Call a function that receives an array using a pointer - c

Question

So I've this:
int main()
{
int workers;
printf("How many workers are there?\n");
scanf("%d", &workers);
printf("What are their preferences?\n");
int *pref = malloc(workers * sizeof(int));
if (pref == NULL)
return -1;
fillPreferences(pref, workers);
return 0;
}
I want now to fill the "pref" 2d array in this function:
void fillPreferences(int pref[][], int size)
{
for (int i=0;i<size;i++)
{
for (int j=0;j<size;j++)
{
scanf(" %d", &pref[i][j]);
}
}
}
It doesn't work, probably because I'm using the pointer wrong. How can I use malloc and then call a function and receive the values in the 2d array by doing pref[i][j]? (Note that I'm not looking to do something like scanf(..., &pref+i) or whatever. I need to actually use that 2d array.
Thanks :)

Answer 1

When you write a[i], it is turned into *(a+i). That is, a[i] accesses the memory by a+i address (well, it is a+i*sizeof(element) even).
As such, a[i][j] means *(*(a+i)+j). Two memory accesses. For this to work, your a should be an array of arrays. That is, you need to malloc its elements first and then malloc a memory to hold them.
In your particular case, i doubt you need it. What you need is make it 1D-array (which is it already) and calculate index from your two indices in whatever fashion you wish.

Answer 2

Your pref array is 1D so you can make it in this way:
#include <stdio.h>
#include <stdlib.h>
void fillPreferences(int **pref, int size)
{
int prefNum=0;
for (int i=0;i<size;i++)
{
puts("Number of preferences");
scanf("%d",&prefNum);
pref[i]=malloc(sizeof(int)*prefNum);
puts("Enter preferences");
for(int j=0;j<prefNum;j++){
scanf(" %d", &pref[i][j]);
}
}
}
int main()
{
int workers;
printf("How many workers are there?\n");
scanf("%d", &workers);
printf("What are their preferences?\n");
int **pref = malloc(workers * sizeof(int *));
if (pref == NULL)
return -1;
fillPreferences(pref, workers);
// Show values
printf("%d %d %d",pref[0][0],pref[1][0],pref[2][0]);
return 0;
}

Answer 3

You allocate memory for a 1D array, but the function you have is designed to accept a 2D array and fill it (although the function definition is incorrect and won't compile).
Corrected code:
#include <stdio.h>
#include <stdlib.h>
void fillPreferences(int** pref, int size)
{
for (int i = 0; i < size; i++)
{
for (int j = 0; j < size; j++)
{
scanf("%d", &pref[i][j]);
}
}
}
int main()
{
int workers;
printf("How many workers are there?\n");
scanf("%d", &workers);
printf("What are their preferences?\n");
int **pref = malloc(workers * sizeof(int*));
if (pref == NULL)
return -1;
for(int i = 0; i < workers; i++)
{
pref[i] = malloc(workers * sizeof(int));
if(pref[i] == NULL)
{
for(int j = 0; j < i; j++)
free(pref[j]);
free(pref);
return -1;
}
fillPreferences(pref, workers);
/* Don't forget to `free` everything after its use! */
return 0;
}