CIS241

System-Level Programming and Utilities

C - 2D Arrays

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Expanding dimensions

We've done 1-D arrays so far

What's a 2-D array called?

• A matrix!

Declaring

We can declare a 2D array like this:

• int mat[2][3];

What are the dimensions?

• 2 rows, 3 cols (row-major)

Where does this memory live?

• On the stack!

It’s one contiguous block of memory

• int mat[2][3];

• What does mat[1][0] refer to?

• Or mat[1][2]?

Dynamic allocation (easy way)

There are multiple ways to dynamically allocate a matrix

Here, we'll walk through the easiest - treat a 2D array as a 1D array!

int* mat = (int*)malloc(w * h * sizeof(int));

How to access the c-th column of the r-th row?

• mat[r * w + c]

This is a cross-language way to handle multi-dimensional arrays

• Can’t use [][] notation, though

What if I want [ ][ ] syntax?

Option 1: Allocate individual arrays

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

Can use [][], but no longer contiguous in memory.

Option 2: Assign pointers into an array

Assigning pointers into an array

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

Can use [][] AND is contiguous