Swift 3 for loop with increment

Question

How do I write the following in Swift3?

for (f = first; f <= last; f += interval)          
{
    n += 1
}

This is my own attempt

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Answer 1

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:

• sequence(first:next:),
• sequence(state:next:),

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(first: T, next: @escaping (T) -> T?) -> 
         UnfoldSequence

func sequence(state: State, 
                        next: @escaping (inout State) -> T?) ->
           UnfoldSequence

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:

• Express for loops in swift with dynamic range

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