Swift Alternatives to C-style for-loops

Posted on April 6th, 2016

Starting in Swift 3.0, C-style for-loops will be gone from Swift! You can read the full Swift Evolution proposal here.

Last week, I talked to an iOS developer who was upset by this (it is a long-held habit after all!) and was confused by what to use as an alternative. @twostraws did a great write-up on the new Swift 2.2 features and what to use instead, so I’m going to write it down here for my own (and your) reference.

Here are the more-readable Swift alternatives.

Looping n times

/* 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 */

// Instead of this:
for var i = 0; i < 10; i++ {
    print(i)
}

// use this: 
for i in 0..<10 {
    print(i)
}

/* 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 */

// Instead of this:

for var i = 0; i < 10; i++ {

print(i)

}

// use this:

for i in 0..<10 {

print(i)

}

Looping n times in reverse

/* 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 */

// instead of this
for var i = 10; i > 0; i-- {
    print(i)
}

// use this
for i in (1...10).reverse() {
    print(i)
}

/* 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 */

// instead of this

for var i = 10; i > 0; i-- {

print(i)

}

// use this

for i in (1...10).reverse() {

print(i)

}

Looping with Stride

/* 0, 2, 4, 6, 8 */

// instead of this
for var i = 0; i < 10; i += 2 {
    print(i)
}

// use this
for i in 0.stride(to: 10, by: 2) {
    print(i)
}

/* 0, 2, 4, 6, 8 */

// instead of this

for var i = 0; i < 10; i += 2 {

print(i)

}

// use this

for i in 0.stride(to: 10, by: 2) {

print(i)

}

Looping through Array Values

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 2, 3, 45, 6, 8, 83, 100 */

// instead of this
for var i = 0; i < someNumbers.count; i++ {
    print(someNumbers[i])
}

// use this
for number in someNumbers {
    print(number)
}

// or this
someNumbers.forEach { number in
    print(number)
}

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 2, 3, 45, 6, 8, 83, 100 */

// instead of this

for var i = 0; i < someNumbers.count; i++ {

print(someNumbers[i])

}

// use this

for number in someNumbers {

print(number)

}

// or this

someNumbers.forEach { number in

print(number)

}

Reverse Looping through Array Values

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 100, 83, 8, 6, 45, 3, 2 */

// instead of this
for var i = someNumbers.count - 1; i >= 0; i-- {
    print(someNumbers[i])
}

// use this
for number in someNumbers.reverse() {
    print(number)
}

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 100, 83, 8, 6, 45, 3, 2 */

// instead of this

for var i = someNumbers.count - 1; i >= 0; i-- {

print(someNumbers[i])

}

// use this

for number in someNumbers.reverse() {

print(number)

}

Looping Through an Array with Index

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/*
 1: 2
 2: 3
 3: 45
 4: 6
 5: 8
 6: 83
 7: 100
 */

// instead of this
for var i = 0; i < someNumbers.count; i++ {
    print("\(i + 1): \(someNumbers[i])")
}

// use this
for (index, number) in someNumbers.enumerate() {
    print("\(index + 1): \(number)")
}

// or this
someNumbers.enumerate().forEach { (index, number) in
    print("\(index + 1): \(number)")
}

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

1: 2

2: 3

3: 45

4: 6

5: 8

6: 83

7: 100

// instead of this

for var i = 0; i < someNumbers.count; i++ {

print("\(i + 1): \(someNumbers[i])")

}

// use this

for (index, number) in someNumbers.enumerate() {

print("\(index + 1): \(number)")

}

// or this

someNumbers.enumerate().forEach { (index, number) in

print("\(index + 1): \(number)")

}

Looping Through Array Indices

Thanks Pyry Jahkola for the tip in the comments!

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 0, 1, 2, 3, 4, 5, 6 */

// instead of this
for var i = 0; i < someNumbers.count; i++ {
    print(i)
}

// use this
for index in someNumbers.indices {
    print(index)
}

let someNumbers = [2, 3, 45, 6, 8, 83, 100]

/* 0, 1, 2, 3, 4, 5, 6 */

// instead of this

for var i = 0; i < someNumbers.count; i++ {

print(i)

}

// use this

for index in someNumbers.indices {

print(index)

}

Don’t Forget Higher Order Functions!!!

I won’t go into detail here, but instead of looping, consider if Swift’s higher order functions such as map, flatMap, filter, reduce, etc can do the job and prioritize using them instead!

Conclusion

I tend to use pretty simple for-loops, so these seem good enough for me. However, if you have another configuration that I’m missing, let me know in the comments!

Wfpfirst Testfirst

Thanks! I’m new to Swift and programming so these tricks very helpful.
Alex

One little addition: if you don’t care about the index, you can use _ instead:
for _ in 0..<10 {
doSomething()
}
Pyry Jahkola

Looping through the indices of an Array (or in fact, any collection):

for i in array.indices { // better than `0 ..< array.count` print(array[i]) }

1
2
3

for i in array.indices { // better than `0 ..< array.count`
print(array[i])
}
- NatashaTheRobot
  
  Thanks! Added it.
Stefan

The code snippets in the first example are not equal. The first block prints 0 through 9, whereas the second one prints 1 through 10.
- NatashaTheRobot
  
  Thanks. Updated!
  - anon
    
    The commented output still shows 1 through 10 — threw me off momentarily.
    - NatashaTheRobot
      
      Fixed it!
Apokrupto

I had Swift 2.2 refuse this, then I had to figure out what the author was doing

for var index = string.startIndex; index < string.endIndex; index = index.successor().successor() {
Matthew K

Here is one I ran into:

for ( i = 0; i < n; ++i) { … }

var i = 0
while i < n {
…
i += 1
}
n3x07

thanks a lot. I really didn’t like the change at all. Now I have a project around 100 warnings in xc0de, not cool at all.
Carlos Sanchez

How would one write the following loop in Swift?
int i;
for(i = 0; i <N; i++){
int j;
for(j = i; j < N; j++){
//Do stuff
}
}
Abhay Deshpande

awesome !! thank you natasha