fill the matrix. Allocating it and filling it with user data are two separate things.

And remember to check for a NULL return from alloc_matrix in main!

Please read this explanation of why you should not cast calloc(), malloc(), and realloc().

Thank you very much :)