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!