if - The Rust Programming Language

# if

Rust’s take on `if` is not particularly complex, but it’s much more like the `if` you’ll find in a dynamically typed language than in a more traditional systems language. So let’s talk about it, to make sure you grasp the nuances.

`if` is a specific form of a more general concept, the ‘branch’, whose name comes from a branch in a tree: a decision point, where depending on a choice, multiple paths can be taken.

In the case of `if`, there is one choice that leads down two paths:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    if x == 5 {
        println!("x is five!");
    }
    #}

If we changed the value of `x` to something else, this line would not print. More specifically, if the expression after the `if` evaluates to `true`, then the block is executed. If it’s `false`, then it is not.

If you want something to happen in the `false` case, use an `else`:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    if x == 5 {
        println!("x is five!");
    } else {
        println!("x is not five :(");
    }
    #}

If there is more than one case, use an `else if`:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    if x == 5 {
        println!("x is five!");
    } else if x == 6 {
        println!("x is six!");
    } else {
        println!("x is not five or six :(");
    }
    #}

This is all pretty standard. However, you can also do this:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    let y = if x == 5 {
        10
    } else {
        15
    }; // y: i32
    #}

Which we can (and probably should) write like this:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    let y = if x == 5 { 10 } else { 15 }; // y: i32
    #}

This works because `if` is an expression. The value of the expression is the value of the last expression in whichever branch was chosen. An `if` without an `else` always results in `()` as the value.