Shift elements in array by index

Question

Given array of n elements, i.e.

var array = [1, 2, 3, 4, 5]

I can write an extension to the Array so I can modify array to achieve this output: [2, 3, 4, 5, 1]:

mutating func shiftRight() {
  append(removeFirst())
}

Is there a way to implement such a function that would shift array by any index, positive or negative. I can implement this function in imperative style with if-else clauses, but what I am looking for is functional implementation.

The algorithm is simple:

1. Split array into two by the index provided
2. append first array to the end of the second

Is there any way to implement it in functional style?

The code I've finished with:

extension Array {
  mutating func shift(var amount: Int) {
    guard -count...count ~= amount else { return }
    if amount < 0 { amount += count }
    self = Array(self[amount ..< count] + self[0 ..< amount])
  }
}

Answer 1

Here is a straightforward and practical solution as an array extension. Works with any argument

extension Array {
    mutating func shift(by positions: Int) {
        guard count > 0 else { return }
        let shift = (positions % count + count) % count
        self = Array(self[shift..<count] + self[0..<shift])
    }
}

var array = [1, 2, 3, 4, 5]

array.shift(by: 2)  // array becomes [3, 4, 5, 1, 2]

array.shift(by: -2) // array becomes [1, 2, 3, 4, 5]

Answer 2

There’s now a function in Swift Algorithms library specifically for that purpose: rotate(toStartAt:). Details here

Answer 3

I know I late to the party, but this answer based on the question works great?

extension Array {
  mutating func shiftRight(p: Int) {
    for _ in 0..<p {
      append(removeFirst())
    }
  }
}

start [5, 0, 4, 11, 0]
shift [5, 0, 4, 11, 0] shift 0
shift [0, 4, 11, 0, 5] shift 1
shift [4, 11, 0, 5, 0] shift 2
shift [11, 0, 5, 0, 4] shift 3

Even better, if you ask it to shift more elements than there are in the array, it simply keeps circling.

Answer 4

You can use ranged subscripting and concatenate the results. This will give you what you're looking for, with names similar to the standard library:

extension Array {
    func shiftRight(var amount: Int = 1) -> [Element] {
        guard count > 0 else { return self }
        assert(-count...count ~= amount, "Shift amount out of bounds")
        if amount < 0 { amount += count }  // this needs to be >= 0
        return Array(self[amount ..< count] + self[0 ..< amount])
    }

    mutating func shiftRightInPlace(amount: Int = 1) {
        self = shiftRight(amount)
    }
}

Array(1...10).shiftRight()
// [2, 3, 4, 5, 6, 7, 8, 9, 10, 1]
Array(1...10).shiftRight(7)
// [8, 9, 10, 1, 2, 3, 4, 5, 6, 7]

Instead of subscripting, you could also return Array(suffix(count - amount) + prefix(amount)) from shiftRight().

Answer 5

With Swift 5, you can create shift(withDistance:) and shiftInPlace(withDistance:) methods in an Array extension with the following implementation in order to solve your problem:

extension Array {

    /**
     Returns a new array with the first elements up to specified distance being shifted to the end of the collection. If the distance is negative, returns a new array with the last elements up to the specified absolute distance being shifted to the beginning of the collection.

     If the absolute distance exceeds the number of elements in the array, the elements are not shifted.
     */
    func shift(withDistance distance: Int = 1) -> Array<Element> {
        let offsetIndex = distance >= 0 ?
            self.index(startIndex, offsetBy: distance, limitedBy: endIndex) :
            self.index(endIndex, offsetBy: distance, limitedBy: startIndex)

        guard let index = offsetIndex else { return self }
        return Array(self[index ..< endIndex] + self[startIndex ..< index])
    }

    /**
     Shifts the first elements up to specified distance to the end of the array. If the distance is negative, shifts the last elements up to the specified absolute distance to the beginning of the array.

