Need some help in writing a recursive implementation for my Palindrome app

First of all I am not asking for people to "do my homework" like I have seen others on here ask for. I have managed to code a working iterative version of a program that determines if a string is a palindrome or not. Spaces, punctuation and special characters are ignored while determining if the string is a palindrome. This version does work but when I try and apply recursive statements in the "isPalindrome()" method I get Stack Overflow errors. I know what these errors are, it's just that applying a recursive method in a program like this is quite hard for me to get my head around (I only got taught about them 2 weeks ago). Anyway here is the code I have managed to compile (and run) so far:

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/** Palindrome.java: A sigle application class that determines if a word or a string
  * is a palindrome or not. This application is designed to ignore spaces between
  * chars, punctuation marks and special characters while determining if the word or
  * string is a palindrome or not.
  * 
  **/

import java.util.Scanner;
import java.util.StringTokenizer;
import java.util.regex.*;

public class Palindrome{

  static String palindrome, str, str2, str3;

  /** The main method of the Palindrome application. Takes input from the
    * user, removes spaces from their input, turns their string input into
    * lowercase and then all non letter characters are taken out of the user's
    * input. Finally the recursive method determines if the string entered in
    * by the user is a palindrome.
    * 
    * @param args Takes in a string array of arguements
    **/
  public static void main(String[] args){
    Scanner input = new Scanner(System.in);
    while(input.hasNext()){ 
      str = removeSpaces(input.nextLine());
      str2 = str.toLowerCase();
      str3 = normalise(str2);
    }
    System.out.println(isPalindrome(str3));

  }

  /** The default constructor
    **/
  public Palindrome(){
  }

  /** isPalindrome(): A boolean method that is passed through a String input
    * and uses a for loop, two inner while loops and an if-else to determine
    * whether the users input is a palindrome.
    *
    * @param s The string input to be tested
    * @return true The users input is a palindrome
    * @return false The user开发者_JS百科s input isn't a palindrome
    **/
  public static boolean isPalindrome(String s){

    int first, last;

    for(first = 0, last = s.length()-1 ; first < last ; first++ , last-- ){
      while( (int)s.charAt(first) < 'a' || (int)s.charAt(first) > 'z' ){
        first++;
      }
      while( (int)s.charAt(last ) < 'a' || (int)s.charAt(last ) > 'z' ){
        last--;
      }
    }
    if( first > last || s.charAt(first) != s.charAt(last) ){
      //return isPalindrome(s.substring(0, s.length()-1)) == false;
      return false;
    }
    else{
      //return isPalindrome(s.substring(0, s.length()-1)) == true;
      return true;
    }
  }

  /**
   * This method takes out punctuation marks in the string parsed
   * through, using Java's regular expressions (regex) and Java's
   * inbuilt method replaceAll(). The regex expression is passed
   * through the replaceAll() method to remove all non alpha-numeric
   * characters from the string passed through the method's parameter.
   * 
   * @param t The string that will have punctuation stripped from it.
   *
   * @return t The string has had all non alpha-numeric characters
   * removed and the new string is then returned.
   */
  public static String normalise(String t){
    t = t.replaceAll("[^a-zA-Z0-9]", "");
    return t;
  }

  /** removeSpaces(): A method that deletes spaces from the users input
    * and then decrements the string length count so any indexes aren't missed
    * when it is incremented.
    * 
    * @param s The string which is going to have it's spaces removed.
    * @return temp The new string is then returned after the spaces have been taken out.
    **/
  public static String removeSpaces(String s){
    StringBuilder temp = new StringBuilder(s); //creates a new StringBuilder with the inputted String
    for(int i = 0; i < temp.length(); i++){ //do this for the entire length of the StringBuilder

      if(temp.charAt(i) == ' '){ //if the char at i is a space

        temp.deleteCharAt(i); //remove the char
        i--; //subtract 1 from the counter so we don't miss an index when we increment it
      }
    }
    return temp.toString(); //return the new String
  }
}

