Comparing values between two arrays

Question

I'm trying to set up a function that checks if a word or a text is a palindrome. To do that, it splits the text so that every letter is an element of a new array, it takes rid of the white spaces and it makes the reverse array. Then it checks if every element of the two arrays, at the same positions, are equal. If not it returns false, if yes it returns true. Here the function:

function palindrome(str) {
  var low = str.toLowerCase();
  var newArray = low.split("");
  var noSpace = newArray.filter(function(val) {
    return val !== " ";
  });
  var reverse = noSpace.reverse();
  
  function check (a, b) {
    console.log(`checking '${a}' against '${b}'`);
    var partial;
    var result = 1;
    for (var i = 0; i < a.length; i++) {
      console.log(`comparing '${a[i]}' and '${b[i]}'`);
      if (a[i] !== b[i]) {
        result = 0;
      } else {
        partial = 1;
        result *= partial;
      }
    }
    return result;
  }
  
  var result = check(noSpace, reverse);
  if (result == 1) {
    return true;
  } else {
    return false;
  }
  
   
}


palindrome("r y e");

I don't know what's wrong but it seems that the function keeps on returning a true value no matter what word or text I pass to the function. What is wrong with that?

Answer 1

Your issue seems to be because reverse() changes the actual array as well. So doing

var reverse = noSpace.reverse();

Will reverse noSpace and assign a reference to it on the variable reverse. That is, both arrays will be the same (reversed) array.

To bypass that, I've used .slice() to create a copy of the original array, and then called .reverse() on that new array, ridding you of any conflicts.

Here's a working snippet of what it looks like:

function palindrome(str) {
    var str_array = str.toLowerCase().split("");
    var no_space = str_array.filter(function(val) {
        return val !== " ";
    });

    // By applying '.slice()', we create a new array
    // reference which can then be reversed and assigned
    // to the 'reverse' variable
    var reverse = no_space.slice().reverse();

    function check(a, b) {
        var partial;
        var result = 1;
        for(var i=0; i < a.length; i++) {
            if(a[i] !== b[i]) {
                // We don't need to keep
                // comparing the two, it
                // already failed
                return 0;
            } else {
                // I've kept this part even though
                // I don't really know what it is
                // intended for
                partial = 1;
                result *= partial;
            }
        }
        return result;
    }
    return check(no_space, reverse) === 1;
}

console.log(palindrome("a b a"));
console.log(palindrome("r y e"));

Answer 2

The way you have coded for palindrome is way too complicated.

But there is one problem with your code: when you do a reverse() it changes the original array as well.

So you will need to make sure that you copy it via slice().

Also you can directly send a boolean result rather than doing a 1 and 0.

Answer 3

At result *= partial;, 1 * 1 will always equal 1

Answer 4

I didn't correct your code, but here is a optimized solution for you.

function palindrom(string) {
    var arr = string.split("");

    var lengthToCheck = Math.floor(arr.length / 2);
    for (var i = 0; i < lengthToCheck; i++) {
        if (arr[i] != arr[arr.length - (1 + i)]) {
            return false;
        }
    }

    return true;
}

First I split the array after every charater of the passed String. After that I get the half of the length of the array as it's enough to check just one half. With the for-loop I compare the first half with the second half. As soon as I found two characters that do not match I return false. In case the whole first half matches the second half of the array, the for-loop will be completed and after that true will be returned.

Answer 5

What's actually happening is .reverse() reverses an array in place, it then stores a reference to that array which is not what you're calling in your check() method.

Simple fix would be to change your if statement:

if (a[i] !== b.reverse()[i])