Swift L-System Algae - swift

Question

So this is just something I'm doing for fun in the IBM Swift sandbox. I did a L-system algae in Scala recently and though it would be cool to do it in Swift to see how the language compares.
https://github.com/I2obiN/L-System-Algorithms-and-Fractals/blob/master/algae.scala
That's the Scala one to show you what I'm aiming for, and this is what I have in Swift;
/**
* Created by t.hood on 26/01/16
* L-System Algae
*
*/
import Foundation
// Mapping function for string
func stringmap(x: String) -> String {
var str = x;
// Replace characters in string
str = x.stringByReplacingOccurrencesOfString("A", withString: "AB") +
str.stringByReplacingOccurrencesOfString("B", withString: "A");
// Return mapped string;
return str;
}
func lsys() {
// Declarations
var iteration : Int = 2;
var x = 0;
var lsystem: String = "A";
let charA: Character = "A";
let charB: Character = "B";
while(x != iteration) {
print(lsystem)
// Iterate through characters in string
for chars in lsystem.characters {
lsystem = stringmap(lsystem);
}
// Inc count ..
x+=1
}
}
// Run ..
lsys();
The problem I'm having is in my mapping function. I need it to map x, print the result, then do the next map on str. The problem is I have the operator + between both maps and I can't get swift to print str after the first operation.
Anyone any ideas on how to get around this? If I could print str after the first replacements I think it would work.

Answer 1

I created a project in Xcode so I could use the debugger to see what's going on. Here's what I found.
I moved your print statement into the for loop so it would show the value of lsystem every time the loop executed.
The while loop executes twice, when x == 0 and 1
The first time through the while loop lsystem == 'A', so the for loop executes once. In stringmap(), the 'A' becomes 'ABA'.
'ABA' is printed.
The second time through the while loop, the for loop gets executed three times.
For loop 1: 'ABA' is sent to stringmap() and 'ABBABAAA' is returned.
For loop 2: 'ABBABAAA' is sent to stringmap() and 'ABBBABBABABABAAAAAAAA' is returned.
For loop 3: 'ABBBABBABABABAAAAAAAA' is sent to stringmap() and 'ABBBBABBBABBABBABBABABABABABABABABAAAAAAAAAAAAAAAAAAAAA' is returned.
I modified your stringmap() function to iterate through the string character by character and apply the grammar to each character, appending each change to a new string that was then returned to lsys().
Seven iterations of the while loop returned this, which agrees with what I see in the Wikipedia article about L-system.
Iteration 0: A
Iteration 1: AB
Iteration 2: ABA
Iteration 3: ABAAB
Iteration 4: ABAABABA
Iteration 5: ABAABABAABAAB
Iteration 6: ABAABABAABAABABAABABA
Iteration 7: ABAABABAABAABABAABABAABAABABAABAAB
Here's the code. This worked in a playground on my iMac, I'd expect it to work in the IBM Swift sandbox too.
func stringmap(x: String) -> String
{
var returnString = ""
for char in x.characters
{
switch (char)
{
case "A" :
returnString += "AB"
break
case "B":
returnString += "A"
break
default:
break
}
}
return returnString
}
func lsys()
{
// Declarations
let iteration : Int = 7;
var x = 0;
var lsystem: String = "A";
print("Iteration \(x): \(lsystem)")
while(x != iteration)
{
lsystem = stringmap(lsystem)
x++
print("Iteration \(x): \(lsystem)")
}
}
lsys()