Initialize a 2d array

Question

I'm trying to create an undefined 2D m x m array as global variable. But after I type the m value, I get a

segmentation fail(core image dumped)

error. Can anyone help me with this? Thank you very much. Here's my code:

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

double **A;

int main(void) {

    int m, i, j;

    scanf("%d", &m);

    A = (double **)malloc(m * m * sizeof(double));

    for (i = 0; i < m; i++) {
        for (j = 0; j < m; j++) {
            A[i][j] = i + j;
        }
    }

    return 0;
}

can someone find a good dupe? I'm too tired to google the proper keywords. — Jean-François Fabre, Aug 25 '18 at 20:03
what the hell, not the perfect dupe, but ... https://stackoverflow.com/questions/36890624/malloc-a-2d-array-in-c — Jean-François Fabre, Aug 25 '18 at 20:05
Despite what you may think, you are allocating a 1D array, so use `double *` instead of `double **`. Access via: `A[(i * m) + j]` instead of `A[i][j]` — Craig Estey, Aug 25 '18 at 20:21
Possible duplicate of [Malloc a 2D array in C](https://stackoverflow.com/questions/36890624/malloc-a-2d-array-in-c) — Alex Celeste, Aug 25 '18 at 20:43
A real 2D array is what dodobhoot posted in his modified answer. But, this involves an extra layer of pointer indirection which makes it slow. See this instead: https://stackoverflow.com/questions/14808908/c-pointer-to-two-dimensional-array This lets you do `A[i][j]` _without_ the indirect pointer penalty. — Craig Estey, Aug 25 '18 at 22:24

dodobhoot · Accepted Answer · 2018-08-25T20:58:10.120

1

If you want to allocate memory block of size m * m your way then you need to use single pointer arithmetic to access elements.

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

double *A;

int main(void) {

    int m;
    scanf("%d", &m);

    A = malloc(m * m * sizeof(double));

    for (int i = 0; i < m; i++) {
        for(int j = 0; j < m; j++) {
            *(A + i * m + j) = i + j;
        }
    }

    return 0;
}

Another way to do this is to use an array of pointers where each pointer points to a memory of size m as explained in here or check the below code

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

double **A;

int main(void) {

    int m;
    scanf("%d", &m) ;

    A = malloc(m * sizeof(double*));

    for(int i = 0; i < m; i++){
        A[i] = malloc(m * sizeof(double));
    }

    for (int i = 0; i < m; i++){
        for(int j = 0; j < m; j++){
            A[i][j] = i + j;
        }
    }

    return 0;
}

edited Aug 25 '18 at 20:58

answered Aug 25 '18 at 20:20

dodobhoot

493
6
15

also, OP should check that `scanf` returned a valid value, else `m` could be `2453463646` – Jean-François Fabre Aug 25 '18 at 20:24
You can get the original syntax back by typing the result as pointer-to-array, making the allocated block an array-of-arrays, and keeping the nice multidim syntax OP wanted without needing separate allocations. Your solution is an important step to understanding how that works, though. – Alex Celeste Aug 25 '18 at 20:39
Thanks for this. I will test sokm – Ericli Aug 25 '18 at 20:46
yeah, a real 2D access alternative would be a plus. But I understand that you don't want to invest on your answer since you got a really unjustified downvote... – Jean-François Fabre Aug 25 '18 at 20:47
Thanks for this. I will test soon. – Ericli Aug 25 '18 at 20:47
2D access alternative added. – dodobhoot Aug 25 '18 at 21:01

Initialize a 2d array

1 Answers1