How to iterate for loop in reverse order in swift?

Question

When I use the for loop in Playground, everything worked fine, until I changed the first parameter of for loop to be the highest value. (iterated in descending order)

Is this a bug? Did any one else have it?

for index in 510..509
{
    var a = 10
}

The counter that displays the number of iterations that will be executions keeps ticking...

Diamonddiamondback

possible duplicate of Reverse Range in Swift – Misinform

Such a brilliant decision to drop c style loops. Bring them back – Ecumenicist

See my answer with Swift 5 that shows up to 4 different ways to iterate in reverse order. – Acaroid

simply .reversed(), as in for index in (509 ... 510).reversed() { ... } – Orson

Answer 1

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.

Lissalissak

With Swift 2.0 it is now: 5.stride(to: 1, by: -1) or 5.stride(through: 1, by: -1) – Wherever

With Swift 3.0 we're back to stride as a free function. – Lissalissak

@Shaun it is a reasonable and safe choice. I can't tell you often R programmers are tripped by its : operator implicitly iterating downwards, causing their loops to execute when they thought it would skip over them. It's very convenient for interactive use, but terribly error prone in scripts, to the point where style guides recommend against using it. – Jewfish

Answer 2

Apply the reverse function to the range to iterate backwards:

For Swift 1.2 and earlier:

// Print 10 through 1
for i in reverse(1...10) {
    println(i)
}

It also works with half-open ranges:

// Print 9 through 1
for i in reverse(1..<10) {
    println(i)
}

Note: reverse(1...10) creates an array of type [Int], so while this might be fine for small ranges, it would be wise to use lazy as shown below or consider the accepted stride answer if your range is large.

---

To avoid creating a large array, use lazy along with reverse(). The following test runs efficiently in a Playground showing it is not creating an array with one trillion Ints!

Test:

var count = 0
for i in lazy(1...1_000_000_000_000).reverse() {
    if ++count > 5 {
        break
    }
    println(i)
}

---

For Swift 2.0 in Xcode 7:

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

Note that in Swift 2.0, (1...1_000_000_000_000).reverse() is of type ReverseRandomAccessCollection<(Range<Int>)>, so this works fine:

var count = 0
for i in (1...1_000_000_000_000).reverse() {
    count += 1
    if count > 5 {
        break
    }
    print(i)
}

---

For Swift 3.0 reverse() has been renamed to reversed():

for i in (1...10).reversed() {
    print(i) // prints 10 through 1
}

Halibut

reversed may copy the collection, though. At least, that's how I understand the documentation on Data. – Noah

Answer 3

Updated for Swift 3

The answer below is a summary of the available options. Choose the one that best fits your needs.

reversed: numbers in a range

Forward

for index in 0..<5 {
    print(index)
}

// 0
// 1
// 2
// 3
// 4

Backward

for index in (0..<5).reversed() {
    print(index)
}

// 4
// 3
// 2
// 1
// 0

reversed: elements in SequenceType

let animals = ["horse", "cow", "camel", "sheep", "goat"]

Forward

for animal in animals {
    print(animal)
}

// horse
// cow
// camel
// sheep
// goat

Backward

for animal in animals.reversed() {
    print(animal)
}

// goat
// sheep
// camel
// cow
// horse

reversed: elements with an index

Sometimes an index is needed when iterating through a collection. For that you can use enumerate(), which returns a tuple. The first element of the tuple is the index and the second element is the object.

let animals = ["horse", "cow", "camel", "sheep", "goat"]

Forward

for (index, animal) in animals.enumerated() {
    print("\(index), \(animal)")
}

// 0, horse
// 1, cow
// 2, camel
// 3, sheep
// 4, goat

Backward

for (index, animal) in animals.enumerated().reversed()  {
    print("\(index), \(animal)")
}

// 4, goat
// 3, sheep
// 2, camel
// 1, cow
// 0, horse

Note that as Ben Lachman noted in his answer, you probably want to do .enumerated().reversed() rather than .reversed().enumerated() (which would make the index numbers increase).