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I have blanked out the recursive statements in the recursive method for now. If someone can tell me what exactly I have done wrong and also help me in implementing a solution that would be really good. I would rather stick with the iterative version because I understand the mechanics of it, but have been asked to do a recursive version (I have been Java coding since after my mid year break last year but am a relative novice at recursion) which is proving to be quite a challenge. If you alter the code and it ends up working with the recursive version please explain how, when, why etc with your alterations. Am not looking for someone to just do this for me, I'm wanting to learn and it seems that I have learned best by example (I did get a B pass last year by analysing examples and reading explanations of implementations). Many thanks :).

EDIT: I think I have got the recursion going ok now, just the logic is the thing confusing me at the moment. Here is the recoded version of the isPalindrome() method:

  public static boolean isPalindrome(String s){

    int first, last;
    boolean isPalindr = true;

    if (s.length() <= 1){
      return true; // Base case
    }

    for(first = 0, last = s.length()-1 ; first < last ; first++ , last-- ){
//      while( (int)s.charAt(first) < 'a' || (int)s.charAt(first) > 'z' ){
//        first++;
//      }
//      while( (int)s.charAt(last ) < 'a' || (int)s.charAt(last ) > 'z' ){
//        last--;
//      }
//  }
      if( first == last || s.charAt(first) == s.charAt(last) ){
        //return isPalindrome(s.substring(first, last));
        return isPalindrome(s.substring(first, last)) == true;
        //isPalindr = false;
      }
      else{
        return isPalindrome(s.substring(first, last)) == false;
        //isPalindr = true;
      }
    }
    return isPalindr;
  }

If someone can help me with the logic I think this will be fixed :).

---

Removing all of the code that has nothing to do with the problem leaves us with this:

public static boolean isPalindrome(String s){
   for loop {
      isPalindrome(); 
   }
}

isPalindrome calls isPalindrome calls isPalindrome, etc... infinitum.

The difference between this and a proper recursive function is that a recursive function will have some sort of conditional statement, breaking the cycle of the function calling itself. The flow of execution will go like this:

isPalindrome(1) begins execution and calls isPalidrome(2)
 isPalindrome(2) begins execution and calls isPalidrome(3)
  isPalindrome(3) begins execution and calls isPalidrome(4)
   isPalindrome(4) begins execution and calls isPalidrome(5)
    isPalindrome(5) begins execution and returns to isPalindrome(4)
   isPalindrome(4) resumes execution and returns to isPalindrome(3)
  isPalindrome(3) resumes execution and returns to isPalindrome(2)
 isPalindrome(2) resumes execution and returns to isPalindrome(1)
isPalindrome(1) resumes execution and returns.

If that explanation doesn't help, think of it like this. Suppose someone was handing you plates, one at a time, to see if you can hold 25 plates at a time. It would go something like this:

Plate 1 is given to you.  Are there 25 plates?  No.  Add another plate.
 Plate 2 is stacked on top of Plate 1.  Are there 25 plates?  No.  Add another plate.
  Plate 3 is stacked on top of Plate 2.  Are there 25 plates?  No.  Add another plate.
   ...
    Plate 24 is stacked on top of Plate 23.  Are there 25 plates?  No.  Add another plate.
     Plate 25 is stacked on top of Plate 24.  Are there 25 plates?  Yes.  Mission Accomplished.  Now, let's put the plates back.
     Plate 25 is removed.
    Plate 24 is removed.
   ...
  Plate 3 is removed.
 Plate 2 is removed.
Plate 1 is removed.

