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javascriptif-statementrecursionpalindrome
Blas Oronoz
Blas Oronoz

Reputation: 48

Getting False on a recursive function with if conditions trying to solve the "Palindrome" Challenge

So I was doing this popular challenge "palindrome" and I was copying this solution from the "Frontend Masters" Javascript Series and I'm getting a different output. I want to know if there is something that changes or am I missing something. This is my first question on StackOverflow because this is just MindF***k.

What is going on?


'use strict'

function isPalindrome(str) {
  if (str.length <= 1) return true;
  var first = str[0];
  var last = str[str.length - 1];
  if (first === last) {
    console.log(str.substring(1, str.length - 1))
    isPalindrome(str.substring(1, str.length - 1));
  }
  return false;
}

console.log(isPalindrome("abcdcba")) // Return false on my machine

I try this on the RunJS app as well as the VScode terminal and also I run Node on the file.

KEEP RETURNING FALSE !!

Upvotes: 0

Views: 194

Answers (2)

Jose Marin
Jose Marin

Reputation: 964

I fixed your code...

'use strict'

function isPalindrome(str) {
  if (str.length <= 1) return true;
  var first = str[0];
  var last = str[str.length - 1];
  if (first === last) {
    console.log(str.substring(1, str.length - 1))
    return isPalindrome(str.substring(1, str.length - 1));
  }else return false;
}

console.log(isPalindrome("abcdcba")) //

I see recursive functions having two different parts:

  1. Going forward when the function starts calling itself and stacking function calls.
  2. in a reverse way going backward when all the stack functions start returning values. In your example this phase is kickoff just after if (str.length <= 1) return true or if first !== last.

I think you you did it totally right going forward, but there was a minor details in the come back. You need to remember that ones you have your solution you need to return values through all the calls back to the initial one.

A recursive function is kind of split in two:

function recursive{

  //forward processing
  ..
  resultValue = recursive()
  ..
  // backward processing if needed
  return kindOfResultValue //resultValue or a transformation if needed
}

Note: Remember to check all conditional branches of your recursive function to return always a value after calling itself

Upvotes: 0

David
David

Reputation: 219026

The function will return true if and only if the length of the input is <= 1:

if (str.length <= 1) return true;

Which it isn't:

isPalindrome("abcdcba")

The only other return statement in the function is:

return false;

It looks like you meant to return the recursive result:

return isPalindrome(str.substring(1, str.length - 1));

Otherwise the function never does anything with the result from calling itself recursively, and just defaults to returning false on the last line.

Upvotes: 3

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