Question

How to dynamically allocate a two dimensional (2D) array in C

Answer 1

#include <stdio.h>
#include <stdlib.h>
 
#define ROWS 3
#define COLS 4
 
void print_arr2d(int** arr2d) {
    for (int i = 0; i < ROWS; i++) {
        for (int j = 0; j < COLS; j++) {
            printf("%3d", arr2d[i][j]);
        }
        printf("\n");
    }
}

int** allocate_arr2d() {
    int** arr2d = malloc(ROWS * sizeof(int*));
 
    for (int i = 0; i < ROWS; ++i) {
        arr2d[i] = calloc(COLS, sizeof(int));
    }
 
    return arr2d;
}

void free_arr2d(int** arr2d) {
    for (int i = 0; i < ROWS; ++i) {
        free(arr2d[i]);
    }
 
    free(arr2d);
}
 
int main(void) {
 
    int** arr2d = allocate_arr2d();
 
    print_arr2d(arr2d);
 
    free_arr2d(arr2d);
 
    return 0;
}

 
 
/*
run:
 
  0  0  0  0
  0  0  0  0
  0  0  0  0
 
*/

 

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Answer 2

#include <stdio.h>
#include <stdlib.h>
 
#define ROWS 3
#define COLS 4
 
void print_arr2d(int** arr2d) {
    for (int i = 0; i < ROWS; i++) {
        for (int j = 0; j < COLS; j++) {
            printf("%3d", arr2d[i][j]);
        }
        printf("\n");
    }
}

int** allocate_arr2d() {
    int** arr2d = malloc(ROWS * sizeof(int*));
 
    for (int i = 0; i < ROWS; ++i) {
        arr2d[i] = malloc(COLS * sizeof(int));
    }
 
    for (int i = 0; i < ROWS; ++i) {
        for (int j = 0; j < COLS; ++j) {
            arr2d[i][j] = i * COLS + j;
        }
    }
    
    return arr2d;
}

void deallocate_arr2d(int** arr2d) {
    for (int i = 0; i < ROWS; ++i) {
        free(arr2d[i]);
    }
 
    free(arr2d);
}
 
int main(void) {
 
    int** arr2d = allocate_arr2d();
 
    print_arr2d(arr2d);
 
    deallocate_arr2d(arr2d);
 
    return 0;
}

 
 
/*
run:
 
  0  1  2  3
  4  5  6  7
  8  9 10 11
 
*/

 

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