Recursive Palindrome

[sparksterz]

Ok, I had a lab assigned to me which detailed many different situations in which I had to use recursion in order to solve them. Most of them were pretty simple. However I've been stuck on this Palindrome problem for hours.

I know most people's Palindrome issues are with proving whether a string is a Palindrome or not. This is NOT one of those problems. According to my teacher I must "Write a method that recursively generates random palindromes. The desired length of the palindrome in characters should be a parameter in your method"

At first I thought it would be a piece of cake and I set program up like this:

public class Recursion
{
	public static String Palindrome(int length)
	{
		//Alphabet to pick letters from
		String alpha="abcdefghijklmnopqrstuvwxyz"; 
		String s="";
		String sBackwards=" ";
		if(length==0)
			sBackwards+=reverse(s);
			return sBackwards; 
			//recursion over-rides sBackwards
		else
		{
			Random r = new Random();
			//places a char from alpha into s
			s+=(alpha.charAt((int)r.nextInt(26))); 
			System.out.print(s);
			return Palindrome(length-1); //recursion
		}
	}
	public static void main(String args[])
	{
		System.out.println(Palindrome(10));
	}
}

This code accurately generates a string of 10 random characters by using recursion. I did not include the method to reverse the string as I have tested it and know that it works.

Example of output: cjxpkgaljs
Neccessary output: cjxpkgaljs sjlagkpxjc

However what I found to be the problem was reversing the string in the Palindrome method. I have a reverse method which will reverse a string. However I have the issue that everytime the method calls itself it creates String s=""; because of that I can never take the full complete string and reverse it. Rather I can only print out 1 character at a time.

Does anyone have a direction they can point me in so that I can try to solve this problem?

The method must use recursion however the Palindrome method does not have to be static. I know it's rather rude that my first post is a post requiring help from people however I would like to try to get help else where as my teacher doesn't have office hours tommorow. But the whole lab isn't due until Sept. 12th, I just want to see if I can solve this problem without meeting her in her office as she isn't exactly the easiest to understand...

Thanks for the help.

[kimbokasteniv]

Hmmm, my guess would be that you either pass s as another parameter or instantiate s outside of the method. If you choose the latter you will have to reset s once length = 0.
That's my guess anyway, hope it helps.

[sparksterz]

Yes I had thought about that as well but where can I create the string I tried doing it directly outside of the method so that it was just within the class Recursion and I got flagged with errors. I will have to ask her if I can pass a string as a parameter. I'll let you guys know.

[Paul.Esson]

How about having a method that thakes 2 params

1. How many iterations left.
2. Current String.

Something like ( but prolly not since i have just done this in my head and it would as always be bug ridden. )

string palindrome(String currentString, int iterations){
	iterations--;	

	Random r = new Random();
	char newLetter = alpha.charAt((int)r.nextInt(26));
	currentString = newLetter + currentString + newLetter;

	if ( iterations != 0 ){
		currentString = palindrome(currentString , iterations);	
	}
	return currentString; 
}

then when you call it you would do something like

String myPalindrome = palindrome("", 10);

[kimbokasteniv]

Yes I had thought about that as well but where can I create the string I tried doing it directly outside of the method so that it was just within the class Recursion and I got flagged with errors. I will have to ask her if I can pass a string as a parameter. I'll let you guys know.

If you got errors when compiling, then I suspect that you did not declare s as a static variable.

You may already be aware of this, but it seems you would have to divide length by two otherwise your palindrome would be twice the requested length. This might bring up another issue of whether the requested length is an odd or even number.

[namitiet07]

to reverse a string by using recursion

[darkagn]

Hi namitiet07 and welcome to daniweb,

Did you have a question regarding this thread?

[jwenting]

nope. Just another of those idiots who start accounts for no reason other than to post nonsense that we've been plagued with lately.