defining a 2D array with malloc and modifying it

Question

How do i define a 2D array using malloc ? (lets say 10X20).

second, can i increase number of rows or cols without creating a new increased array and copying all data

Answer 1

A 2D array is an 1D array of 1D arrays. As an array is simply a pointer, an array of arrays is an array of pointers. So, you use malloc to allocate an array of pointers (each representing a collumn), then use it again to allocate the individual arrays (each representing a row).

To expand/shrink the array, you use realloc(reference). Here's an example code:

#include 
#include 

int** alloc_matrix(int w, int h) {
    int** matrix;

    matrix = (int**) malloc(sizeof(int*) * h);
    for(int i = 0; i < h; i++) {
        matrix[i] = (int*) malloc(sizeof(int) * w);
    }

    return matrix;
}

void realloc_matrix(int*** matrix, int new_w, int new_h) {
    *matrix = (int**) realloc((void *)*matrix, sizeof(int*) * new_h);

    for(int i = 0; i < new_h; i++) {
        (*matrix)[i] = (int*) realloc((void *)(*matrix)[i], sizeof(int) * new_w);
    }
}

int main(int argc, char* argv[]) {
    // Allocating matrix
    int** m = alloc_matrix(10, 15);

    // Filling with some data
    for(int y = 0; y < 15; y++) {
        for(int x = 0; x < 10; x++) {
            m[y][x] = y * x; // Notice the index is [y][x] - NOT [x][y]!
        }
    }

    // Printing
    for(int y = 0; y < 15; y++) {
        for(int x = 0; x < 10; x++) {
            printf("%d\t", m[y][x]); // Notice the index is [y][x] - NOT [x][y]!
        }
        printf("\n");
    }

    // Reallocating
    realloc_matrix(&m, 20, 10);

    // Filling with some data
    for(int y = 0; y < 10; y++) {
        for(int x = 0; x < 20; x++) {
            m[y][x] = y * x; // Notice the index is [y][x] - NOT [x][y]!
        }
    }

    // Printing
    for(int y = 0; y < 10; y++) {
        for(int x = 0; x < 20; x++) {
            printf("%d\t", m[y][x]); // Notice the index is [y][x] - NOT [x][y]!
        }
        printf("\n");
    }
}

I'm sorry if I've got anything wrong. My C-fu is kinda rusty :)