Conditionals

Syntax

if condition {
    // runs when condition is true
} else if other_condition {
    // runs when the first is false and this one is true
} else {
    // runs otherwise
}

• No parentheses are needed around the condition, but the braces are always required, even for a single line.
• The condition must be a bool. Rust has no "truthiness": a number, string, or empty collection is never treated as true or false automatically. Write the comparison you mean, such as count != 0 or !name.is_empty().

if is an expression, meaning it produces a value. You can use it on the right-hand side of let, much like the ternary a ? b : c operator in other languages (Rust doesn't have ?:):

let label = if n % 2 == 0 { "even" } else { "odd" };

When if is used this way, every branch must produce the same type, and an else is required.

Example

fn main() {
    let temperature = 23;

    if temperature > 30 {
        println!("Hot");
    } else if temperature > 15 {
        println!("Pleasant");
    } else {
        println!("Cold");
    }

    let n = 7;
    let kind = if n % 2 == 0 { "even" } else { "odd" };
    println!("{n} is {kind}");

    let cart_items = 0;
    if cart_items != 0 {
        println!("Ready to check out");
    } else {
        println!("Your cart is empty");
    }

    let age = 20;
    let has_id = true;
    if age >= 18 && has_id {
        println!("Entry allowed");
    }
}

Expected output

Pleasant
7 is odd
Your cart is empty
Entry allowed

Common mistakes

• Using a number as a condition. if items { ... } where items is an integer fails:

error[E0308]: mismatched types

The label under it reads "expected bool, found integer". Write if items != 0 instead.

• Branches of different types. let value = if ok { 5 } else { "five" }; fails with:

error[E0308]: `if` and `else` have incompatible types

• Putting a semicolon after the values in an if expression ({ "even"; }). That turns the value into a statement, and the branch produces () instead.

Practice

Store a score of 85 in a variable. Use an if expression to set a grade variable to "A" for 90 or more, "B" for 80 or more, "C" for 70 or more, and "F" otherwise, then print Score 85: grade B.

fn main() {
    let score = 85;
    let grade = if score >= 90 {
        "A"
    } else if score >= 80 {
        "B"
    } else if score >= 70 {
        "C"
    } else {
        "F"
    };
    println!("Score {score}: grade {grade}");
}