Control Flow

Rust has most of the control flow expressions you may expect, and a few you may not.

if

Compared to other curly-brace languages (like C++, Java, and JavaScript), Rust’s if statements have two notable differences:

• Lack of the requirement for the condition expression to be enclosed in parentheses.

  • Example

• if structures in Rust can be expressions as well as just normal statements. That is, they can resolve to a value like a ternary expression in JavaScript.

  • Example

while & loop

Rust has three looping structures. loop is the simplest: it just loops forever until it hits a break. while, like its namesake in other languages, loops while a condition holds (or until it hits a break). Similarly in style to the if statement, the while condition does not need to be enclosed in parentheses.

let mut x = 0; loop { if x >= 10 { break; } x += 1; }
let mut x = 0; while x < 10 { x += 1; }

The third looping structure is the for loop. It operates on any iterable type.

Arrays:

for i in [2, 4, 6, 8] { println!("{i}"); }

Output:

2 4 6 8

Ranges:

for i in 0..5 { println!("{i}"); }

Output:

0 1 2 3 4

There are other iterable structures, like [Vec](<https://doc.rust-lang.org/std/vec/struct.Vec.html>) and [HashSet](<https://doc.rust-lang.org/std/collections/struct.HashSet.html>), which you can explore if you wish.

match

Instead of the switch statement found in many other common languages, Rust opted for the more “functional” match construct.

`let operator = "*";

match operator { "+" => println!("add"), "-" => println!("subtract"), "*" => println!("multiply"), "/" => println!("divide"), _ => println!("unknown"), // _ is the catch-all pattern }`

While matching against input cases using Rust’s pattern matching syntax, you can also extract pieces from the input data using destructuring.

`enum MediaType { Movie, Series { episodes: u32 }, }

let media_type: MediaType = /* ... */;

match media_type { MediaType::Movie => println!("It's a movie!"), // single line terminated with comma MediaType::Series { episodes } => { // multi-line enclosed in curly braces println!("It's a TV show!"); println!("It has {episodes} episodes!"); } }`

match expressions, since they are expressions, can also resolve to a value:

let unit_count = match media_type { MediaType::Series { episodes } => episodes, _ => 1, };

if let

A special version of the if expression can perform destructuring as well. It’s like a conditional let.

This is equivalent to the previous listing:

let unit_count = if let MediaType::Series { episodes } = media_type { episodes } else { 1 };