Rust If...Else

if lets your program make decisions — run one block of code or another, depending on a condition.

Basic if

fn main() {
    let age = 20;

    if age >= 18 {
        println!("You can vote");
    }
}

▶ Try it Yourself

A couple of things to notice, since they trip people up coming from other languages:

• No parentheses around the condition — if age >= 18, not if (age >= 18).
• The condition must be a real bool. if age (a number) won’t compile — you need if age != 0 or similar.

if / else

fn main() {
    let age = 15;

    if age >= 18 {
        println!("You can vote");
    } else {
        println!("Not old enough yet");
    }
}

▶ Try it Yourself

else if for more branches

fn main() {
    let score = 72;

    if score >= 90 {
        println!("Grade: A");
    } else if score >= 75 {
        println!("Grade: B");
    } else if score >= 60 {
        println!("Grade: C");
    } else {
        println!("Grade: F");
    }
}

▶ Try it Yourself

Rust checks each condition top to bottom and runs the first one that matches, just like you’d expect.

Using if as an expression

Here’s something Rust does that a lot of languages don’t: if can produce a value. That means you can use it on the right side of a let:

fn main() {
    let age = 20;
    let status = if age >= 18 { "adult" } else { "minor" };

    println!("{status}");
}

▶ Try it Yourself

This works the same way returning a value from a function does — no semicolon after the value in each branch. The one requirement: both branches must produce the same type (here, both are &str).

Caution

If you forget the else branch when using if as an expression, Rust won’t compile it — it doesn’t know what value to use when the condition is false. if used purely for side effects (like printing) doesn’t need an else; if used as an expression does.

Recap

• if condition { ... } — no parentheses needed; the condition must be a bool.
• Chain more checks with else if, and fall back with else.
• if can be used as an expression to produce a value — both branches must match in type, and you need an else.

Quick check

Quick check

1. Which of these is required for a Rust if condition?

It must be wrapped in parenthesesIt must be a real boolIt must be an integer

2. When using if as an expression with let, what's required of both branches?

They must both print somethingThey must both be one line longThey must produce the same type

Score: 0 / 2

Read more

• Control Flow — The Rust Book