C - Multi-Dimensional Arrays

Overview

Multi-dimensional arrays are arrays of arrays. The most common are two-dimensional arrays, often used to represent matrices or tables.

Learning Objectives

• Declare and index 2D arrays.
• Understand row-major order and passing 2D arrays to functions.

Prerequisites

• C - Arrays

Declaration

int matrix[3][4]; // 3 rows, 4 columns

Example

#include <stdio.h>

int main(void) {
    int mat[2][3] = {{1,2,3},{4,5,6}};
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            printf("%d ", mat[i][j]);
        }
        printf("\n");
    }
}

More Examples

#include <stdio.h>

void print2D(int a[][3], int rows) {
  for (int i = 0; i < rows; i++) {
    for (int j = 0; j < 3; j++) printf("%d ", a[i][j]);
    printf("\n");
  }
}

Checks for Understanding

1. How do you pass a 2D array to a function?

Expected Output

For the 2x3 example:

1 2 3 
4 5 6 

Common Pitfalls

• When passing a 2D array to a function, all but the first dimension must be specified.
• Row-major layout: a[i][j] is contiguous in the j direction; cache-friendliness depends on traversal order.
• Very large arrays on the stack can overflow; prefer static or dynamic allocation.
• Variable-length arrays (VLAs) require care with dimensions and may not be supported by all compilers.

Exercises

1. Write a function to add two 2x3 matrices and print the result.
2. Search for a value in a 2D array and return its coordinates.