Question

I have a simple function called make5 that makes every element in a 2d matrix equal to 5, shown below:

int make5(int r, int c, double **d)
{
  int i, j;

  for (i = 0; i < r; i++) {
    for (j = 0; j < c; j++) {
      d[i][j] = 5;
    }
  }

  return 0;
}

I’d like to be able to run this function on different matrices, using the following calls:

make5(2, 3, a);
make5(2, 4, b);

where a and b have been declared as pointers to arrays of arrays. But when I try this I keep getting a segmentation fault error. How can I change make5 so I can run it on both a and b?

Answer 1

The function works just fine. You must be allocating the memory incorrectly or passing wrong parameters to the function.

Working example:

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

int make5(int r, int c, double **d)
{
  int i, j;

  for (i = 0; i < r; i++) {
    for (j = 0; j < c; j++) {
      d[i][j] = 5;
    }
  }

  return 0;
}

#define ROWS 4
#define COLS 3

int main(void)
{
  double** p = malloc(ROWS * sizeof(double*));
  int i, j;

  for (i = 0; i < ROWS; i++)
    p[i] = malloc(COLS * sizeof(double));

  for (i = 0; i < ROWS; i++)
    for (j = 0; j < COLS; j++)
      p[i][j] = 1;

  for (i = 0; i < ROWS; i++)
  {
    for (j = 0; j < COLS; j++)
      printf("%f ", p[i][j]);
    printf("\n");
  }

  make5(ROWS, COLS, p);

  for (i = 0; i < ROWS; i++)
  {
    for (j = 0; j < COLS; j++)
      printf("%f ", p[i][j]);
    printf("\n");
  }

  return 0;
}

Output (ideone):

1.000000 1.000000 1.000000 
1.000000 1.000000 1.000000 
1.000000 1.000000 1.000000 
1.000000 1.000000 1.000000 
5.000000 5.000000 5.000000 
5.000000 5.000000 5.000000 
5.000000 5.000000 5.000000 
5.000000 5.000000 5.000000