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Iterators

An iterator is any object with has_next() and next() methods. for-in loops work with any iterator automatically.

Iterator interface

Method Returns
has_next() true if more elements remain, false when exhausted
next() Next value, or null when exhausted

Built-in iterators

Every collection has an .iterator() method that returns an iterator:

let arr = [10, 20, 30]
let iter = arr.iterator()

while (iter.has_next()) {
    println(iter.next())   // 10, 20, 30
}

Array — yields elements in order
Dict — yields keys (order not guaranteed)
Set — yields elements (order not guaranteed)
String — for-in yields one character at a time

Dict entries

dict.entries() yields [key, value] pairs:

let d = {"name": "Alice", "age": 30}
let iter = d.entries()

while (iter.has_next()) {
    let pair = iter.next()
    println("${pair[0]}: ${pair[1]}")
}

for-in with iterators

for-in works directly with iterators — no need to call .iterator() explicitly:

for value in [1, 2, 3] {
    println(value)
}

for char in "hello" {
    println(char)
}

Manual iteration control

Use has_next() / next() when you need to stop early or skip elements:

let iter = [1, 2, 3, 4, 5].iterator()

// Take only first 3
let count = 0
while (iter.has_next() && count < 3) {
    println(iter.next())
    count = count + 1
}

Custom iterators

Make any struct iterable by defining has_next() and next() methods. for-in will call them automatically.

struct CountUp {
    current
    limit

    fn has_next(self) {
        return self.current < self.limit
    }

    fn next(self) {
        let v = self.current
        self.current = self.current + 1
        return v
    }
}

fn main() {
    let counter = CountUp { current: 0, limit: 5 }
    for n in counter {
        println(n)   // 0, 1, 2, 3, 4
    }
}

Fibonacci iterator

struct Fibonacci {
    a
    b

    fn has_next(self) {
        return true
    }

    fn next(self) {
        let value = self.a
        let next = self.a + self.b
        self.a = self.b
        self.b = next
        return value
    }
}

fn main() {
    let fib = Fibonacci { a: 0, b: 1 }
    for i in range(0, 10) {
        println(fib.next())   // 0, 1, 1, 2, 3, 5, 8, 13, 21, 34
    }
}

Notes

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