How to swap two characters in String Swift4?

Question

in Swift4, the String is Collections. You will no longer use characters property on a string.

func swapCharacters(input:String,index1:Int,index2:Int)-> String { // logic to swap }

let input = "ABCDEFGH"

If I call the function with (input,3,8) then the output should be

Output : ABCHEFGD

Note: In Swift4, Strings are collections.

Dunaville

what have you tried? Why swap B and C, because you always want to switch B and C or because they are at index 1 and 2 or because they are in the middle or because they are next to each other? – Dimple

@kumar Reddy on what algo are you selecting B & C for swap? What if string goes ABCDFGHRFYUJITR? – Eristic

updated the question. – Dunaville

Possible duplicate of How to replace nth character of a string with another – Evalynevan

Answer 1

Fairly straightforward since String is a collection:

func swapCharacters(input: String, index1: Int, index2: Int) -> String {
    var characters = Array(input)
    characters.swapAt(index1, index2)

    return String(characters)
}

let input = "ABCDEFGH"
print(swapCharacters(input: input, index1: 3, index2: 7)) //ABCHEFGD

or, to provide a direct array-like operation:

extension String {
    mutating func swapAt(_ index1: Int, _ index2: Int) {
        var characters = Array(self)
        characters.swapAt(index1, index2)
        self = String(characters)
    }
}

var input = "ABCDEFGH"
input.swapAt(3, 7)
print(input) //ABCHEFGD

Villeinage

Whats is the time complexity of your solution? I see you are creating new String on each mutation. Is it O(n)? – Mockheroic

@Mockheroic It's obviously O(n) depending on the length of the String. If you need lots of similar operations, you should use arrays of characters instead of a String because the conversion from String to [Character] is the slow thing here. – Villeinage

If this is the case, do you think replacing Sequence at the given range would be a better option? At least it wouldn’t be O(n). – Mockheroic

@Mockheroic I don't think it would help much. String characters are not the same length - String can store them in different ways, and that means that replacing a character can actually mean changing the whole sequence of bytes. With short strings the difference will be probably negligible anyway. – Villeinage

But it’s worth mentioning that if you are dealing with utf8 sequence then replacing a range might be of benefit. – Mockheroic

Answer 2

If you are looking for String manipulation without converting it to array. I won't say it's great solution, but does its job.

func swapCharacters(input: String, index1: Int, index2: Int) -> String {
var result = input
let index1Start = input.index(input.startIndex, offsetBy: index1)
let index1End = input.index(after: index1Start)

let index2Start = input.index(input.startIndex, offsetBy: index2)
let index2End = input.index(after: index2Start)

let temp = input[index2Start..<index2End]
result.replaceSubrange(index2Start..<index2End,
                       with: input[index1Start..<index1End])
result.replaceSubrange(index1Start..<index1End, with: temp)

return result

}

print(swapCharacters(input: "ABCDEFGH", index1: 3, index2: 7))

prints: ABCHEFGD

Juetta

Answer 3

Your question doesn't make sense. Strings in Swift are not indexed by Int. You need to first figure out for yourself what a Character really is (an Extended Grapheme Cluster). Then you need to figure out why Int cannot be used as the index for Strings. And then you can come up with a real problem. Read up here:

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html

Permeability

I am solving data structure problems in Swift while doing the permutations for a string i need to swap the elements. So how should I do that without accessing the index? – Dunaville

"Int cannot be used as the index for Strings"?? For a given Swift string value are there not a countable and ordered set of indexes? That Swift choose not to use integers is just that, a choice - maybe a good one, maybe not, but there is no cannot here surely? – Bruges