Conditional Statements

Our programs so far have been linear; they execute every line of code from top to bottom in the same order every time. To create powerful and intelligent programs, we need them to be able to make decisions.

CONDITIONAL STATEMENTS allow us to control the flow of our program’s execution. They use the relational and logical operators from the previous lesson to evaluate conditions. If a condition is true (evaluates to a non-zero value), one block of code is executed. If it’s false (evaluates to 0), that block is skipped, and optionally, another block is executed.

The primary tool for this is the if statement.

The if statement is the simplest form of a conditional. It executes a block of code ONLY if its condition is true.

Syntax: if (condition) { // Code to execute if ‘condition’ is true }

The else statement provides an alternative block of code to execute when the if condition is false.

Syntax: if (condition) { // Code to execute if ‘condition’ is true } else { // Code to execute if ‘condition’ is false }

To check for multiple, mutually exclusive conditions, you can chain else if statements. The program will check each condition in order. As soon as it finds a true condition, it runs that block and skips the rest of the entire chain. The final else is optional and acts as a “catch-all” if none of the preceding conditions were true.

Note on logical combinations: We can use logical operators (&&, ||) to create more complex conditions. For example, to check if a grade is a B (80-89):

Full Source

/**
 * @file 5_conditional_statements.c
 * @brief Part 1, Lesson 5: Conditional Statements
 * @author dunamismax
 * @date 06-15-2025
 *
 * This lesson introduces conditional statements, which allow a program
 * to make decisions and execute different code blocks based on specific conditions.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * Our programs so far have been linear; they execute every line of code from
 * top to bottom in the same order every time. To create powerful and intelligent
 * programs, we need them to be able to make decisions.
 *
 * CONDITIONAL STATEMENTS allow us to control the flow of our program's execution.
 * They use the relational and logical operators from the previous lesson to
 * evaluate conditions. If a condition is true (evaluates to a non-zero value),
 * one block of code is executed. If it's false (evaluates to 0), that block
 * is skipped, and optionally, another block is executed.
 *
 * The primary tool for this is the `if` statement.
 */

#include <stdio.h>

int main(void)
{
    // --- Part 1: The `if` Statement ---

    /*
     * The `if` statement is the simplest form of a conditional. It executes a
     * block of code ONLY if its condition is true.
     *
     * Syntax:
     * if (condition) {
     *     // Code to execute if 'condition' is true
     * }
     */
    int score = 100;

    printf("--- Part 1: The 'if' Statement ---\n");
    printf("Initial score: %d\n", score);

    // This condition `score == 100` is true, so the printf inside the braces will run.
    if (score == 100)
    {
        printf("Congratulations on a perfect score!\n");
    }

    // This condition `score > 100` is false, so this block is skipped.
    if (score > 100)
    {
        printf("This message will not be printed.\n");
    }
    printf("\n");

    // --- Part 2: The `if-else` Statement ---

    /*
     * The `else` statement provides an alternative block of code to execute
     * when the `if` condition is false.
     *
     * Syntax:
     * if (condition) {
     *     // Code to execute if 'condition' is true
     * } else {
     *     // Code to execute if 'condition' is false
     * }
     */
    int number_to_check;

    printf("--- Part 2: The 'if-else' Statement ---\n");
    printf("Enter a whole number: ");
    scanf("%d", &number_to_check);

    // We use the modulus operator `%` to check for an even number.
    // If a number divided by 2 has a remainder of 0, it's even.
    if (number_to_check % 2 == 0)
    {
        printf("%d is an even number.\n", number_to_check);
    }
    else
    {
        printf("%d is an odd number.\n", number_to_check);
    }
    printf("\n");

    // --- Part 3: The `if-else if-else` Chain ---

    /*
     * To check for multiple, mutually exclusive conditions, you can chain
     * `else if` statements. The program will check each condition in order.
     * As soon as it finds a true condition, it runs that block and skips the
     * rest of the entire chain. The final `else` is optional and acts as a
     * "catch-all" if none of the preceding conditions were true.
     */
    int user_grade;

    printf("--- Part 3: The 'if-else if-else' Chain ---\n");
    printf("Enter your numerical grade (0-100): ");
    scanf("%d", &user_grade);

    if (user_grade >= 90)
    {
        printf("Your letter grade is A.\n");
    }
    else if (user_grade >= 80)
    {
        printf("Your letter grade is B.\n");
    }
    else if (user_grade >= 70)
    {
        printf("Your letter grade is C.\n");
    }
    else if (user_grade >= 60)
    {
        printf("Your letter grade is D.\n");
    }
    else
    {
        printf("Your letter grade is F.\n");
    }

    /*
     * Note on logical combinations:
     * We can use logical operators (`&&`, `||`) to create more complex conditions.
     * For example, to check if a grade is a B (80-89):
     */
    if (user_grade >= 80 && user_grade < 90)
    {
        printf("This is a solid B.\n");
    }

    return 0;
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * In this lesson, you've learned how to give your program a brain.
 *
 * Key Takeaways:
 * - The `if` statement executes code only if a condition is true.
 * - The `if-else` statement provides an alternative path for when the condition is false.
 * - The `if-else if-else` chain allows you to test a series of conditions in order,
 *   executing only the first one that is true.
 * - These constructs are the fundamental building blocks for creating logic
 *   and making your programs smart and responsive.
 *
 * 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 5_conditional_statements 5_conditional_statements.c`
 * 4. Run the executable:
 *    - On Linux/macOS:   `./5_conditional_statements`
 *    - On Windows:       `5_conditional_statements.exe`
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 5_conditional_statements 5_conditional_statements.c
./5_conditional_statements