Replacing a char in a string based on a condition

I came across this question in CodeWars recently & tried to solve it using Swift 5 (which I’m new to):

Given a string of digits, you should replace any digit below 5 with ‘0’ and any digit 5 and above with ‘1’. Return the resulting string.
Note: input will never be an empty string

I come from a Python background where we could using indexing with if statements to solve this particular problem like this:

def bin(x):
    newstr = ""
    for num in x:
        if int(num) < 5:
            newstr += "0"
        else:
            newstr += "1"
    return newstr

But when I tried to use indexing in Swift( like the Python style), I failed…which, after a while of researching made me come across this method: String.replacingOccurrences(of: , with: )

So if my Swift code for the above ques is like this:

func fakeBin(digits: String) -> String {
  
  for i in digits{
    
    if (Int(i)<5){
      some code to replace the char with '0'
    }
    else{some code to replace the char with '1'}
  
    return digits
  }

How do I use the above method String.replacingOccurrences(of: , with: ) in my code and achieve the desired output? If this is not possible, how else can we do it?

Note: I have already looked up for solutions on the website & found one:How do I check each individual digit within a string? But wasn’t of any use to me as the questioner wanted the help in Java

>Solution :

A Swift String is a collection of Characters. You can map each character to "0" or "1", and join the result to a new string:

func fakeBin(digits: String) -> String {
    return String(digits.map { $0 < "5" ? "0" : "1" })
}