Rust if Expression

In Rust, the if expression is a control flow structure used to execute code based on conditions. Unlike many other languages, Rust's if can return a value, making it very flexible and powerful.

Basic Syntax

The basic syntax of Rust's if expression is: if condition { code block }. The condition must be of boolean type (bool), otherwise the compiler will report an error. The following example shows how to use the if expression to check if a number is positive.

let number = 5;
if number > 0 {
    println!("The number is positive");
}

Using else and else if

You can use else and else if to handle multiple conditions; there can be multiple else if branches, but only one else branch. The following example demonstrates how to print different messages based on whether a number is positive, negative, or zero.

let number = -3;
if number > 0 {
    println!("Positive");
} else if number < 0 {
    println!("Negative");
} else { // Executes the else branch when all conditions do not match
    println!("Zero");
}

At most, only one branch will be executed, and after executing one branch, other branches will not be executed:

let x = 10;

if x > 5 {
    println!("Branch A");  // This executes
} else if x > 8 {
    println!("Branch B");  // ❌ Will not execute (condition is true, but it is mutually exclusive)
} else {
    println!("Branch C");  // ❌ Will not execute
}

if Expression Returning a Value

Rust's if expression can return a value, allowing you to use it in assignments. Each branch must return a value of the same type. The following example shows how to use the if expression to assign a string.

let condition = true;
let result = if condition {
    "Condition is true"
} else {
    "Condition is false"
};
println!("{}", result); // Output: Condition is true

When if is used as an expression, it must have an else branch.

Nested if Expressions

You can nest other if expressions within an if expression to handle more complex logic. The following example shows how to check if a number is within a specific range.

let x = 10;
if x > 0 {
    if x < 20 {
        println!("x is between 0 and 20");
    }
}