CODEWITHSUNDEEP
javarecursionpalindrome
Mic
Mic

Reputation: 11

Palindrome with recursion; index out of bound

I'm trying to write a function that checks whether a string variable is a palindrome or not. I can't find the reason this code throws IndexOutOfBoundException. I've tried to use examples so as to follow the result, but I still can't see where the exception is from.

public static boolean palindrome(String s) {
    if (s.charAt(0) == s.charAt(s.length() - 1)) {
        if (s.length() > 2) {       
            StdOut.print(s.substring(1, s.length() - 2));
            palindrome(s.substring(1, s.length() - 2));
        } else
            return true;
    }
    return false;
}

Upvotes: 1

Views: 141

Answers (4)

barak manos
barak manos

Reputation: 30136

Try this:

public static boolean palindrome(String s)
{
    if (s.length() < 2)
        return true;
    if (s.charAt(0) != s.charAt(s.length()-1))
        return false;
    return palindrome(s.substring(1,s.length()-1));
}

Upvotes: 2

Mureinik
Mureinik

Reputation: 311188

There are several problems here:

  1. As noted, the substring call is wrong - the endIndex is exclusive, so you should end at s.length() - 1)
  2. You are calling palindrome, and then moving on with the function, eventually returning false - instead, you should return the result of palindrome applied on the substring:

So, after taking both these mistakes in to considetation, the method should read as follows:

public static boolean palindrome(String s) {
    if (s.charAt(0) == s.charAt(s.length() - 1)) {
        if (s.length() > 2) {
            // note the return and the corrected substring
            return palindrome(s.substring(1, s.length() - 1));
        } else
            return true;
    }
    return false;
}

Upvotes: 1

Eran
Eran

Reputation: 393781

Your substring is wrong. You are removing an extra character from the end.

public static boolean palindrome(String s) {
    if (s.charAt(0) == s.charAt(s.length() - 1)) {
        if (s.length() > 2) {       
            StdOut.print(s.substring(1, s.length() - 1));
            palindrome(s.substring(1, s.length() - 1));
        } else
            return true;
    }
    return false;
}

Upvotes: 1

NPE
NPE

Reputation: 500287

palindrome can end up calling itself with an empty string. When that happens, the first line with give an IndexOutOfBoundsException in s.charAt(0).

P.S. On closer inspection, the bounds in the substring calls are also incorrect: since the end index is exclusive, the second argument should be s.length() - 1. You'll need to fix that too.

Upvotes: 4

Related Questions