Program for dynamic matrix allocation

Question

I am learning C. I have a program that dynamically creates and fills two matrices X and Y, with random numbers using rand() as shown below

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

void main(){

 int i, j, k, m, n;
 double **x;
 double **y;
 printf("Enter a number as the size of two square matrices\n");
 scanf("%d", &m);
 x = (double**)malloc(m * sizeof(double));  
 y = (double**)malloc(m * sizeof(double));
 /* initialize random seed: */
 srand(time(NULL));
 for(i = 0; i < m; i++) {
    x[i] = (double*)malloc(m * sizeof(double));
    y[i] = (double*)malloc(m * sizeof(double));
    for(i = 0; i < m; i++) {
        for(j = 0; j < m; j++) {
            x[i][j] = rand();
            y[i][j] = rand();
        }
        printf("\n");
    }
  }
  printf("\n\n");
}

When I run this program and give 2 as size of my matrix, I am seeing "Segmetation Fault" as error. Please note that the idea is to fill the two matrices with random elements of type double. Let me know if the above code is correct.

Corrected:Edit1

 x = (double**)malloc(m * sizeof(double*)); 
 y = (double**)malloc(m * sizeof(double*));
 /* initialize random seed: */
 srand(time(NULL));
 for(i = 0; i < m; i++) {
    x[i] = (double*)malloc(m * sizeof(double));
    y[i] = (double*)malloc(m * sizeof(double));

    for(j = 0; j < m; j++) {
        x[i][j] = rand();
        y[i][j] = rand();
    }
    printf("\n");

  }
  printf("\n\n");
}

Above is working correctly now. Need some explanation on x = (double**)malloc(m * sizeof(double*)); and x[i] = (double*)malloc(m * sizeof(double));

why * for sizeof(double*) in x = (double**)malloc(m * sizeof(double*));

Edit2

void main(){

 int i, j, k, m, n; 
 printf("Enter a number as the size of two square matrices\n");
 scanf("%d", &m);

 double (*x)[m] = malloc(sizeof(double[m][m]));
 double (*y)[m] = malloc(sizeof(double[m][m]));
 /* initialize random seed: */
 srand(time(NULL));
 for(i = 0; i < m; i++) {
    for(j = 0; j < m; j++) {
        x[i][j] = rand();
        y[i][j] = rand();
    }
    printf("\n");

  }
  printf("\n\n");
}

As per Jens Gustedt matrix allocation changed... I have removed

 double **x;
 double **y;
 x = (double**)malloc(m * sizeof(double*)); 
 y = (double**)malloc(m * sizeof(double*));
 x[i] = (double*)malloc(m * sizeof(double));
 y[i] = (double*)malloc(m * sizeof(double));

No it's not correct. Getting a seg fault clearly tells you there's a problem. Suggest you run your program in a debugger and/or add debugging print statements to try to find your problem. Also you may want to look at the "Related" links in this question. This type of matrix allocation has been asked many times already. — kaylum
user3121023 thanks for pointing out. I have added * and removed inner i loop. — noob man
@user3121023 can you explain the lines x = (double**)malloc(m * sizeof(double)); and x[i] = (double*)malloc(m * sizeof(double)); — noob man
These are only fake matrices and this technique belongs into a museum. In modern C, since C99, simply allocate a 2D matrix as double (*matrix)[m] = malloc(sizeof(double[m][m]));. So you only need one call to malloc for the whole matrix and have it all in consecutive memory. — Jens Gustedt

Unknown Unknown · Accepted Answer · 2016-01-19T13:03:01

These are only fake matrices and this technique belongs into a museum. In modern C, since C99, simply allocate a 2D matrix as double (*matrix)[m] = malloc(sizeof(double[m][m]));. So you only need one call to malloc for the whole matrix and have it all in consecutive memory.
– Jens Gustedt

Program for dynamic matrix allocation

1 Answers