JavaScript palindrome checker. What is wrong with this code? The input "almostoma" should have given a false output, but my code gives a true output

Question

It works just fine with other palindromes (e.g "Able was I saw Elba.", "A man, a plan, a canal. Panama."), but fails with "almostoma".

function palindrome(str) {
  str = str.toLowerCase();
  str = str.split(" ").join("");
  str = str.replace(",", "");
  str = str.replace("_", "");
  str = str.replace(".", "");
  for (var i = 0; i < str.length; i++) {
    if (str[i] == str[str.length - i - 1]) {
      return true;
    } else {
      return false;
    }
  }
}
console.log(palindrome("almostoma"));

Your code is returning true directly if the first and the last characters match. You do not compare the other chars anymore. You should not return true if the characters match, but return false if they not match. When your loop is done you return true cause all characters match — Thallius, Jul 07 '23 at 11:01
This function will return true for any string where the first and last characters are the same. Notice that you `return true` when `str[i] == str[str.length - i - 1]` -- this exits the function. What you should do is `continue` when that condition is true and only `return true` outside of your loop. — glhrmv, Jul 07 '23 at 11:01
Related: [Does return stop a loop?](https://stackoverflow.com/a/11714515) — Nick Parsons, Jul 07 '23 at 11:02
thank you @glhrmv for your help. I will try correcting my code. — Patrick, Jul 07 '23 at 11:49

score 2 · Answer 1 · edited Jul 07 '23 at 11:44

2

What your code does is that as soon as the if (str[i] == str[str.length - i - 1]) is true, the program returns and it stops executing. Meaning only the last character is checked for equality

What I advise is to check for inequality only:

function palindrome(str) {
  str = str.toLowerCase();
  str = str.split(" ").join("");
  str = str.replace(",", "");
  str = str.replace("_", "");
  str = str.replace(".", "");
  for (var i = 0; i < str.length; i++) {
    if (str[i] != str[str.length - i - 1]) {
      return false;
    }
  }
  return true;
}
console.log(palindrome("almostoma"));

edited Jul 07 '23 at 11:44

mplungjan

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answered Jul 07 '23 at 11:04

Manan Dhiman

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Thank you! I tried doing it your way and made some modifications to fit the requirements, and it worked! – Patrick Jul 07 '23 at 12:14

score 2 · Answer 2 · answered Jul 07 '23 at 11:59

See the other answers and comment for the error you made

Here is a different approach

const palindrome = (str) => {
  str = String(str).toLowerCase(); // handle numbers too
  str = str.replace(/[^\p{L}\p{N}]/gu, ''); // remove all non characters unicode aware
  return str === [...str].reverse().join(''); // compare the reversed string
}

console.log(palindrome("almostoma"));
console.log(palindrome("Able was I saw Elba."))
console.log(palindrome("A man, a plan, a canal. Panama."))
console.log(palindrome(88000088))

This is much simpler. no need for a loop, just see if the revered string equals the input string. — Chris Barr, Jul 07 '23 at 12:03

JavaScript palindrome checker. What is wrong with this code? The input "almostoma" should have given a false output, but my code gives a true output

2 Answers2