Passing double pointer (of dynamically allocated double array) to function

Question

double** makeit(int, int);
void showit(double**, int, int, int);

int main()
{
    int i,j;
    int x,y;
    printf("x=");
    scanf("%d",&x);
    printf("y=");
    scanf("%d",&y);

    double (*mas2d)[x];
    mas2d=makeit(x,y);
    printf("%4.0f ir  %4.0f \n",mas2d[0][0],mas2d[1][0]);
    showit(&mas2d, x, y);

    return 0;
}

double** makeit(int x, int y)
{
    double (*masp)[x];
    int i,j;
    double skc;
    masp= malloc((x*y)*sizeof(double));
    skc=1;

    for (i=0;i<x;i++)
    {
        for (j=0;j<y;j++)
        {
            masp[i][j]=skc;
            skc++;
        }
    }
    return masp;
}

void showit(double** mas[], int x, int y)
{
    int i,j;
    printf("%4.0f ir  %4.0f \n",mas[0][0],mas[1][0]);
    printf("x===%d",x);

    for(i=0;i<x;i++)
    {
        printf("\n");

        for(j=0;j<y;j++) 
        {
            printf("%4.0f \n",mas[i][j]);
        }
    }
}

What I do 1. I dynamically allocate double array mas2d in function makeit. 2. I want to send that mas2d array pointer to function showit and print it there.

What's the problem I can print mas2d array pointer from main function with no problems, but when I pass it to separate function showit, I just cant get it work... I've been trying to send it as a 3D pointer and maybe 100 other ways with no luck at all.

Here you call show like this showit(&mas2d, x, y); but you declare it like this =>> void showit(double**, int, int, int); , Why ? — Michi
– Michi, Commented May 27, 2016 at 17:50
Push up your compiler's warning level and fix the code until no more warnings are issued. — alk
– alk, Commented May 27, 2016 at 17:56
@rokastokas Take a look Here maybe will help somehow. The way I see you need a Function returning a Pointer to an Array — Michi
– Michi, Commented May 27, 2016 at 17:58
@alk, sorry i forgot to change those names from my language, fixed. — rokastokas
– rokastokas, Commented May 27, 2016 at 19:17

dbush · Accepted Answer · 2016-05-27 18:09:42Z

1

The cleanest way to do this is to have all instances of the 2D use the type double **. When you do your allocations, you'll first need to allocate an array of x double *, then for each row allocate y double.

double** makeit(int x, int y){
    double **masp;
    int i,j;
    double skc;

    masp= malloc(x*sizeof(double *));
    skc=1;

    for (i=0;i<x;i++) {
        masp[i]= malloc(y*sizeof(double));
        for (j=0;j<y;j++){
            masp[i][j]=skc;
            skc++;
        }
    }

    return masp;
}

answered May 27, 2016 at 18:09

dbush

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1 Comment

rokastokas Over a year ago

thank you!! i understand allocating your way is proper way, but still, is it possible to do this in my way? maybe wrong passing, or smtg?

alk · Accepted Answer · 2016-05-27 20:41:04Z

Try this (see details on changes as comments to the source):

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

double (*makeit(size_t x, size_t y))[] /* Make it return what it creates,
                                                      a pointer to an array of double.  */
{
  double (*masp)[x];
  size_t i, j; /* Index arrays and address storage using size_t. */
  double skc = 1.; /* Always initialise on definition if possible; Use a double literal 
                                                                to initialise a double. */
  masp = malloc(y * sizeof *masp); /* Allocate y times what masp points to,
                                                         that is an array of x doubles. */
  if (NULL == masp) /* Return is we got nothing. */
  {
    return NULL;
  }
  /* skc = 1; */ /* Not needed any more (see definition of skc). */

  for (i = 0; i < x; i++)
  {
    for (j = 0; j < y; j++)
    {
        masp[i][j] = skc;
        skc++;
    }
  }
  return masp;
}

void showit(size_t x, size_t y, double (*mas)[x]) /* Pass the address of a VLA
                                              after passing it primary dimension (x). */
{
  size_t i, j; /* Index arrays and address storage using size_t. */
  printf("%4.0f ir  %4.0f \n",mas[0][0], mas[1][0]);
  printf("x===%zu", x); /* Use the appropriate conversion specifier for size_t */

  for (i = 0; i < x; i++)  
  {
    printf("\n");

    for (j = 0; j < y; j++)
    {
        printf("%4.0f \n", mas[i][j]);
    }
  }
}

int main(void) /* It ought to be this at least. */
{
  //int i, j; /* Unused. */
  size_t x,y; /* Index array and address storage using size_t. */

  printf("x=");
  fflush(stdout); /* Make sure the output is shown. */
  scanf("%zu",&x); /* Use the appropriate specifier for size_t */

  printf("y=");  /* Make sure the output is shown. */
  fflush(stdout); /* Use the appropriate specifier for size_t */
  scanf("%zu",&y); /* Make sure the output is shown. */

  double (*mas2d)[x] = makeit(x, y); /* Always initialise on definition if possible. */

  if (NULL == mas2d) /* Do error checking! */
  {
    perror("makeit() failed");
    exit(EXIT_FAILURE);
  } 

  printf("%4.0f ir  %4.0f \n", mas2d[0][0], mas2d[1][0]); 

  showit(x, y, mas2d); /* Adjust order of parameters suiting the changes. */

  return 0;
}

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