Swift Arrays: Write a rotate-right function

Question

Task:

Write a function which rotates all elements of a given array to the right.

Example: [1, 2, 3] => [3, 1, 2]

My solution:

func rotateRight(array ary: [Int]) -> [Int] {
 guard ary.isEmpty == false else {
 return ary
 }
 
 var copy = ary
 let last = ary.last
 
 var i = ary.count - 1
 
 while i > 0 {
 copy[i] = copy[i - 1]
 i -= 1
 }
 
 copy[0] = last!
 
 return copy
}
var ary1 = [1, 2, 3]
print(rotateRight(array: ary1)) // [3, 1, 2]
var ary2 = [1, 1, 2, 3, 3, 4]
print(rotateRight(array: ary2)) // [4, 1, 1, 2, 3, 3]
var ary3 = [Int]()
print(rotateRight(array: ary3)) // []
var ary4 = [1]
print(rotateRight(array: ary4)) // [1]

I have to make a copy of the array because Swift-parameter are immutable. Do I handle the issue in a proper way? Or should it be done differently?

Is there a better way to solve the described task?

• ielyamani: guard let last = ary.last else { return ary }; return [last] + ary.dropLast()
• ielyamani: return ary.isEmpty ? ary : ary.suffix(1) + ary.prefix(ary.count - 1)

Answer 1

A comparison against true or false is never needed, ary.isEmpty == false is more concisely expressed as !ary.isEmpty.

Here I would not use a guard statement at all. That statement was introduced to get rid of the "if-let pyramid of doom," to unwrap a variable without introducing another scope/indentation level. But I would not use a guard statement for every "early return" situation, in particular not if it makes the statement look like a double negation.

if ary.isEmpty {
 return ary
}

is shorter and clearer in my opinion. Alternatively one can use optional binding

if let last = ary.last {
 // ...
} else {
 return ary
}

to test for a non-empty array and get the last element in one step, that allows to get rid of the ugly forced unwrap copy[0] = last! in your code.

As already mentioned in the comment, array slicing methods can be used instead of copying all elements to their new place:

if let last = ary.last {
 return [last] + ary.dropLast()
} else {
 return ary
}

Another option is to concatenate two array slices, this can easily be generalized to shifting array elements by more than one position to the right or left:

if ary.isEmpty {
 return ary
}
let frontIndex = ary.count - 1
return Array(ary[frontIndex...]) + ary[0..<frontIndex]

Naming: The parameter name ary reminds my of unary/binary/arity and is not significantly shorter than array. My suggestion is to use array with an empty argument label:

func rotateRight(_ array: [Int]) -> [Int] {
 // ... 
}
var array1 = [1, 2, 3]
print(rotateRight(array1)) // [3, 1, 2]

Of course, if you don't want to reinvent the wheel then you can simply use the rotate(toStartAt:) method of the swift-algorithms package.