Return multiple of string characters - ios

Question

I want to create a function where an input of a string is taken in and the output returns a string of how many times the number is repeated. For example if my string is "1111223444" it will return "41221334", because there are four 1's, two 2's, one 3, and three 4's. So an input of "2234467" would return "2213241617". I am not sure if a dictionary would be the best way to implement it and I am really confused. I have started the function however don't know where to go from here. Any tips or resources will be helpful.
func stringOutput(input: String) -> String {
var result = ""
var lastCharacter: Character
var count = 0
var countDict: [String: Int] = [:]
for item in input {
countDict[item] as Character
}
return result
}

Answer 1

Your function computes the next term of the Look-and-say sequence. Here is an implementation (essentially taken from Leetcode 38: The “count-and-say” sequence on Code Review). Instead of traversing the string, we directly search for the next index of a character different from the current one. Neither the first nor the last run has be be treated specially:
extension String {
func lookAndSay() -> String {
var result = ""
var fromIndex = startIndex // Start of current run
while fromIndex != endIndex {
let char = self[fromIndex] // Current character
// Find start of next run
let toIndex = self[fromIndex...].firstIndex(where: { $0 != char }) ?? endIndex
// Compute length of run, and append it to the result
let len = distance(from: fromIndex, to: toIndex)
result += "\(len)\(char)"
// Continue with next run
fromIndex = toIndex
}
return result
}
}
print("1111223444".lookAndSay()) // 41221334
print("2234467".lookAndSay()) // 2213241617

Answer 2

Try the snippet below.
func stringOutput(input: String) -> String {
var result = ""
var lastKnownCharacter: Character? = nil
var lastKnownCharacterCount: Int = 0
for character in input {
if lastKnownCharacter == nil {
lastKnownCharacter = character
lastKnownCharacterCount = 1
} else if lastKnownCharacter == character {
lastKnownCharacterCount += 1
} else {
result.append("\(lastKnownCharacterCount)\(lastKnownCharacter!)")
lastKnownCharacter = character
lastKnownCharacterCount = 1
}
}
result.append("\(lastKnownCharacterCount)\(lastKnownCharacter!)")
return result
}