Levenshtein distance in Swift3

Question

I'm using a tutorial from Rosetta Code to calculate Levenshtein distance. It seems their code is in Swift2 so I get this error Binary operator '+' cannot be applied to operands of type '[Int]' and 'Repeated<String.CharacterView>' when doing this: var cur = [i + 2] + empty where let empty = repeatElement(s, count: 0). How can I go about this?

Answer 1

There were a couple of changes to make.

• The construction of the Array empty.
• enumerate() is now enumerated()
• successor() doesn't exist anymore so I replaced it with +1

So the function is now

Swift 4:

func levDis(_ w1: String, _ w2: String) -> Int {
    let empty = [Int](repeating:0, count: w2.count)
    var last = [Int](0...w2.count)

    for (i, char1) in w1.enumerated() {
        var cur = [i + 1] + empty
        for (j, char2) in w2.enumerated() {
            cur[j + 1] = char1 == char2 ? last[j] : min(last[j], last[j + 1], cur[j]) + 1
        }
        last = cur
    }
    return last.last!
}

Swift 3:

func levDis(w1: String, w2: String) -> Int {

    let (t, s) = (w1.characters, w2.characters)

    let empty = Array<Int>(repeating:0, count: s.count)
    var last = [Int](0...s.count)

    for (i, tLett) in t.enumerated() {
        var cur = [i + 1] + empty
        for (j, sLett) in s.enumerated() {
            cur[j + 1] = tLett == sLett ? last[j] : min(last[j], last[j + 1], cur[j])+1
        }
        last = cur
    }
    return last.last!
}

Answer 2

Updated and improved answer to Swift 4, based on @Spads answer.

extension String {
    func levenshteinDistanceScore(to string: String, ignoreCase: Bool = true, trimWhiteSpacesAndNewLines: Bool = true) -> Float {

        var firstString = self
        var secondString = string   

        if ignoreCase {
            firstString = firstString.lowercased()
            secondString = secondString.lowercased()
        }
        if trimWhiteSpacesAndNewLines {
            firstString = firstString.trimmingCharacters(in: .whitespacesAndNewlines)
            secondString = secondString.trimmingCharacters(in: .whitespacesAndNewlines)
        }

        let empty = [Int](repeating:0, count: secondString.count)
        var last = [Int](0...secondString.count)

        for (i, tLett) in firstString.enumerated() {
            var cur = [i + 1] + empty
            for (j, sLett) in secondString.enumerated() {
                cur[j + 1] = tLett == sLett ? last[j] : Swift.min(last[j], last[j + 1], cur[j])+1
            }
            last = cur
        }

        // maximum string length between the two
        let lowestScore = max(firstString.count, secondString.count)

        if let validDistance = last.last {
            return  1 - (Float(validDistance) / Float(lowestScore))
        }

        return 0.0
    }
}

infix operator =~
func =~(string: String, otherString: String) -> Bool {
    return string.levenshteinDistanceScore(to: otherString) >= 0.85
}

Answer 3

Since @Daniel Illescas answer is not working, here is working version with Int return type and with assert.

extension String {

    func levenshteinDistance(to string: String, ignoreCase: Bool = true, trimWhiteSpacesAndNewLines: Bool = true) -> Int {
        
        var firstString = self
        var secondString = string
        
        if ignoreCase {
            firstString = firstString.lowercased()
            secondString = secondString.lowercased()
        }
        
        if trimWhiteSpacesAndNewLines {
            firstString = firstString.trimmingCharacters(in: .whitespacesAndNewlines)
            secondString = secondString.trimmingCharacters(in: .whitespacesAndNewlines)
        }
        
        let empty = [Int](repeating: 0, count: secondString.count)
        var last = [Int](0...secondString.count)
        
        for (i, tLett) in firstString.enumerated() {
            var cur = [i + 1] + empty
            for (j, sLett) in secondString.enumerated() {
                cur[j + 1] = tLett == sLett ? last[j] : Swift.min(last[j], last[j + 1], cur[j]) + 1
            }
            
            last = cur
        }
        
        if let validDistance = last.last {
            return validDistance
        }
        
        assertionFailure()
        return 0
    }
}

Answer 4

func ~=(string: String, otherString: String) -> Bool {
    return string.levenshteinDistanceScore(to: otherString) >= 0.85 
}