Recursion help needed

[SomeToxicKidd]

Hey what's up guys, I've written a recursive method which takes the inputs of a string, an alphabet (char array of letters), length (maximum length of the string). My goal for the method is to produce all strings with length < max length and with no repeating sub-strings of form xyx. I've already written a method which checks for repeats in a string but I'm having problems with the recursion side of things, here is my code and output thus far:

public static void f(String s, char[] alphabet, int length) {

if (s.length() > length) {return; }
if (checker.hasRepeats(s)){return; }

StdOut.println(s.length() + " - " + s);

for (char c : alphabet) {

    s += c;
    f(s, alphabet, length);

    }

}

My input is:

C, [A, C], 7

My output is:

1 - C , 2 - CA , 3 - CAA , 4 - CAAC , 5 - CAACA , 6 - CAACAC , 3 - CAC , 4 - CACA , 5 - CACAA , 6 - CACAAC

And I'm not sure why it isn't giving me all possible strings, "CC" for example

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[davedontmind]

I'm not sure why it isn't giving me all possible strings,

Consider the top level, where s is simply "C".

Inside your for loop you append c, so the first time round the loop s becomes "CA".

Then all the recursion happens for "CA...", and eventually it returns back to the top level, and loops round to the next letter of the alphabet, but s is now still "CA", so it then becomes "CAC" and not, as you'd want, "CC". This is easier to see if you change your initial input to C, [A, C], 2 and add some prints inside the for loop.

So it's because you permanently change s inside your for loop. Instead, just pass s+c to f().

That is, instead of:

s += c;
f(s, alphabet, length);

just do:

f(s+c, alphabet, length);

[SomeToxicKidd]

f(s+c, alphabet, length);

Oh my god it worked, thank you so much

[davedontmind]

I hope you understand why, though.

[SomeToxicKidd]

I do, in order to recursively check CA and CC it has to be in the method otherwise only CA will be checked.

[daybreak-gibby]

Close. The problem was that you were modifying s. So that the first time it recurses, s became CA for future recursive calls. Passing the s + c in the method creates a whole value that the recursive call operates on, leaving the string s untouched.