Arrays

So far, we have only used variables that can hold one value at a time. For example, int score = 100;. But what if you need to store the scores for an entire class of 30 students? Declaring 30 separate variables (score1, score2, etc.) would be extremely inefficient.

An ARRAY is a data structure that can store a fixed-size, sequential collection of elements of the SAME data type. Think of it as a row of numbered boxes, where each box holds a value of the same type.

To declare an array, you specify the data type of its elements, the name of the array, and the number of elements it can hold (its size) in square brackets [].

The following line declares an array named high_scores that can hold exactly 5 integers.

ACCESSING ARRAY ELEMENTS: You access individual elements of an array using an INDEX in square brackets. C uses ZERO-BASED INDEXING. This is a critical concept. It means the first element is at index 0, the second at index 1, and so on. For an array of size 5, the valid indices are 0, 1, 2, 3, and 4.

Accessing an index outside of this range (like high_scores[5]) leads to UNDEFINED BEHAVIOR, a common source of bugs and crashes in C programs.

Manually assigning each element is tedious. You can initialize an array when you declare it using an initializer list in curly braces {}.

If you provide an initializer list, you can let the compiler figure out the size of the array by leaving the square brackets empty.

The real power of arrays is realized when you combine them with loops. A for loop is perfect for iterating through an array, performing an action on each element.

Full Source

/**
 * @file 8_arrays.c
 * @brief Part 1, Lesson 8: Arrays
 * @author dunamismax
 * @date 06-15-2025
 *
 * This lesson introduces arrays, a fundamental data structure for storing
 * collections of elements of the same type.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * So far, we have only used variables that can hold one value at a time. For
 * example, `int score = 100;`. But what if you need to store the scores for
 * an entire class of 30 students? Declaring 30 separate variables (`score1`,
 * `score2`, etc.) would be extremely inefficient.
 *
 * An ARRAY is a data structure that can store a fixed-size, sequential
 * collection of elements of the SAME data type. Think of it as a row of
 * numbered boxes, where each box holds a value of the same type.
 */

#include <stdio.h>

int main(void)
{
    // --- Part 1: Declaring and Initializing an Array ---

    /*
     * To declare an array, you specify the data type of its elements, the name
     * of the array, and the number of elements it can hold (its size) in
     * square brackets `[]`.
     *
     * The following line declares an array named `high_scores` that can hold
     * exactly 5 integers.
     */
    int high_scores[5]; // An array of 5 integers.

    /*
     * ACCESSING ARRAY ELEMENTS:
     * You access individual elements of an array using an INDEX in square brackets.
     * C uses ZERO-BASED INDEXING. This is a critical concept. It means the
     * first element is at index 0, the second at index 1, and so on.
     * For an array of size 5, the valid indices are 0, 1, 2, 3, and 4.
     *
     * Accessing an index outside of this range (like `high_scores[5]`) leads to
     * UNDEFINED BEHAVIOR, a common source of bugs and crashes in C programs.
     */
    printf("--- Part 1: Assigning and Accessing Array Elements ---\n");
    high_scores[0] = 980; // Assign 980 to the first element
    high_scores[1] = 950; // Assign 950 to the second element
    high_scores[2] = 875;
    high_scores[3] = 820;
    high_scores[4] = 799; // The fifth (and last) element

    printf("The highest score is: %d\n", high_scores[0]);
    printf("The third highest score is: %d\n", high_scores[2]);
    printf("\n");

    // --- Part 2: Initializer Lists ---

    /*
     * Manually assigning each element is tedious. You can initialize an array
     * when you declare it using an initializer list in curly braces `{}`.
     *
     * If you provide an initializer list, you can let the compiler figure out
     * the size of the array by leaving the square brackets empty.
     */
    printf("--- Part 2: Using an Initializer List ---\n");
    double temperatures[] = {72.5, 75.0, 69.8, 80.1, 85.3}; // Size is 5
    char grades[] = {'A', 'B', 'A', 'C', 'B', 'A'};         // Size is 6

    printf("The first temperature recorded was: %.1f\n", temperatures[0]);
    printf("The fourth student's grade was: %c\n", grades[3]);
    printf("\n");

    // --- Part 3: Iterating Over an Array ---

    /*
     * The real power of arrays is realized when you combine them with loops.
     * A `for` loop is perfect for iterating through an array, performing an
     * action on each element.
     */
    printf("--- Part 3: Iterating Over an Array with a 'for' loop ---\n");
    printf("All recorded temperatures:\n");

    // We will loop from index 0 up to (but not including) 5.
    // The loop counter `i` is used as the array index.
    for (int i = 0; i < 5; i++)
    {
        printf("  Day %d: %.1f\n", i + 1, temperatures[i]);
    }
    printf("\n");

    // Let's use a loop to calculate the sum of all high scores.
    int total_score = 0;
    for (int i = 0; i < 5; i++)
    {
        total_score = total_score + high_scores[i];
    }
    printf("Sum of all high scores: %d\n", total_score);

    return 0;
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * In this lesson, you've learned to manage collections of data.
 *
 * Key Takeaways:
 * - An ARRAY is a fixed-size collection of elements of the same type.
 * - C uses ZERO-BASED INDEXING, where the first element is at index 0.
 * - Accessing an array element is done with `array_name[index]`.
 * - You can initialize an array with a list of values like `int arr[] = {1, 2, 3};`.
 * - Loops, especially `for` loops, are essential for processing each element in an array.
 *
 * NOTE: For now, we manually kept track of the array's size (e.g., `i < 5`). In
 * future lessons, we will learn more robust ways to handle array sizes.
 *
 * HOW TO COMPILE AND RUN THIS CODE:
 *
 * 1. Open a terminal or command prompt.
 * 2. Navigate to the directory where you saved this file.
 * 3. Use the GCC compiler to create an executable file:
 *    `gcc -Wall -Wextra -std=c11 -o 8_arrays 8_arrays.c`
 * 4. Run the executable:
 *    - On Linux/macOS:   `./8_arrays`
 *    - On Windows:       `8_arrays.exe`
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 8_arrays 8_arrays.c
./8_arrays