stride: numbers

Stride is way to iterate without using a range. There are two forms. The comments at the end of the code show what the range version would be (assuming the increment size is 1).

startIndex.stride(to: endIndex, by: incrementSize)      // startIndex..<endIndex
startIndex.stride(through: endIndex, by: incrementSize) // startIndex...endIndex

Forward

for index in stride(from: 0, to: 5, by: 1) {
    print(index)
}

// 0
// 1
// 2
// 3
// 4

Backward

Changing the increment size to -1 allows you to go backward.

for index in stride(from: 4, through: 0, by: -1) {
    print(index)
}

// 4
// 3
// 2
// 1
// 0

Note the to and through difference.

stride: elements of SequenceType

Forward by increments of 2

let animals = ["horse", "cow", "camel", "sheep", "goat"]

I'm using 2 in this example just to show another possibility.

for index in stride(from: 0, to: 5, by: 2) {
    print("\(index), \(animals[index])")
}

// 0, horse
// 2, camel
// 4, goat

Backward

for index in stride(from: 4, through: 0, by: -1) {
    print("\(index), \(animals[index])")
}

// 4, goat
// 3, sheep 
// 2, camel
// 1, cow  
// 0, horse 

Notes

• @matt has an interesting solution where he defines his own reverse operator and calls it >>>. It doesn't take much code to define and is used like this:

  for index in 5>>>0 {
      print(index)
  }
  
  // 4
  // 3
  // 2
  // 1
  // 0
  

• Check out On C-Style For Loops Removed from Swift 3

Passport

Answer 4

Swift 4 onwards

for i in stride(from: 5, to: 0, by: -1) {
    print(i)
}
//prints 5, 4, 3, 2, 1

for i in stride(from: 5, through: 0, by: -1) {
    print(i)
}
//prints 5, 4, 3, 2, 1, 0

Etherege

Answer 5

With Swift 5, according to your needs, you may choose one of the four following Playground code examples in order to solve your problem.

---

#1. Using ClosedRange reversed() method

ClosedRange has a method called reversed(). reversed() method has the following declaration:

func reversed() -> ReversedCollection<ClosedRange<Bound>>

Returns a view presenting the elements of the collection in reverse order.

Usage:

let reversedCollection = (0 ... 5).reversed()

for index in reversedCollection {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

As an alternative, you can use Range reversed() method:

let reversedCollection = (0 ..< 6).reversed()

for index in reversedCollection {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

---

#2. Using sequence(first:next:) function

Swift Standard Library provides a function called sequence(first:next:). sequence(first:next:) has the following declaration:

func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T>

Returns a sequence formed from first and repeated lazy applications of next.

Usage:

let unfoldSequence = sequence(first: 5, next: {
    $0 > 0 ? $0 - 1 : nil
})

for index in unfoldSequence {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

---

#3. Using stride(from:through:by:) function

Swift Standard Library provides a function called stride(from:through:by:). stride(from:through:by:) has the following declaration:

func stride<T>(from start: T, through end: T, by stride: T.Stride) -> StrideThrough<T> where T : Strideable

Returns a sequence from a starting value toward, and possibly including, an end value, stepping by the specified amount.

Usage:

let sequence = stride(from: 5, through: 0, by: -1)

for index in sequence {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

As an alternative, you can use stride(from:to:by:):

let sequence = stride(from: 5, to: -1, by: -1)

for index in sequence {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

---

#4. Using AnyIterator init(_:) initializer

AnyIterator has an initializer called init(_:). init(_:) has the following declaration:

init(_ body: @escaping () -> AnyIterator<Element>.Element?)

Creates an iterator that wraps the given closure in its next() method.