     If the absolute distance exceeds the number of elements in the array, the elements are not shifted.
     */
    mutating func shiftInPlace(withDistance distance: Int = 1) {
        self = shift(withDistance: distance)
    }

}

Usage:

let array = Array(1...10)
let newArray = array.shift(withDistance: 3)
print(newArray) // prints: [4, 5, 6, 7, 8, 9, 10, 1, 2, 3]

var array = Array(1...10)
array.shiftInPlace(withDistance: -2)
print(array) // prints: [9, 10, 1, 2, 3, 4, 5, 6, 7, 8]

let array = Array(1...10)
let newArray = array.shift(withDistance: 30)
print(newArray) // prints: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

let array = Array(1...10)
let newArray = array.shift(withDistance: 0)
print(newArray) // prints: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

var array = Array(1...10)
array.shiftInPlace()
print(array) // prints: [2, 3, 4, 5, 6, 7, 8, 9, 10, 1]

var array = [Int]()
array.shiftInPlace(withDistance: -2)
print(array) // prints: []

Answer 6

I took a stab at writing some extensions for this. It has some nice features:

• Shifting by an amount greater than count causes a wrap-around.
• Shifting by negative amounts flips the direction
• Exposes functions as the bit-shift binary operators (<<, <<=, >>, >>=)

extension Array {
    public func shiftedLeft(by rawOffset: Int = 1) -> Array {
        let clampedAmount = rawOffset % count
        let offset = clampedAmount < 0 ? count + clampedAmount : clampedAmount
        return Array(self[offset ..< count] + self[0 ..< offset])
    }

    public func shiftedRight(by rawOffset: Int = 1) -> Array {
        return self.shiftedLeft(by: -rawOffset)
    }

    public mutating func shiftLeftInPlace(by rawOffset: Int = 1) {
        if rawOffset == 0 { return /* no-op */ }

        func shiftedIndex(for index: Int) -> Int {
            let candidateIndex = (index + rawOffset) % self.count

            if candidateIndex < 0 {
                return candidateIndex + self.count
            }

            return candidateIndex
        }

        // Create a sequence of indexs of items that need to be swapped.
        //
        // For example, to shift ["A", "B", "C", "D", "E"] left by 1:
        // Swapping 2 with 0: ["C", "B", "A", "D", "E"]
        // Swapping 4 with 2: ["C", "B", "E", "D", "A"]
        // Swapping 1 with 4: ["C", "A", "E", "D", "B"]
        // Swapping 3 with 1: ["C", "D", "E", "A", "B"] <- Final Result
        //
        // The sequence here is [0, 2, 4, 1, 3].
        // It's turned into [(2, 0), (4, 2), (1, 4), (3, 1)] by the zip/dropFirst trick below.
        let indexes = sequence(first: 0, next: { index in
            let nextIndex = shiftedIndex(for: index)
            if nextIndex == 0 { return nil } // We've come full-circle
            return nextIndex
        })

        print(self)
        for (source, dest) in zip(indexes.dropFirst(), indexes) {
            self.swapAt(source, dest)
            print("Swapping \(source) with \(dest): \(self)")
        }
        print(Array<(Int, Int)>(zip(indexes.dropFirst(), indexes)))
    }

    public mutating func shiftRightInPlace(by rawOffset: Int = 1) {
        self.shiftLeftInPlace(by: rawOffset)
    }
}

public func << <T>(array: [T], offset: Int) -> [T] { return array.shiftedLeft(by: offset) }
public func >> <T>(array: [T], offset: Int) -> [T] { return array.shiftedRight(by: offset) }
public func <<= <T>(array: inout [T], offset: Int) { return array.shiftLeftInPlace(by: offset) }
public func >>= <T>(array: inout [T], offset: Int) { return array.shiftRightInPlace(by: offset) }

You can see it in action here.

Here is a more general solution, which implements this functionality lazily for any type that meets the requirements:

extension RandomAccessCollection where
    Self: RangeReplaceableCollection,
    Self.Index == Int,
    Self.IndexDistance == Int {
    func shiftedLeft(by rawOffset: Int = 1) -> RangeReplaceableSlice<Self> {
        let clampedAmount = rawOffset % count
        let offset = clampedAmount < 0 ? count + clampedAmount : clampedAmount
        return self[offset ..< count] + self[0 ..< offset]
    }

    func shiftedRight(by rawOffset: Int = 1) -> RangeReplaceableSlice<Self> {
        return self.shiftedLeft(by: -rawOffset)
    }

    mutating func shiftLeft(by rawOffset: Int = 1) {
        self = Self.init(self.shiftedLeft(by: rawOffset))
    }

    mutating func shiftRight(by rawOffset: Int = 1) {
        self = Self.init(self.shiftedRight(by: rawOffset))
    }

