Controlling the Flow

Most programs need to make decisions. "If the user is logged in, show their profile; otherwise, show the login page." C uses if, else, and else if for this.

1. The Simple if

The code block { } only runs if the condition in the parentheses is True (non-zero).

if (balance < 0) {
    printf("Warning: Your account is overdrawn!\n");
}

2. The if-else Duo

Provides two mutually exclusive paths. One of them must run.

if (score >= 40) {
    printf("Congratulations, you passed!\n");
} else {
    printf("I'm sorry, you failed. Keep studying!\n");
}

3. The else if Ladder

Used for more than two choices. C checks them in order, top to bottom. As soon as it finds a true one, it runs it and skips the rest of the ladder.

if (temp > 30) {
    printf("It's a hot day.\n");
} else if (temp > 20) {
    printf("It's a pleasant day.\n");
} else if (temp > 10) {
    printf("It's a bit chilly.\n");
} else {
    printf("It's freezing!\n");
}

4. Nested Decisions

You can put an if inside another if.

if (hasAccount) {
    if (passwordCorrect) {
        printf("Access Granted.\n");
    } else {
        printf("Wrong password.\n");
    }
}

What is "True" in C?

Unlike many languages, early C didn't have a dedicated bool type. Instead:

• 0 is False.
• Any non-zero value (1, -5, 100, 0.5) is True.

if (5) { printf("This always prints!"); }
if (0) { printf("This never prints."); }

The Ternary Operator Shorthand

For very simple decisions, you can use ? :.

int price = (isMember) ? 10 : 20;

This reads: "Is isMember true? If yes, price is 10. If no, price is 20."

The Semicolon Trap: Never put a semicolon immediately after the if parentheses: if(x > 5); { ... }. This tells C the if statement is finished, so the code in { } will run every time, regardless of x!