C - create an array with malloc, then loop through it

Question

I'm learning C at the moment and tried to write this function

int *create_matrix(int n) {
   int *matrix = malloc(n*n*sizeof(int));
   srand(time(NULL));
   int i, j;
   for (i = 0; i < n; i++) {
      for (j = 0; j < n; j++) {
        matrix[i][j] = rand()%10;
      }
   }
   return matrix;
}

why does this fail to compile? its complaining about matrix[i][j] is not an pointer/array. but I've just declared it as an pointer six lines above...

use matrix[i * n + j], you don't have a multidimensional array — Gunther Van Butsele, Oct 15 '14 at 11:22
Since you declared `int *matrix`, `matrix` is a pointer, but `matrix[i]` is an int, thus `matrix[i][j]` doesn't make sense (it's like saying `21[i] = rand()%10;`). — Dettorer, Oct 15 '14 at 11:37
Thank you everyone for all the explaination! I clearly see my mistake now and it seems i'll need a little bit more practice with pointers... — thebackfisch, Oct 15 '14 at 20:30

score 7 · Answer 1 · edited May 23 '17 at 12:20

It's a 1D array, so you have to treat it as a 1D array, and not as 2D.

You can of course still store your n x n elements in it:

    matrix[i * n + j] = rand() % 10;

If you prefer, you can set up a 2D structure by following the advice given in How do I work with dynamic multi-dimensional arrays in C?

By the way, you probably don't want to be calling srand() every time you create a matrix. If you call create_matrix() twice in quick succession, you could end up getting the same "random" matrix. Call srand() once at the start of your program.

CinCout · Answer 2 · 2014-10-15T13:32:37.847

2

You've declared matrix as a one-dimensional array, and not as a two-dimensional array. So use it like:

for(int i=0; i<n*n; ++i)
    matrix[i] = //whatever;

If you really need two-dimensional array, you need to use double pointers:

int **matrix = malloc(n * sizeof(int *));  // notice the double pointer
// ....
// ....
for (i = 0; i < n; i++)
{
    matrix[i] = malloc(sizeof(int) * n);
    // ....
}

edited Oct 15 '14 at 13:32

answered Oct 15 '14 at 11:23

CinCout

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You also need to allocate the rows if you are going for a two-dimensional approach. – Tom Fenech Oct 15 '14 at 11:34
I think that's obvious! I was just suggesting a way how it could be approached. – CinCout Oct 15 '14 at 12:38
It's obvious to me and perhaps to you but the issue here is that you've half-suggested another way of doing something, making your answer incomplete. It would be a lot more useful to show the full approach, or simply leave it out entirely as others have already explained it in more depth. – Tom Fenech Oct 15 '14 at 13:22

score 1 · Accepted Answer · answered Oct 15 '14 at 11:26

You declare matrix as a one-dimensional array. You need to either store the data in it as such, i.e.

matrix[i * n + j] = rand() % 10;

Or declare it as a two-dimensional array and allocate memory accordingly.

    int** matrix = malloc(n * sizeof(int*));
    int i, j;
    for (i = 0; i < n; i++) {
        matrix[i] = malloc(n * sizeof(int));
        for (j = 0; j < n; j++) {
            matrix[i][j] = rand() % 10;
        }
    }

score 1 · Answer 4 · answered Oct 15 '14 at 11:26

What it's really complaining about is, the matrix[i] is not a pointer/array. You've only created a 1-dimensional array, you either can access it directly like

matrix[i * n + j] = rand() % 10;

or go ahead, do it cleanly, and redo the array, so it will be 2-dimensional. You'll need to use pointer to a pointer (for every member in line - one row)

int **matrix

but you'll have to loop trough the

*matrix

to malloc each row by itself.

Look up on 2-dimensional arrays.

//EDIT: Also, move the srand() to the beginning of the file. You don't want to srand() with every new matrix.

score 1 · Answer 5 · answered Oct 15 '14 at 11:27

You want two dimensions, then you need a pointer to pointer (not a single pointer):

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

int **create_matrix(int n)
{
   int **matrix = malloc(n * sizeof(int *));
   srand(time(NULL));
   int i, j;
   for (i = 0; i < n; i++) {
      matrix[i] = malloc(sizeof(int) * n);
      for (j = 0; j < n; j++) {
         matrix[i][j] = rand()%10;
      }
   }
   return matrix;
}

int main(void)
{    
    int **matrix = create_matrix(5);
    return 0;
}

C - create an array with malloc, then loop through it

5 Answers5