Java Exercise

[Anni-Frid Lyngstad]

Hello everyone,

I'm trying to learn how to program in Java, but I'm stuck on an exercise.

I have to print all combinations of the string 'XYZ' in lexicographic order (the order that words are found in dictionary). I'm able to do that in Java. The output is:
XXX
XXY
XXZ
XYX
XYY
XYZ
(etc...)

Here is my program:

import java.util.Arrays;

 class Main

{

    public static void lexicographic (char[] chars, String output)

    {

        if (output.length() == chars.length)

        {

            System.out.print(output + " ");

            return;

        }

        for (char c : chars) {

            lexicographic(chars, output + c);

        }

    }

     public static void main(String[] args)

    {

        String str = "XYZ";

        char[] chars = str.toCharArray();

        Arrays.sort(chars); 

        lexicographic(chars, "");

    }

}

However, if I want to remove, let's say, all combininations that includes 'X' and 'Y', what should I do?

Thank you in advance!
Anni-Frid

[Carey Brown]

Wouldn't this work?

String str = "Z";

Please don't double space your lines of code.

[Norm Radder]

, what should I do?

Is that a question about what the algorithm is to solve the problem?
Or do you have the algorithm for the solving the problem and are asking how to code it in java?
If the latter, post it with your questions about how the problems you are having writing the code.

[Anni-Frid Lyngstad]

Thank you for your replies.

Let me start again from the beginning (at first I tried to simplify the problem but now I realize that the way that I put it changes everything).

First of all, I have to write a code in Java that prints all the combinations of the string 'WXYZ' in lexicographic order (I removed the 'W' to make the problem simpler but now I realize that I should not have).

That should give me:

WWWW (first line)
WWWX
WWWY
WWWZ
WWXW
etc...
ZZZZ (last line)

That part I can get it done easily (with the code I posted above).

However, I have to remove (i.e. the code should not print) all the combinations that contain 'W' and 'Z' (at the same time).

For example, the combination WWWZ should not be in the output since it contains 'W' and 'Z'.

I don't know how to write that in Java.

@Corey: Sorry, I will be more careful next time.

[Norm Radder]

Can you use the String class methods?  For example use indexOf two times: once for W and once for Z

[Carey Brown]

Just to take the requirements you just filled in and stubbing out the code you should have something like this. This would be "a" way of doing it.

import java.util.Arrays;

class Lexi
{
	public static void lexicographic( char[] chars, String output, String findAllChars )
	{
		if( output.length() == chars.length && ! hasAllCharsIn( output, findAllChars ) )
		{
			System.out.print( output + " " );
			return;
		}

		for( char c : chars )
		{
			lexicographic( chars, output + c, findAllChars );
		}
	}

	private static boolean hasAllCharsIn( String str, String findAllChars )
	{
	// TODO	
	}
	
	public static void main( String[] args )
	{
		String str = "WXYZ";
		char[] chars = str.toCharArray();
		Arrays.sort( chars );
		lexicographic( chars, "", "WZ" );
	}
}

[Carey Brown]

Oops. There's a simple error in there, see if you can spot it. Still, stubbing things out helps narrow down the task.

[Anni-Frid Lyngstad]

We need a return statement at line 21 but I am struggling to put it properly...

[Anni-Frid Lyngstad]

Oh I didn't see the "to do" comment that you have added in the code. At first I thought you put a random "1000" there (I need to put my glasses on).

But even when I add a "return false;" statement (because of the boolean type), I get the same result, i.e. it prints all the combinations without properly removing those with W and Z.

And don't get me wrong, I don't understand the use of the "return false;" statement in that context.

[Anni-Frid Lyngstad]

@ Norm: Tomorrow I'll try to use indexOf, looks like a good idea!

[Campbell Ritchie]

Welcome to the Ranch

Why would you want indexOf()? What about contains()? But that doesn't answer my first question. “Trying out” a method looks like guessing, and it looks as though you hadn't yet worked out what(←read that link) you want to do. You cannot write code until you have worked out what you are doing, and then you write the algorithm on paper. Once you have the correct algorithm, the coding will become easy.

[Piet Souris]

Suppose that your 'lexicographic' method has a parameter 'List<Character> doNotPrint', then it is efficient to take this into account in the 'for' loop, like

for (char c : chars) {
    if (!doNotPrint.contains(c)) {
        lexicographicHelper(chars, output + c, doNotPrint);
    }
}

Another thing:
it is not very userfriendly to bother the user with a method that requires a sorted char[] and an output string. A possibility is to make this method private, rename it as say 'lexicographicHelper'. Have a user method 'lexicograpic(String string, List<Character> doNotPrint)', that calls the Helper method.