Printing out all the combinations of a string

Question

iOS 11, Swift 4.0

Trying to write a recursive function to show all the possible combinations of a string. I got this, but its not quite right since I get only 20 pa

Answer 1

Not a direct answer to your question but you can get all permutations (translated from java to Swift) as follow:

public extension RangeReplaceableCollection {
    func permutations() -> [SubSequence] {
        isEmpty ? [] : permutate(.init())
    }
    private func permutate(_ subSequence: SubSequence) -> [SubSequence] {
        var permutations = isEmpty ? [subSequence] : []
        indices.forEach {
            permutations += (self[..<$0] + self[$0...].dropFirst())
                .permutate(subSequence + CollectionOfOne(self[$0]))
        }
        return permutations
    }
}

---

let str = "ABCD"
print(str.permutations())   // "["ABCD", "ABDC", "ACBD", "ACDB", "ADBC", "ADCB", "BACD", "BADC", "BCAD", "BCDA", "BDAC", "BDCA", "CABD", "CADB", "CBAD", "CBDA", "CDAB", "CDBA", "DABC", "DACB", "DBAC", "DBCA", "DCAB", "DCBA"]\n"

---

Per-mutating a substring

print("ABCD".dropLast().permutations())   // ["ABC", "ACB", "BAC", "BCA", "CAB", "CBA"]\n"

Answer 2

Since you said: Trying to write a recursive function to show all the possible combinations of a string

I think you could so something like this:

// Takes any collection of T and returns an array of permutations
func permute<C: Collection>(items: C) -> [[C.Iterator.Element]] {
    var scratch = Array(items) // This is a scratch space for Heap's algorithm
    var result: [[C.Iterator.Element]] = [] // This will accumulate our result

    // Heap's algorithm
    func heap(_ n: Int) {
        if n == 1 {
            result.append(scratch)
            return
        }

        for i in 0..<n-1 {
            heap(n-1)
            let j = (n%2 == 1) ? 0 : i
            scratch.swapAt(j, n-1)
        }
        heap(n-1)
    }

    // Let's get started
    heap(scratch.count)

    // And return the result we built up
    return result
}

// We could make an overload for permute() that handles strings if we wanted
// But it's often good to be very explicit with strings, and make it clear
// that we're permuting Characters rather than something else.

let string = "ABCD"
let perms = permute(string.characters) // Get the character permutations
let permStrings = perms.map() { String($0) } // Turn them back into strings
print(permStrings) // output if you like

Extracted from this answer which helped me a lot once: Calculate all permutations of a string in Swift

This would give you a number of permutations.

I hope this helps you.