2D Arrays

This is mostly useful for implementing matrices or tables of data. There are three methods for declaring a 2D array:

Normal declaration

int A[3][4];

An array of 3 * 4 size will be created inside the main memory, with three rows and four columns. The memory will be allocated like a single dimension array, but the compiler allows us to access it as a 2D array with the row number and column number, for example with A[1][2] = 15;
We can also directly mention the list of elements
This one is fully stack allocated.

int A[3][4] = {{1, 2, 3, 4}, {2, 4, 6, 8}, {3, 5, 7, 9}};

With pointers

Declaring an array of int pointers, and pointing them each to an array of size four, created in heap.
Even with this structure, you can access it just like a normal array.
This is partially in the heap, as int *A is created in the stack.

int *A[3];
A[0] = (int *)malloc(4 * sizeof(int));
A[1] = (int *)malloc(4 * sizeof(int));
A[2] = (int *)malloc(4 * sizeof(int));

A[1][2] = 15;

With double pointers

This one is fully heap allocated.


#include <stdlib.h>

int main() {
  int **A;
  A = (int **)malloc(3 * sizeof(int *));
  A[0] = (int *)malloc(4 * sizeof(int));
  A[1] = (int *)malloc(4 * sizeof(int));
  A[2] = (int *)malloc(4 * sizeof(int));

  // Accessing the 2D array
  for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 4; j++) {
      A[i][j] = 5;
    }
  }
}