Swift 2.2 Decrementing Specific for Loop in Swift 3

Swift 2.2 decrementing specific for loop in Swift 3

Your code isn't counting the number of 3-letter words in the array. It is counting the number of 3-letter words at the end of the array. It will return 0 for your sample input array.

When a C-style for loop is very complex, the final fallback solution is to translate it to a while loop. Any C-style for loop can be mechanically converted into an equivalent while loop, which means you can do it even if you don't fully understand what it is doing.

This for loop:

for initialization; condition; increment {
    // body
}

is equivalent to:

initialization
while condition {
    // body
    increment
}

So, your code is equivalent to:

let array = ["hello", "world", nil, "foo", nil, "bar", "Peter Griffin"]
var threeLetterWords = 0

var i = array.count - 1
while i >= 0 && array[i]?.characters.count == 3 {
    i -= 1
    threeLetterWords += 1
}
print("Found words: \(threeLetterWords)") // says `Found words: 0`

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Here is how to use a for loop and guard to do the equivalent of your code:

let array = ["hello", "world", nil, "foo", nil, "bar", "Peter Griffin"]
var num3LetterWords = 0

for word in array.reversed() {
    guard word?.characters.count == 3 else { break }
    num3LetterWords += 1
}

print(num3LetterWords)

How to iterate for loop in reverse order in swift?

Xcode 6 beta 4 added two functions to iterate on ranges with a step other than one:
stride(from: to: by:), which is used with exclusive ranges and stride(from: through: by:), which is used with inclusive ranges.

To iterate on a range in reverse order, they can be used as below:

for index in stride(from: 5, to: 1, by: -1) {
    print(index)
}
//prints 5, 4, 3, 2

for index in stride(from: 5, through: 1, by: -1) {
    print(index)
}
//prints 5, 4, 3, 2, 1

Note that neither of those is a Range member function. They are global functions that return either a StrideTo or a StrideThrough struct, which are defined differently from the Range struct.

A previous version of this answer used the by() member function of the Range struct, which was removed in beta 4. If you want to see how that worked, check the edit history.

Decrement index in a loop after Swift C-style loops deprecated

Here is an easier (and more Swifty) approach.

for i in (0 ..< 5).reversed() {
    print(i) // 4,3,2,1,0
}

let array = ["a", "b", "c", "d", "e"]
for element in array.reversed() {
    print(element) // e,d,c,b,a
}

array.reversed().forEach { print($0) } // e,d,c,b,a

print(Array(array.reversed())) // e,d,c,b,a

Converting a C-style for loop that uses division for the step to Swift 3

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)
}

How can I do a Swift for-in loop with a step?

The Swift synonym for a "step" is "stride" - the Strideable protocol in fact, implemented by many common numerical types.

The equivalent of (i = 1; i < max; i+=2) is:

for i in stride(from: 1, to: max, by: 2) {
    // Do something
}

Alternatively, to get the equivalent of i<=max, use the through variant:

for i in stride(from: 1, through: max, by: 2) {
    // Do something
}

Note that stride returns a StrideTo/StrideThrough, which conforms to Sequence, so anything you can do with a sequence, you can do with the result of a call to stride (ie map, forEach, filter, etc). For example:

stride(from: 1, to: max, by: 2).forEach { i in
    // Do something
}

Can you tell a for-loop to conditionally advance by more than one step?

No, you can't change how a for-in loop iterates from within the loop.

A while loop with your own index counter is probably the simplest solution in this case. Though you may be able to make use of sequence(first:next:) in a for-in loop.

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