Module 09: Arrays & Multidimensional Matrices

1. 1D Contiguous Memory 1 / 4

1D Array Contiguous Memory & Pointer Decay

Why arr[i] is mathematically identical to *(arr + i)

An array is a contiguous block of homogeneous elements stored side-by-side in memory. In C, the name of the array automatically decays to a pointer to its first element (&arr[0]).

array_math.c

int arr[4] = {10, 20, 30, 40};

// Address calculation: Address of arr[i] = BaseAddress + (i * sizeof(type))
printf("%d\\n", arr[2]);     // 30
printf("%d\\n", *(arr + 2)); // 30 (Dereferencing base pointer + 2 elements)
printf("%d\\n", 2[arr]);     // 30 (Valid C: *(2 + arr) == *(arr + 2))

2. 2D Row-Major Order 2 / 4

2D Arrays & Row-Major Order Memory Mapping

How multi-dimensional grids flatten onto 1D linear RAM

📐

Row-Major Formula

For int mat[ROWS][COLS];, element mat[r][c] is located at:
Linear 1D Index = (r × COLS) + c

matrix_passing.c

#define COLS 3

// Column count MUST be specified so compiler can compute (r * COLS + c) byte strides:
void print_matrix(int mat[][COLS], int rows) {
    for (int r = 0; r < rows; ++r) {
        for (int c = 0; c < COLS; ++c) {
            printf("%d ", mat[r][c]);
        }
        printf("\\n");
    }
}

3. Variable-Length Arrays 3 / 4

Variable-Length Arrays (VLAs in C99)

Runtime sizing on the Stack and its security implications

C99 introduced VLAs, allowing array dimensions determined at runtime:

vla_danger.c

void process(int n) {
    int vla[n]; // Allocated on Stack frame at runtime
    // DANGER: If n is excessively large (e.g. 10,000,000), it instantly blows the Stack (Stack Overflow Crash)!
    // This is why C11 made VLAs optional and professional systems engineers avoid them.
}

4. Array of Pointers 4 / 4

Array of Pointers vs Pointer to an Array

int *p[4] vs int (*p)[4]

pointer_array_syntax.c

// 1. Array of 4 Pointers (Ragged 2D array / jagged array)
int *arr_of_ptrs[4];

// 2. Single Pointer to an Array of 4 Integers
int (*ptr_to_arr)[4];

Module 09: Arrays & Multidimensional Memory Layout

1D Array Contiguous Memory & Pointer Decay

2D Arrays & Row-Major Order Memory Mapping

Variable-Length Arrays (VLAs in C99)

Array of Pointers vs Pointer to an Array

1. Out-of-Bounds Undefined Behavior

2. Practice Exercises & Solutions