Usage:

var index = 5

guard index >= 0 else { fatalError("index must be positive or equal to zero") }

let iterator = AnyIterator({ () -> Int? in
    defer { index = index - 1 }
    return index >= 0 ? index : nil
})

for index in iterator {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

If needed, you can refactor the previous code by creating an extension method for Int and wrapping your iterator in it:

extension Int {

    func iterateDownTo(_ endIndex: Int) -> AnyIterator<Int> {
        var index = self
        guard index >= endIndex else { fatalError("self must be greater than or equal to endIndex") }

        let iterator = AnyIterator { () -> Int? in
            defer { index = index - 1 }
            return index >= endIndex ? index : nil
        }
        return iterator
    }

}

let iterator = 5.iterateDownTo(0)

for index in iterator {
    print(index)
}

/*
Prints:
5
4
3
2
1
0
*/

Acaroid

sequence(first: 5, next: { ($0 > 0) ? ($0 - 1) : nil }) // is simpler than ($0 - 1 >= 0) – Genoese

Answer 6

For Swift 2.0 and above you should apply reverse on a range collection

for i in (0 ..< 10).reverse() {
  // process
}

It has been renamed to .reversed() in Swift 3.0

Kinsella

Answer 7

In Swift 4 and latter

    let count = 50//For example
    for i in (1...count).reversed() {
        print(i)
    }

Elegit

Answer 8

Swift 4.0

for i in stride(from: 5, to: 0, by: -1) {
    print(i) // 5,4,3,2,1
}

If you want to include the to value:

for i in stride(from: 5, through: 0, by: -1) {
    print(i) // 5,4,3,2,1,0
}

Coloration

Answer 9

If one is wanting to iterate through an array (Array or more generally any SequenceType) in reverse. You have a few additional options.

First you can reverse() the array and loop through it as normal. However I prefer to use enumerate() much of the time since it outputs a tuple containing the object and it's index.

The one thing to note here is that it is important to call these in the right order:

for (index, element) in array.enumerate().reverse()

yields indexes in descending order (which is what I generally expect). whereas:

for (index, element) in array.reverse().enumerate() (which is a closer match to NSArray's reverseEnumerator)

walks the array backward but outputs ascending indexes.

Farhi

Answer 10

as for Swift 2.2 , Xcode 7.3 (10,June,2016) :

for (index,number) in (0...10).enumerate() {
    print("index \(index) , number \(number)")
}

for (index,number) in (0...10).reverse().enumerate() {
    print("index \(index) , number \(number)")
}

Output :

index 0 , number 0
index 1 , number 1
index 2 , number 2
index 3 , number 3
index 4 , number 4
index 5 , number 5
index 6 , number 6
index 7 , number 7
index 8 , number 8
index 9 , number 9
index 10 , number 10


index 0 , number 10
index 1 , number 9
index 2 , number 8
index 3 , number 7
index 4 , number 6
index 5 , number 5
index 6 , number 4
index 7 , number 3
index 8 , number 2
index 9 , number 1
index 10 , number 0

Ablebodied

Answer 11

You can consider using the C-Style while loop instead. This works just fine in Swift 3:

var i = 5 
while i > 0 { 
    print(i)
    i -= 1
}

Brier

Answer 12

This will works decrement by one in reverse order.

let num1 = [1,2,3,4,5]
for item in nums1.enumerated().reversed() { 

    print(item.offset) // Print the index number: 4,3,2,1,0
    print(item.element) // Print the value :5,4,3,2,1

}

Or you can use this index, value property

let num1 = [1,2,3,4,5]
for (index,item) in nums1.enumerated().reversed() { 

    print(index) // Print the index number: 4,3,2,1,0
    print(item) // Print the value :5,4,3,2,1

}

Enochenol

Answer 13

var sum1 = 0
for i in 0...100{
    sum1 += i
}
print (sum1)

for i in (10...100).reverse(){
    sum1 /= i
}
print(sum1)

Abukir

Answer 14

You can use reversed() method for easily reverse values.

var i:Int
for i in 1..10.reversed() {
    print(i)
}

The reversed() method reverse the values.

Aguirre

Answer 15

Reversing an array can be done with just single step .reverse()

var arrOfnum = [1,2,3,4,5,6]
arrOfnum.reverse()

Overblouse

Answer 16

For me, this is the best way.

var arrayOfNums = [1,4,5,68,9,10]

for i in 0..<arrayOfNums.count {
    print(arrayOfNums[arrayOfNums.count - i - 1])
}

Dirt

The best native way – Nutty