next()

The next element in the underlying sequence, if a next element exists; otherwise, nil.

Repeatedly calling this method returns, in order, all the elements of the underlying sequence. As soon as the sequence has run out of elements, all subsequent calls return nil.

You must not call this method if any other copy of this iterator has been advanced with a call to its next() method.

The following example shows how an iterator can be used explicitly to emulate a for-in loop. First, retrieve a sequence’s iterator, and then call the iterator’s next() method until it returns nil.

let numbers = [2, 3, 5, 7]
var numbersIterator = numbers.makeIterator()

while let num = numbersIterator.next() {
    print(num)
}
// Prints "2"
// Prints "3"
// Prints "5"
// Prints "7"