    //Swift 3
    static func << (c: Self, offset: Int) -> RangeReplaceableSlice<Self> { return c.shiftedLeft(by: offset) }
    static func >> (c: Self, offset: Int) -> RangeReplaceableSlice<Self> { return c.shiftedRight(by: offset) }
    static func <<= (c: inout Self, offset: Int) { return c.shiftLeft(by: offset) }
    static func >>= (c: inout Self, offset: Int) { return c.shiftRight(by: offset) }
}

Answer 7

An easy solution,

 public func solution(_ A : [Int], _ K : Int) -> [Int] {

    if A.count > 0 {
        let roundedK: Int = K % A.count

        let rotatedArray = Array(A.dropFirst(A.count - roundedK) + A.dropLast(roundedK))

        return rotatedArray
    }

    return []
}

Answer 8

Here's a functional implementation for "in place" rotation that doesn't require extra memory nor a temporary variable and performs no more than one swap per element.

extension Array 
{
    mutating func rotateLeft(by rotations:Int) 
    { 
       let _ =                                              // silence warnings
       (1..<Swift.max(1,count*((rotations+1)%(count+1)%1))) // will do zero or count - 1 swaps
       .reduce((i:0,r:count+rotations%count))               // i: swap index r:effective offset
       { s,_ in let j = (s.i+s.r)%count                     // j: index of value for position i
         swap(&self[j],&self[s.i])                          // swap to place value at rotated index  
         return (j,s.r)                                     // continue with next index to place
       }
    }
}

It optimally supports zero, positive and negative rotations as well as rotations of larger magnitude than the array size and rotation of an empty array (i.e. it cannot fail).

Uses negative values to rotate in the other direction (to the right).

Rotating a 3 element array by 10 is like rotating it by 1, the fist nine rotations will bring it back to its initial state (but we don't want to move elements more than once).

Rotating a 5 element array to the right by 3, i.e. rotateLeft(by:-3) is equivalent to rotateLeft(by:2). The function's "effective offset" takes that into account.

Answer 9

The fastest way is (but takes double memory!):

input:

var arr = [1,2,3,4,5]
let k = 1 (num steps to rotate)
let n = arr.count ( a little but faster )

rotation LEFT:

    var temp = arr
    for i in 0..<n {
        arr[(n-i+k)%n] = temp[i]
    }

result: [2, 1, 4, 3, 5]

rotation RIGHT:

    var temp = arr
    for i in 0..<n {
    arr[(i+k)%n] = temp[i]
    }

result: [4, 1, 2, 3, 5]

Answer 10

In objective C you can simply get left shifted array like this:

- (NSMutableArray *)shiftedArrayWithOffset:(NSInteger)offset
{
    NSMutableArray *bufferArray = [[NSMutableArray alloc] initWithArray:originalArray];
    for (int i = 0; i < offset; i++)
    {
        id object = [bufferArray firstObject];
        [bufferArray removeObjectAtIndex:0];
        [bufferArray addObject:object];
    }
    return bufferArray;
}

Answer 11

Following the Nate Cook answers , I need also to shift an array returning reverse order, so I made:

//MARK: - Array extension 
Array {
    func shiftRight( amount: Int = 1) -> [Element] {
        var amountMutable = amount
        assert(-count...count ~= amountMutable, "Shift amount out of bounds")
        if amountMutable < 0 { amountMutable += count }  // this needs to be >= 0
        return Array(self[amountMutable ..< count] + self[0 ..< amountMutable])
    }
    func reverseShift( amount: Int = 1) -> [Element] {
        var amountMutable = amount
        amountMutable = count-amountMutable-1
        let a: [Element] = self.reverse()
        return a.shiftRight(amountMutable)
    }

    mutating func shiftRightInPlace(amount: Int = 1) {
        self = shiftRight(amount)
    }

    mutating func reverseShiftInPlace(amount: Int = 1) {
        self = reverseShift(amount)
    }
}

We have for example:

Array(1...10).shiftRight()
// [2, 3, 4, 5, 6, 7, 8, 9, 10, 1]
Array(1...10).shiftRight(7)
// [8, 9, 10, 1, 2, 3, 4, 5, 6, 7]
Array(1...10).reverseShift()
// [2, 1, 10, 9, 8, 7, 6, 5, 4, 3]
Array(1...10).reverseShift(7)
// [8, 7, 6, 5, 4, 3, 2, 1, 10, 9]