Here's how that might be coded:

bool stackPlates(int i){
   plateStack.addPlate();

   if (plateStack.wasDropped == true) { return false; }     // Were the plates dropped?  Return FALSE to indicate failure.
    else if (i < 25) { return stackPlates(i+1); }           // Are there 25 plates yet?  If not, add another.
     else { return true; }                                  // There are 25 plates stacked.  Return TRUE to indicate success.

   plateStack.removePlate(i);
}

Here's stackPlates(int i) called from another function:

bool success = stackPlates(1);

if (success==TRUE) { cout << "CONGRATULATIONS!  YOU STACKED 25 PLATES!"; }
 else { cout << "YOU BROKE THE PLATES!  BETTER LUCK NEXT TIME!"; }

What your function needs to do in order to work properly is do this:

bool isPalindrome(string s, int i) {

   char first = s[i];                   // REPLACE THIS WITH THE CODE TO SKIP SPACES & SPECIAL CHARACTERS
   char last = s[(s.length -1) -i];     // REPLACE THIS WITH THE CODE TO SKIP SPACES & SPECIAL CHARACTERS

   if ( first != last )  { return false; }                // return false if mismatch letter
    else if ( i >= (s.length/2) ) { return true; }        // return true if string fully checked
     else { return isPalindrome(s, i+1); }                // string not fully checked; move to next letter

}

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You're experiencing stack overflows because the else branch at the bottom of the function is executed when (first <= last && "characters are equals"), so you keep recurring on the case where your string is composed by one character.

By the way, I think your code is not using recursion cleanly: you should preprocess your string only one time before starting recurring on the string, and the code that performs the palindrome recursion should be far simpler.

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For any given entry into isPalindrome, it's going to recursively call itself regardless because you have no condition on your else. So, if it meets the criteria "first > last || s.charAt(first) != s.charAt(last)", it's going to recursively call isPalindrome, then the next call is too, even if it hits the else.

I don't know what a Palindrome is or what the real solution to the problem is, but that's why you're getting the stack overflow error. I suspect you need to add another condition to your else such that it will stop recursively calling itself.

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When writing a recursive function the best way to go about this is usually to decide on a base case (:like "" is a palindrome, though so is "a" ... ) and then devise a method to take any state and move it to the base case.

So in the case of the palindrome, it's the same basic idea as before, if the first character and the last character are the same you return true and check the rest of the string ( thus moving closer to the base case ) and if they are not then you return false.

Your stack overflow comes from calling isPalindrome in every case rather than when you need to continue solving the problem, don't forget that if two characters mean that something isn't a palindrome, the rest is rendered irrelevant ( and thus needn't be recursed on )

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Your recoded version is a bit strange, because it's still using a loop when it doesn't need to. In particular, your code will never go beyond the first iteration in your loop, because in the embedded if-else statement, you're going to return a result no matter what, so your function will always exit during the first iteration (unless there are no iterations at all).

Recursion should be approached by

1. Identifying a base case, i.e. a simplest case that can be solved
2. Re-representing a larger problem as a partial solution followed by the same, but smaller problem.

The base case you've handled correctly; any String which is length 1 or less is automatically a Palindrome.

The next step is to consider a larger problem, perhaps some string abcwewe....ba. How can we break this down into a simpler problem? We know that we'd normally check whether something is a palindrome by checking the letters one by one in pairs, starting at the ends, but then we also realise that each time we check the letters, we just repeat the same problem again and solve it the same way.

In the string I gave above, we check and verify that the first letter a is the same as the last letter a, so that's kind of a partial solution. Now we we end up with is the smaller word bcwewe....b, and it's the same problem again: Is this new String a palindrome also?

Thus, all you have to do now is to invoke the recursive call, but this time with the substring beginning with the 2nd character to the 2nd to last character. You can code the answer in just two lines, as below:

public static boolean isPalindrome(String s) {
    if (s.length() <= 1) return true; // base case
    return s.charAt(0) == s.charAt(s.length()-1) && isPalin(s.substring(1,s.length()-1)); // recursive case
}

One point to note is that I'm using the short circuit &&, so if the first condition fails (checking first and last character), then Java will not invoke the recursion.