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cpointersmultidimensional-array
Miles
Miles

Reputation: 2527

2D pointer to act like 2D array

In C, I am trying to declare a 2D array outside of any function so that it can be shared by the functions. I know in the single dimensional case you can do the following(and make it behave LIKE an array):-

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

int *ptr;

int main(){
  int n=5;
  int i;

  ptr = malloc(n*sizeof(*ptr));
  for(i=0; i<n; i++){
    ptr[i] = i;
    printf("%i\n", ptr[i]);
  }
  free(ptr);
  return 0;
}

I am wondering how this would work for the multidimensional case.

Upvotes: 0

Views: 133

Answers (3)

Noctua
Noctua

Reputation: 5208

In stead of a single pointer, you just use a double pointer:

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

int **ptr;

int main() {
  int n = 5;
  int m = 10;
  int i, j;

  ptr = malloc(n * sizeof(int*));
  for(i=0; i<n; i++) {
    ptr[i] = malloc(m * sizeof(int));
    for(j=0; j<m; j++) {
      ptr[i][j] = j;
      printf("%i\n", ptr[i][j]);
    } 
  }

  for(i=0; i<n; i++) {
    free(ptr[i]);
  }
  free(ptr);

  return 0;
}

Things you should be wary of:

  • Allocate each array seperately
  • Free each array seperately, and the whole

Upvotes: 1

Jens Gustedt
Jens Gustedt

Reputation: 78903

As long as you fix the second dimension you can basically use the same "trick":

double (*A)[42];

If the second dimension is to be dynamic, too, you'd have to declare it in function scope:

double (*B)[n];

as a pointer to VLA, variable length array, and then pass the pointer to array to functions as 

void f(size_t m, double (*aB)[m]);

you'd allocate such a 2D array in one go

B = malloc(sizeof(double[m][n]));

and thus you ensure that your matrix is contiguously allocated.

Upvotes: 0

Jack
Jack

Reputation: 133557

If you want to access it using two indices with [] operator then you will need to use a pointer to pointer to int and allocate each single inner 1D array:

int **ptr = malloc(R*sizeof(int*));
for (int i = 0, i < R; ++i)
  ptr[i] = malloc(C*sizeof(int));

ptr[i][j] = 40;

But you can also directly create a single 1D array of R*C elements and then access it by calculating the correct index ptr[R*i + j] or ptr[C*j + i].

Upvotes: 0

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