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. Unlike an if expression, 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 expression 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 expression:

{{#include ../../../../examples/match_expressions/src/main.sw}}

Loops

while

This is what a while loop looks 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.

for

This is what a for loop that computes the sum of a vector of numbers looks like:

for element in vector.iter() {
    sum += element;
}

You need the for keyword, some pattern that contains variable names such as element in this case, the ìn keyword followed by an iterator, and a block of code inside the curly braces ({...}) to execute each iteration. vector.iter() in the example above returns an iterator for the vector. In each iteration, the value of element is updated with the next value in the iterator until the end of the vector is reached and the for loop iteration ends.

Modifying the vector during iteration, by e.g. adding or removing elements, is a logical error and results in an undefined behavior:

// The behavior of this `for` loop is undefined because
// the `vector` gets modified within the loop.
for element in vector.iter() {
    if element == 3 {
        vector.push(6); // Modification of the vector!
    }
}

break and continue

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

{{#include ../../../../examples/break_and_continue/src/main.sw:break_example}}

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

{{#include ../../../../examples/break_and_continue/src/main.sw:continue_example}}

Nested loops

You can also use nested while loops if needed:

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