C — Arrays

Declaration and initialization

#include <stdio.h>

int main() {
    int numbers[5];               // Uninitialized
    int zeros[5] = {0};          // All zeros
    int primes[] = {2, 3, 5, 7, 11}; // Size inferred: 5
    int partial[5] = {10, 20};   // {10, 20, 0, 0, 0}

    // C99 designated initializers
    int scores[5] = {[0] = 95, [3] = 87}; // {95, 0, 0, 87, 0}

    printf("First: %d, Last: %d\n", numbers[0], numbers[4]);
    return 0;
}

Array size

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40, 50};

    // Total bytes
    printf("Bytes: %zu\n", sizeof(arr));       // 20 (5 × 4)

    // Element count
    size_t len = sizeof(arr) / sizeof(arr[0]);
    printf("Length: %zu\n", len);              // 5

    return 0;
}

Accessing elements

#include <stdio.h>

int main() {
    int nums[] = {10, 20, 30, 40, 50};

    // Read elements
    printf("First: %d\n", nums[0]);  // 10
    printf("Third: %d\n", nums[2]);  // 30

    // Modify elements
    nums[1] = 99;
    printf("Modified: %d\n", nums[1]); // 99

    // Iterate over array
    for (int i = 0; i < 5; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");

    return 0;
}

Bounds checking

C does not check array bounds — access beyond the array is undefined:

#include <stdio.h>

int main() {
    int arr[3] = {1, 2, 3};

    // These are undefined behavior:
    // printf("%d\n", arr[10]);  // Reading beyond bounds
    // arr[10] = 99;            // Writing beyond bounds

    // Always track array sizes manually
    return 0;
}

Passing arrays to functions

Arrays decay to pointers — you must pass the size:

#include <stdio.h>

void printArray(const int arr[], int size) {
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

void reverse(int arr[], int size) {
    for (int i = 0; i < size / 2; i++) {
        int temp = arr[i];
        arr[i] = arr[size - 1 - i];
        arr[size - 1 - i] = temp;
    }
}

int main() {
    int nums[] = {1, 2, 3, 4, 5};
    int size = sizeof(nums) / sizeof(nums[0]);

    printArray(nums, size); // 1 2 3 4 5
    reverse(nums, size);
    printArray(nums, size); // 5 4 3 2 1

    return 0;
}

Multi-dimensional arrays

#include <stdio.h>

int main() {
    // 2D array (matrix)
    int matrix[3][4] = {
        {1, 2, 3, 4},
        {5, 6, 7, 8},
        {9, 10, 11, 12}
    };

    // Access elements
    printf("Element [1][2]: %d\n", matrix[1][2]); // 7

    // Iterate
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            printf("%3d", matrix[i][j]);
        }
        printf("\n");
    }

    return 0;
}

Array algorithms

#include <stdio.h>

// Linear search
int linearSearch(int arr[], int size, int target) {
    for (int i = 0; i < size; i++) {
        if (arr[i] == target) return i;
    }
    return -1;
}

// Bubble sort
void bubbleSort(int arr[], int size) {
    for (int i = 0; i < size - 1; i++) {
        for (int j = 0; j < size - 1 - i; j++) {
            if (arr[j] > arr[j + 1]) {
                int temp = arr[j];
                arr[j] = arr[j + 1];
                arr[j + 1] = temp;
            }
        }
    }
}

// Find min and max
void findMinMax(int arr[], int size, int *min, int *max) {
    *min = *max = arr[0];
    for (int i = 1; i < size; i++) {
        if (arr[i] < *min) *min = arr[i];
        if (arr[i] > *max) *max = arr[i];
    }
}

int main() {
    int nums[] = {64, 34, 25, 12, 22, 11, 90};
    int size = sizeof(nums) / sizeof(nums[0]);

    bubbleSort(nums, size);
    printf("Sorted: ");
    for (int i = 0; i < size; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n"); // 11 12 22 25 34 64 90

    int min, max;
    findMinMax(nums, size, &min, &max);
    printf("Min: %d, Max: %d\n", min, max);

    int idx = linearSearch(nums, size, 25);
    printf("Found 25 at index: %d\n", idx);

    return 0;
}

Variable-length arrays (VLA)

C99 allows arrays with runtime-determined size:

#include <stdio.h>

int main() {
    int n;
    printf("Enter array size: ");
    scanf("%d", &n);

    int arr[n]; // VLA — size determined at runtime
    for (int i = 0; i < n; i++) {
        arr[i] = i * 2;
    }

    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    return 0;
}

Mini Practice

Write C code that:

1. Creates a 2D array and prints its diagonal elements
2. Implements bubble sort and sorts an array
3. Searches for an element in an array
4. Finds the second largest element in an array

Up Next

In the next lesson, you'll learn about Strings — working with text in C.