Control Flow

If-else

if runs one block when a condition is true, optionally another when it’s false:

let is_arriving = true

if is_arriving {
    println("hello")
} else {
    println("good bye")
}

Chain conditions with else if:

let hour = 14

if hour < 12 {
    println("good morning")
} else if hour < 17 {
    println("good afternoon")
} else {
    println("good night")
}

if is an expression — it returns a value, so you can use it on the right side of a let:

let label = if temperature > 30 { "hot" } else { "cool" }

Match

match checks a value against a series of patterns and runs the first one that fits. It’s like a more powerful if/else if chain.

let n = 2
let s = match n {
    0 -> "zero"
    1 -> "one"
    2 -> "two"
    3 -> "three"
    _ -> "something else"
}
// s = "two"

The _ is a catch-all. match does much more than literal lookup — it can pull values out of enums and tuples in one step. See Patterns for the full picture.

While loops

Repeat as long as a condition holds:

var n = 3
while n > 0 {
    print("hello")
    n = n - 1
}
// hellohellohello

Use break to exit a loop early, and continue to skip to the next iteration:

var i = 0
while true {
    if i >= 5 { break }
    if i % 2 == 0 {
        i += 1
        continue
    }
    println(i)
    i += 1
}
// 1
// 3

For loops

Iterate over anything that can be iterated. Most commonly, that’s an array or a count:

let arr = [1, 2, 3, 4, 5]
var sum = 0
for n in arr {
    sum = sum + n
}
println(sum)    // 15

Iterating over an integer counts from 0 up to (but not including) that integer:

for i in 5 {
    println(i)   // prints 0, 1, 2, 3, 4
}

For an arbitrary range, use range(begin, end):

for i in range(2, 6) {
    println(i)   // prints 2, 3, 4, 5
}

for works on any type that implements Iterable (see Interfaces), so you can write your own.