Swift 2.2 -> 3.0: Strideable:s stride(...) replaced by global stride(...) functions
In Swift 2.2, we can (as you’ve tried in your own attempt) make use of the blueprinted (and default-implemented) functions stride(through:by:) and stride(to:by:) from the protocol Strideable
/* Swift 2.2: stride example usage */ let from = 0 let to = 10 let through = 10 let by = 1 for _ in from.stride(through, by: by) { } // from ... through (steps: 'by') for _ in from.stride(to, by: by) { } // from ..< to (steps: 'by')
Whereas in Swift 3.0, these two functions has been removed from Strideable in favour of the global functions stride(from:through:by:) and stride(from:to:by:); hence the equivalent Swift 3.0 version of the above follows as
/* Swift 3.0: stride example usage */ let from = 0 let to = 10 let through = 10 let by = 1 for _ in stride(from: from, through: through, by: by) { } for _ in stride(from: from, to: to, by: by) { }
In your example you want to use the closed interval stride alternative stride(from:through:by:), since the invariant in your for loop uses comparison to less or equal to (<=). I.e.
/* example values of your parameters 'first', 'last' and 'interval' */
let first = 0
let last = 10
let interval = 2
var n = 0
for f in stride(from: first, through: last, by: interval) {
print(f)
n += 1
} // 0 2 4 6 8 10
print(n) // 6
Where, naturally, we use your for loop only as an example of the passage from for loop to stride, as you can naturally, for your specific example, just compute n without the need of a loop (n=1+(last-first)/interval).
Swift 3.0: An alternative to stride for more complex iterate increment logic
With the implementation of evolution proposal SE-0094, Swift 3.0 introduced the global sequence functions:
which can be an appropriate alternative to stride for cases with a more complex iterate increment relation (which is not the case in this example).
Declaration(s)
func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldSequence<T, (T?, Bool)> func sequence<T, State>(state: State, next: @escaping (inout State) -> T?) -> UnfoldSequence<T, State>
We’ll briefly look at the first of these two functions. The next arguments takes a closure that applies some logic to lazily construct next sequence element given the current one (starting with first). The sequence is terminated when next returns nil, or infinite, if a next never returns nil.
Applied to the simple constant-stride example above, the sequence method is a bit verbose and overkill w.r.t. the fit-for-this-purpose stride solution:
let first = 0
let last = 10
let interval = 2
var n = 0
for f in sequence(first: first,
next: { $0 + interval <= last ? $0 + interval : nil }) {
print(f)
n += 1
} // 0 2 4 6 8 10
print(n) // 6
The sequence functions become very useful for cases with non-constant stride, however, e.g. as in the example covered in the following Q&A:
Just take care to terminate the sequence with an eventual nil return (if not: “infinite” element generation), or, when Swift 3.1 arrives, make use of its lazy generation in combination with the prefix(while:) method for sequences, as described in evolution proposal SE-0045. The latter applied to the running example of this answer makes the sequence approach less verbose, clearly including the termination criteria of the element generation.
/* for Swift 3.1 */
// ... as above
for f in sequence(first: first, next: { $0 + interval })
.prefix(while: { $0 <= last }) {
print(f)
n += 1
} // 0 2 4 6 8 10
print(n) // 6