Check Palindrome using recursion

Question

I am trying to implement a function that will check to see if a word is a palindrome Below is the code that i have tried to use. the code works for one letter words obviously and words that don't start and end with the same letter. It fails on anything else. Please help

bool is_palindrome(int start, int end, const string & str)
{
    if (str[start] != str[end])
        return false;
    else if (start == end)
        return true;
    else
        return is_palindrome(start++, end--, str);

    return false;
}

here is the main function that tis function calls http://csel.cs.colorado.edu/%7Eekwhite/CSCI2270Fall2011/recursion_lab/palindrome.cxx

The ++ and -- operators are not doing what you think they're doing. — Raymond Chen, Oct 26 '11 at 01:03
"Fails"? Did you step through and find out _where_ it failed? On paper? With a pen? — Lightness Races in Orbit, Oct 26 '11 at 01:03
Please do not undo formatting fixes. Better still, don't make the formatting errors and typos in the first place! — Lightness Races in Orbit, Oct 26 '11 at 01:05
@Sean: There's more than one problem here. Consider each piece of advice given. — Lightness Races in Orbit, Oct 26 '11 at 01:05
You say "It fails": are you running into an infinite loop, by chance? — Chris Frederick, Oct 26 '11 at 01:05
Also, please tag as homework. It helps others determine how to answer your question appropriately. — Michael Price, Oct 26 '11 at 01:59
Possibly worth noting that in a language without tail recursion optimisation, recursion isn't the best way to check for a palindrome. Do this to learn recursion, but don't consider it a good solution. — slim, Aug 14 '17 at 10:35

score 7 · Accepted Answer · answered Oct 26 '11 at 01:04

start++ increments the [local] variable start, and evaluates to the old value. You want to evaluate to the new value, and you don't need to alter the local variable at all.

So write start+1, end-1 instead, and then consider the case where your string has an even number of characters because there is another issue there.

score 2 · Answer 2 · answered Oct 26 '11 at 01:04

2

Couple of things:

Don't you need to use the prefix increment/decrement operators?

return is_palindrome(++start,--end, str);

Also, you need to make the test start >= end for it to work on an even number of characters.

answered Oct 26 '11 at 01:04

500 - Internal Server Error

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score 0 · Answer 3 · edited Jul 04 '15 at 18:00

0

It should be something like:

bool is_palindrome(int start, int end, const string& str)
{
        if (str[start] != str[end])
          return false;
        else if ((start == end) || (start < end))
          return true;
        else
          return is_palindrome(++start, --end, str);

        return false;
}

edited Jul 04 '15 at 18:00

Randika Vishman

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answered Jul 04 '15 at 17:31

Suman

1

score 0 · Answer 4 · answered Aug 14 '17 at 07:07

Try this, not sure isit 100% working but for me it works:

bool is_palindrome(int start, int end, const string& str)
{
    if (str[start] != str[end])
      return false;
    else if (str[start] == str[end])
      {
        if (start <= end)
        return is_palindrome(start+1, end-1, str);
        return true;
      }

}

score -1 · Answer 5 · answered Aug 14 '17 at 10:31

recursively calls is palindrome function to check the string is palindrome or not. bool is_palindrome(int start, int end, const string& str) the start and end variables is used to contol the matching process for the palindrome. after each matching the next iteration is given using the recursion format

Check Palindrome using recursion

5 Answers5