Control Flow

if 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
    }
}

Using 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.

match expressions

Sway supports advanced pattern matching through exhaustive match expressions.

script;

fn foo() {
    // do something
}
fn bar() {
    // do something
}

enum SomeEnum {
    A: u64,
    B: bool,
    C: b256,
}

fn main() -> u64 {
    let x = 5;

    // Match as an expression.
    let a = match 8 {
        7 => {
            4
        },
        9 => {
            5
        },
        8 => {
            6
        },
        _ => {
            100
        },
    };

    // Match as a statement for control flow.
    match x {
        5 => {
            foo()
        },
        _ => {
            bar()
        },
    };

    // Match an enum
    let e = SomeEnum::A(42);
    let v = match e {
        SomeEnum::A(val) => {
            val
        },
        SomeEnum::B(true) => {
            1
        },
        SomeEnum::B(false) => {
            0
        },
        _ => {
            0
        },
    };

    // Match as expression used for a return.
    match 42 {
        0 => {
            24
        },
        foo => {
            foo
        },
    }
}

In the example above, braces around the code block following => in each match arm are not required unless the code block contains multiple statements. They are added in this example due to an issue in the Sway formatter.

Loops

while

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.

break 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
}

Nested loops

You can also use nested while loops if needed:

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