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Basics /
Control Flow

Icon LinkControl Flow

Icon Linkif expressions

Sway supports if, else, and else if expressions that allow you to branch your code depending on conditions.

For example: 

fn main() {
	let number = 6;
 
	if number % 4 == 0 {
		// do something
	} else if number % 3 == 0 {
		// do something else
	} else {
		// do something else
	}
}

Icon LinkUsing if in a let statement

Like Rust, ifs are expressions in Sway. What this means is you can use if expressions on the right side of a let statement to assign the outcome to a variable. 

let my_data = if some_bool < 10 { foo() } else { bar() };

Note that all branches of the if expression must return a value of the same type.

Icon Linkmatch expressions

Sway supports advanced pattern matching through exhaustive match expressions. Unlike an if statement, a match expression asserts at compile time that all possible patterns have been matched. If you don't handle all the patterns, you will get compiler error indicating that your match expression is non-exhaustive.

The basic syntax of a match statement is as follows: 

let result = match expression {
	pattern1 => code_to_execute_if_expression_matches_pattern1,
	pattern2 => code_to_execute_if_expression_matches_pattern2,
	pattern3 | pattern4 => code_to_execute_if_expression_matches_pattern3_or_pattern4
	...
	_ => code_to_execute_if_expression_matches_no_pattern,
}

Some examples of how you can use a match statement: 

script;
 
// helper functions for our example
fn on_even(num: u64) {
	// do something with even numbers
}
fn on_odd(num: u64) {
	// do something with odd numbers
}
 
fn main(num: u64) -> u64 {
	// Match as an expression
	let is_even = match num % 2 {
		0 => true,
		_ => false,
	};
 
	// Match as control flow
	let x = 12;
	match x {
		5 => on_odd(x),
		_ => on_even(x),
	};
 
	// Match an enum
	enum Weather {
		Sunny: (),
		Rainy: (),
		Cloudy: (),
		Snowy: (),
	}
	let current_weather = Weather::Sunny;
	let avg_temp = match current_weather {
		Weather::Sunny => 80,
		Weather::Rainy => 50,
		Weather::Cloudy => 60,
		Weather::Snowy => 20,
	};
 
	let is_sunny = match current_weather {
		Weather::Sunny => true,
		Weather::Rainy | Weather::Cloudy | Weather::Snowy => false,
	};
 
	// match expression used for a return
	let outside_temp = Weather::Sunny;
	match outside_temp {
		Weather::Sunny => 80,
		Weather::Rainy => 50,
		Weather::Cloudy => 60,
		Weather::Snowy => 20,
	}
}

Icon LinkLoops

Icon Linkwhile

Loops in Sway are currently limited to while loops. This is what they look like: 

while counter < 10 {
	counter = counter + 1;
}

You need the while keyword, some condition (value < 10 in this case) which will be evaluated each iteration, and a block of code inside the curly braces ({...}) to execute each iteration.

Icon Linkbreak and continue

break and continue keywords are available to use inside the body of a while loop. The purpose of the break statement is to break out of a loop early: 

fn break_example() -> u64 {
	let mut counter = 1;
	let mut sum = 0;
	let num = 10;
	while true {
		if counter > num {
			break;
		}
		sum += counter;
		counter += 1;
	}
	sum // 1 + 2 + .. + 10 = 55
}

The purpose of the continue statement is to skip a portion of a loop in an iteration and jump directly into the next iteration: 

fn continue_example() -> u64 {
	let mut counter = 0;
	let mut sum = 0;
	let num = 10;
	while counter < num {
		counter += 1;
		if counter % 2 == 0 {
			continue;
		}
		sum += counter;
	}
	sum // 1 + 3 + .. + 9 = 25
}

Icon LinkNested loops

You can also use nested while loops if needed: 

while condition_1 == true {
	// do stuff...
	while condition_2 == true {
		// do more stuff...
	}
}