Converting a C-style for loop that uses division f

Question

I have this loop, decrementing an integer by division, in Swift 2.

for var i = 128; i >= 1 ; i = i/2  {
   //do some thing
}

The C-style for loop is deprecated, so how can I convert this to Swift 3.0?

Answer 1

I'll suggest that you should use a while loop to handle this scenario:

var i = 128
while i >= 1
{
    // Do your stuff
    i = i / 2
}

Answer 2

MartinR's solution is very generic and useful and should be part of your toolbox.

Another approach is to rephrase what you want: the powers of two from 7 down to 0.

for i in (0...7).reversed().map({ 1 << $0 }) {
    print(i)
}

Answer 3

Quite general loops with a non-constant stride can be realized with sequence:

for i in sequence(first: 128, next: { $0 >= 2 ? $0/2 : nil }) {
    print(i)
}

• Advantages: The loop variable i is a constant and its scope is restricted to the loop body.
• Possible disadvantages: The terminating condition must be adapted (here: $0 >= 2 instead of i >= 1), and the loop is always executed at least once, for the first value.

One could also write a wrapper which resembles the C-style for loop more closely and does not have the listed disadvantages (inspired by Erica Sadun: Stateful loops and sequences):

public func sequence<T>(first: T, while condition: @escaping (T)-> Bool, next: @escaping (T) -> T) -> UnfoldSequence<T, T> {
    let nextState = { (state: inout T) -> T? in
        guard condition(state) else { return nil }
        defer { state = next(state) }
        return state
    }
    return sequence(state: first, next: nextState)
}

and then use it as

for i in sequence(first: 128, while: { $0 >= 1 }, next: { $0 / 2 }) {
    print(i)
}