Arrays

What you'll learn

An array stores a fixed number of same-typed elements in one contiguous region of memory. That simple layout makes arrays fast and foundational—and makes precise bounds essential.

By the end, you'll be able to:

• Declare, initialize, index, and update arrays
• Process arrays with safe zero-based loops
• Pass an array together with its length to a function
• Work with row-major multidimensional arrays

Declare & initialize arrays

Put the element count inside brackets. Initializers fill elements from left to right; omitted elements become zero. A designated initializer can target a particular index.

#include <stdio.h>

int main(void) {
    int scores[5] = {88, 92, 75, 100, 84};
    int zeros[4] = {0};             // every element becomes zero
    int inferred[] = {3, 6, 9};     // compiler infers length 3
    int sparse[6] = {[1] = 10, [4] = 40};

    scores[2] = 79;
    printf("%d %d %d %d\n",
           scores[2], zeros[3], inferred[2], sparse[4]);
    return 0;
}

Contiguous memory

If one int occupies four bytes on a platform, a five-element array occupies twenty consecutive bytes. C can find element i by movingi × sizeof(element) bytes from the beginning.

Traverse & process

A zero-based for loop naturally visits every element. Common operations include counting, summing, finding an extreme, searching, and transforming values.

#include <stdio.h>

int main(void) {
    int scores[] = {88, 92, 75, 100, 84};
    size_t count = sizeof scores / sizeof scores[0];
    int total = 0;
    int highest = scores[0];

    for (size_t i = 0; i < count; i++) {
        total += scores[i];
        if (scores[i] > highest) highest = scores[i];
    }

    printf("Average: %.1f\n", (double) total / count);
    printf("Highest: %d\n", highest);
    return 0;
}

Compute element counts

In the scope where an array is actually declared, divide its total byte size by one element's size:

Arrays and functions

An array argument gives a function access to the original elements. The function needs a separate length because no end marker is carried automatically. Use const when the function only reads.

#include <stddef.h>
#include <stdio.h>

double average(const int values[], size_t count) {
    if (count == 0) return 0.0;

    long total = 0;
    for (size_t i = 0; i < count; i++) {
        total += values[i];
    }
    return (double) total / count;
}

int main(void) {
    int values[] = {4, 8, 15, 16, 23, 42};
    size_t count = sizeof values / sizeof values[0];

    printf("%.2f\n", average(values, count));
    return 0;
}

Multidimensional arrays

A two-dimensional array is an array whose elements are themselves arrays. C stores rows contiguously in row-major order.

#include <stdio.h>

#define ROWS 2
#define COLS 3

int main(void) {
    int matrix[ROWS][COLS] = {
        {1, 2, 3},
        {4, 5, 6}
    };

    for (size_t row = 0; row < ROWS; row++) {
        for (size_t col = 0; col < COLS; col++) {
            printf("%d ", matrix[row][col]);
        }
        putchar('\n');
    }
    return 0;
}

Bounds & safety

C does not insert automatic bounds checks. An invalid index accesses storage that does not belong to that element, producing undefined behavior.

int values[5] = {0};

values[0] = 10;  // valid: first element
values[4] = 50;  // valid: last element

// values[5] = 60;  // BUG: one past the array
// values[-1] = 5;  // BUG: before the array

• Carry the element count with the array
• Use size_t for sizes and indexes
• Write conditions as index < count
• Test empty, one-element, and full-capacity cases

Recap & quick check

Arrays give C compact sequences. Next up: Module 8 — Strings & Character Handling, where character arrays become text.