Palindrome Solutions

JavaScript
#1

I’ve been trying to write a palindrome function for the following algorithm challenge: https://www.freecodecamp.org/challenges/check-for-palindromes

Can anybody tell me why the code below isn’t returning any ‘false’ statements?
Thank you in advance.

function palindrome(str) {
strInLowerCase = str.toLowerCase();
readForward = strInLowerCase.replace(/[^a-z 0-9]+$/g);
readBackward = readForward.split().reverse().join("");
if (readBackward == readForward) {
return true;
}
return false;
}

palindrome(“not a palindrome”);

#2

Stick the following right before the if statement and see why it is not returning false:

  console.log(readForward);
  console.log(readBackward)

Your split function is not correct.

Also, your regular expression has an error also. You do not specify what you want to replace with.

#3

I’d like to replace only the alphanumeric characters in the original string with
.replace("", /[^a-z 0-9]+$/g). I’m not quite sure how to go about this.

#4

RegEx is the key to eliminate the special characters. Did you validate your regular expression?

#5

I passed the challenge with this function. There has to be a cleaner, simpler way to write it.

function palindrome(str) {
// turn letters into lowercase, replace letters
var strInLowerCase = str.toLowerCase();
var strPunctuationless = strInLowerCase.replace(/[.,/#!$%^&*;:{}=-_`~()]/g,"");
var strSpaceless = strPunctuationless.replace(/\s{1,}/g,"");
var readForward = strSpaceless;
var readBackward = readForward.split("").reverse().join("");
if (readBackward == readForward) {
return true;
}
return false;
}

palindrome(“not a palindrome”);

#6

Below was my approach to clear the challenge

function palindrome(str) {
  var cleanString = str.replace(/[^A-Z0-9]/ig, "_").replace(/_/ig, "").toLowerCase();  
  return (cleanString == Array.from(cleanString).reverse().join(""));
}
1 Like
#8

Does .replace(/[^A-Z0-9]/ig, “_”) not delete uppercase letters? How did you then change them to lowercase letters with .replace(/_/ig, “”).toLowerCase()?

#9

.replace(/[^A-Z0-9]/ig, “_”) - Will replace all special characters except to “_”
.replace(/_/ig, “”).toLowerCase() - This 2 operations replace all _ to “” and convert all to lowercase.

I did not write a single expression to both at once. We can optimize the RegEx

#10

@samee.ks

var cleanString = str.replace(/[^A-Z0-9]/ig, "").toLowerCase();
#11

Thanks !
That was so lame

#12

I’m still confused. To me, .replace(/[^A-Z0-9]/ig, “_”) gives the command to replace /[^A-Z0-9]/ig with “_”, not to ignore “_”. .replace(/_/ig, “”) looks to be formatted the same as .replace(/[^A-Z0-9]/ig, “_”) yet gives a different command. What am I missing?

#13

For Check for Palindromes, the advanced solution includes .match() methods to replace any non-alphanumeric characters. Why use .match() when it is supposed to return an array with the the corresponding string characters? Like most people, I had only thought of .replace().

#14

match was not used to replace any non-alphanumeric characters. It was used to test if the current character (either str[front] or str[back]) was an alphanumeric character. If it was not, then the next character was examined via incrementing/decrementing front/back. The following could have been used instead of the match statements and worked exactly the same.

while ( str[front].match(/[\W_]/) ) {

could have been replaced with:

while ( /[\W_]/.test(str[front]) ) {

AND

while ( str[back].match(/[\W_]/) ) {

could have been replaced with:

while ( /[\W_]/.test(str[back]) ) {
#15

Ahhh - I see! The entire function make more sense now. Thanks!

#16

function palindrome(str) {
// Good luck!
var lStr = str.toLowerCase(str);
var char_array = lStr.match(/[a-z0-9]/g);
var start = 0;
var end = char_array.length - 1;
for (start; start <= end; start++) {
if (char_array[start] !== char_array[end]) {
console.log(char_array[start]);
console.log(char_array[end]);
return false;
} else {
–end;
}
}
return true;
}
palindrome(‘almostomla’);

#17

Nice and easy :slight_smile:

function palindrome(str) {
// Good luck!
var remove = str.replace(/[^A-Z0-9]/ig, “”).toLowerCase();
var check = remove.split(’’).reverse().join(’’);

if (remove=== check){
return true;
} else return false;
}

palindrome(“eye”);`

#18

Hi guys,

I’d like to share my solution here:

function palindrome(str) {
  str = str.toLowerCase().replace(/\W/g, '').replace('_','');
  var arr = str.split('');
  var firstArr = arr.slice(0,Math.floor(arr.length / 2));
  var secondArr = arr.slice(Math.ceil(arr.length / 2));
  return firstArr.join() == secondArr.reverse().join();
}
#19

My approach takes longer I believe, but is a bit different:

function palindrome(str) {
  // split the input into an array
  var array = [];
  var string = str.toLowerCase();
  // filter array of white space and non alphanumeric values
  string = string.replace(/[^0-9a-z]/gi, '');
  array = string.split("");
  // Create first and last letter to compare
  var firstChar;
  var lastChar;
  var length = Math.floor((array.length)/2);
  // Compare using a for loop at lenght/2
  for(var i = 0; i < length; i++){
    firstChar = array.shift();
    lastChar = array.pop();
    
    if (firstChar !== lastChar){
      return false;
    }
  }
  // If statement, if a != b then false
  return true;
}
#20

A post was split to a new topic: Why does my Palindrome solution pass all but one test?

#21

Here’s my code. How can I make it better?

function palindrome(str) {
// Good luck!
var firstStr = str.toLowerCase();
var secStr = firstStr.replace(/\s/g, “”);
var thirdStr = secStr.replace(/[. ,:-_-/()]+/g, “”);
var arr = [];
arr= thirdStr.split(’’);

newArr = arr.reverse();

var joinArr = newArr.join(’’);

if(joinArr === thirdStr){
return true;
}
else{
return false;
}

}