[Source | Tests]

Iterate over a collection forever, or a set number of times.

for (odd, n) in zip([true, false].cycled(), 1...) {
    // ...
}

for x in (1...3).cycled(times: 3) {
    print(x)
}
// Prints 1 through 3 three times

cycled(times:) provides a more expressive way of repeating a collection's elements a limited number of times than combining repeatElement(_:count:) and joined().

Two new methods are added to collections:

extension Collection {
    func cycled() -> CycledSequence<Self>

    func cycled(times: Int) -> CycledTimesCollection<Self>
}

The new CycledSequence type is a sequence only, given that the Collection protocol design makes infinitely large types impossible/impractical. CycledSequence also conforms to LazySequenceProtocol when the base type conforms.

The CycledTimesCollection type always has Collection conformance, with BidirectionalCollection, RandomAccessCollection, and LazySequenceProtocol conformance when the base type conforms.

Calling these methods is O(1).

There's a slight off-by-one ambiguity around the naming of cycled(times:), since one can reasonably interpret the number of cycles as including the first run-through of elements (the actual semantics) or starting after the first set. This ambiguity is present in all the different potential names for this function: repeated(times:), cycled(repetitions:), etc.

Rust: The cycle method repeats an iterator's elements forever. I don’t see anything that matches the cycled(times:) behavior.

Ruby: Passing a number to cycle limits the number of repetitions, like the cycled(times:) here.

Python: Python’s itertools includes a cycle method, which caches the elements of the iterator on the first pass.