Lesson 47 min read

Conditionals

Traffic lights for your code — red means stop, green means go

Traffic Lights for Your Code

Without conditionals, your program would be a single straight road — no turns, no choices, no decisions. Conditionals are the traffic lights that let your code take different paths depending on what's happening.

But before we dive in, you need to understand one critical thing about C: there is no true boolean type (at least not natively). In C, truth is a number:

0 is false — zero, nada, nothing.
Everything else is true — 1, -5, 42, 999 — all truthy.

This simplicity is elegant, but it opens the door to some sneaky bugs.

Truthiness in C

#include <stdio.h>

int main() {
    if (1)  printf("1 is true\n");
    if (-1) printf("-1 is true\n");
    if (42) printf("42 is true\n");
    if (0)  printf("0 is true\n");  // This never prints!

    // Common pattern: checking if a value exists
    int items_in_cart = 3;
    if (items_in_cart) {
        printf("You have %d items\n", items_in_cart);
    } else {
        printf("Your cart is empty\n");
    }
    return 0;
}

Output

1 is true
-1 is true
42 is true
You have 3 items

if, else if, else

The classic decision structure. C evaluates each condition top-to-bottom and takes the first branch that's true. If none match, the else block runs (if you have one).

Grade Calculator

#include <stdio.h>

int main() {
    int score = 78;

    if (score >= 90) {
        printf("Grade: A\n");
    } else if (score >= 80) {
        printf("Grade: B\n");
    } else if (score >= 70) {
        printf("Grade: C\n");
    } else if (score >= 60) {
        printf("Grade: D\n");
    } else {
        printf("Grade: F\n");
    }

    printf("Score: %d\n", score);
    return 0;
}

Output

Grade: C
Score: 78

switch/case — The Vending Machine

When you're comparing one variable against many specific values, switch is cleaner than a chain of if/else if. Think of it like a vending machine — you press a button (the case) and get a specific result.

But there's a trap: you must use break after each case. Without it, execution "falls through" into the next case like a ball rolling down stairs.

switch/case — Menu System

#include <stdio.h>

int main() {
    int choice = 2;

    printf("=== MENU ===\n");
    printf("1. New Game\n");
    printf("2. Load Game\n");
    printf("3. Settings\n");
    printf("4. Quit\n\n");

    switch (choice) {
        case 1:
            printf("Starting new game...\n");
            break;
        case 2:
            printf("Loading saved game...\n");
            break;
        case 3:
            printf("Opening settings...\n");
            break;
        case 4:
            printf("Goodbye!\n");
            break;
        default:
            printf("Invalid choice!\n");
            break;
    }
    return 0;
}

Output

=== MENU ===
1. New Game
2. Load Game
3. Settings
4. Quit

Loading saved game...

Fall-Through — Bug or Feature?

#include <stdio.h>

int main() {
    // Intentional fall-through: grouping cases
    char grade = 'B';

    switch (grade) {
        case 'A':
        case 'B':
            printf("Great job!\n");
            break;
        case 'C':
            printf("Not bad, keep going.\n");
            break;
        case 'D':
        case 'F':
            printf("Need improvement.\n");
            break;
        default:
            printf("Invalid grade.\n");
    }
    return 0;
}

Output

Great job!

Note: The two most common conditional bugs in C:
1. = vs == — Writing if (x = 5) instead of if (x == 5). The first assigns 5 to x (and is always true since 5 is nonzero). The compiler might not warn you. Some programmers write if (5 == x) to catch this — a typo 5 = x won't compile.
2. Forgetting break in switch — Without break, execution falls through to the next case. Sometimes intentional, usually a bug.

The Ternary Operator

The ternary operator ? : is a one-line if/else. It's great for simple assignments but don't overuse it — nested ternaries become unreadable fast.

Ternary Operator

#include <stdio.h>

int main() {
    int a = 15, b = 23;

    // Instead of a 5-line if/else:
    int max = (a > b) ? a : b;
    int min = (a < b) ? a : b;

    printf("Max: %d\n", max);
    printf("Min: %d\n", min);

    // Inline in printf
    int temp = -3;
    printf("%d is %s\n", temp, (temp >= 0) ? "positive" : "negative");

    // Absolute value
    int abs_val = (temp < 0) ? -temp : temp;
    printf("Absolute value: %d\n", abs_val);
    return 0;
}

Output

Max: 23
Min: 15
-3 is negative
Absolute value: 3

Challenge

Quick check

What does the expression if (x = 0) do in C?

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