Arrays & Strings

Master contiguous memory blocks, 2D matrices, array decay to pointers, string handling, and avoid buffer overflows.

Start Learning View Memory

1D Arrays

A collection of elements of the same data type stored in contiguous memory.

array1d.c

#include <stdio.h>

int main() {
    // Declare array of 5 integers
    int scores[5];
    
    // Initialize elements
    scores[0] = 85;
    scores[1] = 92;
    scores[4] = 95;
    
    // Loop through array
    for (int i = 0; i < 5; i++) {
        printf("Index %d: %d\n", i, scores[i]);
    }
    return 0;
}

Memory Layout (4 bytes per int): Index Value Address ─────────────────────────── 0 85 1000 ┐ 1 92 1004 │ Contiguous 2 ?? 1008 │ Memory Block 3 ?? 1012 │ 4 95 1016 ┘

Declaration Styles

int arr1[10];           // Uninitialized
int arr2[] = {1, 2, 3}; // Size 3
int arr3[5] = {1, 2};   // [1, 2, 0, 0, 0]

Key Rules

Size is constant at compile time
Indices: 0 to (size-1)
No bounds checking!
Name is pointer to first element

2D Arrays

Matrices and tables. Arrays of arrays.

matrix.c

int matrix[2][3] = {
    {1, 2, 3},   // Row 0
    {4, 5, 6}    // Row 1
};

// Access: matrix[row][col]
printf("%d", matrix[1][2]); // Output: 6

Memory Layout

How multi-dimensional arrays are flattened in RAM.

2D Array: int mat[2][2] = {{1,2}, {3,4}}; Logical View: Physical RAM (Row-Major Order): ┌───┬───┐ ┌───┬───┬───┬───┐ │ 1 │ 2 │ │ 1 │ 2 │ 3 │ 4 │ ├───┼───┤ └───┴───┴───┴───┘ │ 3 │ 4 │ [0][0] [0][1] [1][0] [1][1] └───┴───┘ Address increases linearly ----->

                Row-Major Order: C stores arrays row by row. This is crucial for cache performance. Always loop through rows first, then columns.
            

Array Decay

The concept of arrays "decaying" into pointers when passed to functions.

decay.c

void printSize(int arr[]) {
    // Decays to (int *), size is 8 bytes (pointer)
    printf("%lu", sizeof(arr)); 
}

int main() {
    int nums[10];
    // True size: 10 * 4 = 40 bytes
    printf("%lu", sizeof(nums)); 
    
    printSize(nums);
}

Strings

Character arrays terminated by a null character '\0'.

Declaration

char s1[] = "Hello";
// Size is 6 ('H','e','l','l','o','\0')

char s2[10] = "Hi";
// s2[2] through s2[9] are 0

String Functions

strlen(): Length excluding \0
strcpy(): Copy string (Unsafe)
strcat(): Concatenate
strcmp(): Compare

String: "C" Index: 0 1 Char: 'C' '\0' ASCII: 67 0