Control Flow: if Expression

In Rust, if expressions are a fundamental part of control flow, allowing you to execute different blocks of code based on conditions. Here's a breakdown of how to use if, else, and else if in Rust, along with examples:

1. Basic if Expression

An if expression executes a block of code only if a condition is true 3.

fn main() {
    let number = 10;

    if number > 0 {
        println!("{} is greater than 0", number);
    }
}

2. if...else Expression

The if...else expression allows you to execute one block of code if the condition is true and another block of code if the condition is false 3.

fn main() {
    let number = -2;

    if number > 0 {
        println!("{} is greater than 0", number);
    } else {
        println!("{} is less than or equal to 0", number);
    }
}

3. if...else if...else Expression

You can evaluate multiple conditions using else if expressions. This is useful when you need to handle more than two possible outcomes 3.

fn main() {
    let number = -2;

    if number > 0 {
        println!("{} is greater than 0", number);
    } else if number == 0 {
        println!("{} is equal to 0", number);
    } else {
        println!("{} is less than 0", number);
    }
}

4. if as an Expression

In Rust, if blocks are also expressions, meaning they can return values. All branches must return the same type 1.

fn main() {
    let n = 5;

    let big_n = if n < 10 && n > -10 {
        println!("number is a small number, increase ten-fold");
        10 * n
    } else {
        println!("number is a big number, halve the number");
        n / 2
    };

    println!("{} -> {}", n, big_n);
}

5. Matching on Options

if let is useful for checking if an Option contains a value.

fn main() {
    let optional_value: Option<i32> = Some(42);

    if let Some(value) = optional_value {
        println!("The value is: {}", value);
    } else {
        println!("The optional value is None");
    }
}

6. Using if with match

You can combine if statements with match for more complex control flow 4.

fn main() {
    let value = 3;

    match value {
        1 => println!("Value is 1"),
        2 => println!("Value is 2"),
        _ if value > 2 => println!("Value is greater than 2"),
        _ => println!("Value is something else"),
    }
}

These examples cover the main ways to use if, else, and else if in Rust. They demonstrate how to create conditional logic, return values from if expressions, and work with Option types.