Good old palindrome in javascript.

[mrchow19910319]

I am trying to solve this how to detect a word is palindrome quest using javascript where I found some logic error in my code I do not know how to solve. 

 

Let's say that we ignore all the edge cases, like uppercase/lowercase of the word, non-letter characters etc.

And the user only key in a single word with lowercase in it.

 

There are a few ways you can do this. 

 

1. use javascript built in function. (this one works)

function check (string){
	let new_string = string.split('').reverse().join('');

	if (string === new_string){
		return true;
	}
	return false;
}


console.log(check("helloaxd"));
console.log(check("eye"));

I think that we have not much to say, all the work is done using split() and reverse() and join()

 

2. Split old string into array, using for loop to create a new reversed array, then use join to make it a reversed string (this one works)

function check(string){
	let arr = string.split('');
	let new_arr = [];

	for(let i = arr.length-1; i>=0; i--){
		new_arr.push(arr[i]);
	}

	let new_string = new_arr.join('');

	if(string===new_string){
		return true;
	}
	return false;
}


console.log(check("helloasd"));
console.log(check("hasd"));

since the above snippet is still comparing strings, it works. 

now it gets strange.

3. convert everything to array then compare them (DOES NOT WORK)

function palindrome(str) {
  let arr = str.split('');
  let reversed = [];

  for(let i = arr.length-1;i>=0;i--){
    reversed.push(arr[i]);
  }

  console.log("reversed is:" + reversed);
  console.log("original is:" + arr);

  if(reversed===arr){return true;}
  else{return false;}

}



console.log(palindrome("eye"));

this will give you false no matter what. I think that we cannot compare arrays like that can we? 

4. reverse string returned an object?? 

function check(string){
	let new_string = {};

	for(let i = string.length-1; i>=0; i--){
		new_string = new_string + string[i];
	}

	console.log("string is " + string);
	console.log("reversed string is: " + new_string);
	return new_string === string;
}


console.log(check("hello"));

in this snippet, new_string will give you 

[object Object]olleh

What does it mean and why is that so? 

 

[jincio]

So, yeah, you can't compare Arrays. That's why you usually use library like Lodash for doing such thing. For the last case on other hand, you are initializing a new empty object

let new_string = {};

 so when you try to add a string to an object javascript try to convert everything to a string and for that use the method `.toString()` on the obejct, which result in [object Object]. For making the last example work you need to initialize the new object as a new empty string

let new_string = "";

 

[vorticalbox]

Here is another way for you

it is a waste of processing time to create a new string to compare when you compare the input to itself.

const isPalindrome = (str) => {
  const length = str.length
  for(let i = 0; i < length; i++){
  	// has to have +1 else its out of the bound of the array
  	if(str[i] !== str[length-(i+1)]) return false
  }
  return true
}

 

[mrchow19910319]

5 minutes ago, vorticalbox said:

Here is another way for you

it is a waste of processing time to create a new string to compare when you compare the input to itself.

const isPalindrome = (str) => {
  const length = str.length
  for(let i = 0; i < length; i++){
  	// has to have +1 else its out of the bound of the array
  	if(str[i] !== str[length-(i+1)]) return false
  }
  return true
}

 

after finding 7 ways of checking palindrome

Spoiler

just kidding. thanks for the input! 

I think that doing all these algorithms is gonna prepare me for technical interviews.

[vorticalbox]

1 hour ago, mrchow19910319 said:

after finding 7 ways of checking palindrome

  Reveal hidden contents

just kidding. thanks for the input! 

I think that doing all these algorithms is gonna prepare me for technical interviews.

you could run some benchmarks and see which is the fastest  I think mine will be as it returns false on the first fail without needing to build the whole string /array.

[mrchow19910319]

35 minutes ago, vorticalbox said:

you could run some benchmarks and see which is the fastest  I think mine will be as it returns false on the first fail without needing to build the whole string /array.

haha yeah! when I have time I will check that! 

 

what language you main? 

[infamoustrey]

5 hours ago, vorticalbox said:

Here is another way for you

it is a waste of processing time to create a new string to compare when you compare the input to itself.

const isPalindrome = (str) => {
  const length = str.length
  for(let i = 0; i < length; i++){
  	// has to have +1 else its out of the bound of the array
  	if(str[i] !== str[length-(i+1)]) return false
  }
  return true
}

 

o(n) in all of it's beauty

[elpiop]

18 hours ago, vorticalbox said:

Here is another way for you

it is a waste of processing time to create a new string to compare when you compare the input to itself.

const isPalindrome = (str) => {
  const length = str.length
  for(let i = 0; i < length; i++){
  	// has to have +1 else its out of the bound of the array
  	if(str[i] !== str[length-(i+1)]) return false
  }
  return true
}

 

the loop condition only needs to be length/2, not the full length of the string