Add string to beginning of another string - ios

Question

Basic question
I have 2 strings. I want to add one string to another? Here's an example:
var secondString= "is your name."
var firstString = "Mike, "
Here I have 2 strings. I want to add firstString to secondString, NOT vice versa. (Which would be: firstString += secondString.)
More detail
I have 5 string
let first = "7898"
let second = "00"
let third = "5481"
let fourth = "4782"
var fullString = "\(third):\(fourth)"
I know for sure that third and fourth will be in fullString, but I don't know about first and second.
So I will make an if statement checking if second has 00. If it does, first and second won't go in fullString. If it doesn't, second will go intofullString`.
Then I will check if first has 00. If it does, then first won't go inside of fullString, and if not, it will go.
The thing is, I need them in the same order: first, second, third fourth. So in the if statement, I need a way to potentially add first and second at the beginning of fullString.

Answer 1

Re. your basic question:
secondString = "\(firstString)\(secondString)"
or
secondString = firstString + secondString
Here is a way to insert string at the beginning "without resetting" per your comment (first at front of second):
let range = second.startIndex..<second.startIndex
second.replaceRange(range, with: first)
Re. your "more detail" question:
var fullString: String
if second == "00" {
fullString = third + fourth
} else if first == "00" {
fullString = second + third + fourth
} else {
fullString = first + second + third + fourth
}

Answer 2

From the Apple documentation:
String values can be added together (or concatenated) with the addition operator (+) to create a new String value:
let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome now equals "hello there"
You can also append a String value to an existing String variable with the addition assignment operator (+=):
var instruction = "look over"
instruction += string2
// instruction now equals "look over there"
You can append a Character value to a String variable with the String type’s append() method:
let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome now equals "hello there!"
So you are pretty much free to add these in any way shape or form.
Which includes
secondstring += firststring
Edit to accommodate the new information:
Strings in Swift are mutable which means you can always add to a string in-place without recreating any objects.
Something like (pseudo-code)
if(second != "00")
{
fullstring = second + fullstring
//only do something with first if second != 00
if(first != "00")
{
fullstring = first + fullstring
}
}