Check if second string is rotation of another string

Question

I'm trying to write a function that takes in two string parameters and check if the second parameter is a rotated version of the first string.

So the following would be the results:

checkRotationStrings('waterbottle', 'lewaterbott'); // true checkRotationStrings('waterbottle', 'bottlewater'); // true checkRotationStrings('waterbottle', 'erbottlewat'); // true checkRotationStrings('waterbottle', 'lewaterbottx'); // false

I wrote the following code, but there are some edge cases that I can't seem to figure out:

function checkRotationStrings(string, rotatedString) {
  let result;
  let rotationCheck
  let stringArr = string.split('');
  let rotatedStringArr = rotatedString.split('')

  for (let i = 0; i < string.length - 1; i++) {
    if (rotatedString[0] === stringArr[i]) {
       result = stringArr.slice(i);
       rotationCheck = stringArr.slice(0, i).concat(result).join('');
    }
  }
  console.log(rotationCheck)
  if (rotationCheck === string){
    return true;
  } else {
    return false;
  }
}

Any help would be appreciated.

For example, I'm breaking the strings where the first letter in the rotatedString is found in the original string. Let's say it's `waterbottle` and `ttlewaterb`. The letter `t` will be found in index `2` first instead of index `7` which is where I want to slice. — , Apr 25 '18 at 19:43
`checkRotationStrings('waterbottle', 'erbottllewat'); // true` should be `false` — Nina Scholz, Apr 25 '18 at 19:48
I would recommend avoiding using `string` as a variable name, as it is *very* close to the [`String`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) object. — Herohtar, Apr 25 '18 at 19:53
Here you have it https://stackoverflow.com/questions/2553522/interview-question-check-if-one-string-is-a-rotation-of-other-string — algrid, Apr 25 '18 at 21:52

Nina Scholz · Answer 1 · 2018-04-25T20:08:20.083

You could use String#repeat with rotated and two as parameter and check with String#includes.

function checkRotationStrings(string, rotated) {
    return string.length === rotated.length && rotated.repeat(2).includes(string);
}

console.log(checkRotationStrings('waterbottle', 'lewaterbott'));  //  true
console.log(checkRotationStrings('waterbottle', 'bottlewater'));  //  true
console.log(checkRotationStrings('waterbottle', 'erbottllewat')); // false
console.log(checkRotationStrings('waterbottle', 'lewaterbottx')); // false
console.log(checkRotationStrings('waterbottle', 'ttlewaterb'));   // false

Hmm, interesting. How does `rotated + rotated` work? – Apr 25 '18 at 19:50 — , Apr 25 '18 at 19:50

Scott Sauyet · Answer 2 · 2018-04-25T20:15:09.557

This is a somewhat strange solution, as it uses some only for the index parameter. But each iteration in some simply compares two strings, one which is a rotation of the first, and the other is the second one.

const checkRotationStrings = (str, rot) => 
    str.split('').some((s, i) => str.slice(i) + str.slice(0, i) == rot);

[
  ['waterbottle', 'lewaterbott'],   // true 
  ['waterbottle', 'bottlewater'],   // true 
  ['waterbottle', 'erbottllewat'],  // false  -- ORIGINAL
  ['waterbottle', 'erbottlewat'],   // true   -- CORRECTED
  ['waterbottle', 'lewaterbottx']   // false
].forEach(([s, r]) => console.log(`'${s}', '${r}': ${checkRotationStrings(s, r)}`))

And this reminds me how much I hate the name `some`. It really should be `any`. And I have no idea *why* I feel strongly about this! — Scott Sauyet, Apr 25 '18 at 20:00

score 2 · Answer 3 · answered Apr 25 '18 at 19:59

You could use substring and rotate until you find a match. Like this:

function checkRotationStrings(string, rotatedString) {

  let match = false;
  for (let i = 0;
    (i < string.length - 1) & !match; i++) {

    match = rotatedString.substring(i, rotatedString.length) + rotatedString.substring(0, i) === string;

  }
  return match

}


console.log(checkRotationStrings('waterbottle', 'lewaterbott')); // true
console.log(checkRotationStrings('waterbottle', 'bottlewater')); // true 
console.log(checkRotationStrings('waterbottle', 'erbottlewat')); // true 
console.log(checkRotationStrings('waterbottle', 'lewaterbottx')); // false

score 0 · Answer 4 · answered Apr 25 '18 at 20:17

You can do the following:

    checkRotationStrings(str1: string, str2: string) {

    if (str1.length !== str2.length) {

      return false;
    } else {

      for (var i = 0; i < str2.length; i++) {

        if (str2[i] === str1[0]) {

          var substring1 = str2.substring(0,i);
          var substring2 = str2.substring(i,str2.length);
          var concatWord = substring2.concat(substring1);

          if(str1 === concatWord){

            console.log(str1 + " matches " + concatWord)
            return true;

          }else{
            console.log(str1 + " not matches " + concatWord)

          }


        }

      }

      return false;

    }


  }

גלעד ברקן · Answer 5 · 2018-04-26T12:55:26.930

Stack Overflow questions often receive answers in the form of completely new code (and/or ideas) on how to achieve the desired result. Usually, there's at least an attempt to help the questioner with their actual question, which in this case seemed to ask for some help with your code (one issue at least had to do with "slicing," as you commented).

I also love to offer new code or ideas if I particularly like it so I'm only partly critical here, but the answers so far have suffered from a complete lack of relating to the question.

There's nothing wrong with how you have conceived of checking for string rotation. You just have a couple of bugs:

First, since rotationCheck is meant to rotate string in order to compare it with rotatedString, you got the string-building reversed. It should be:

rotationCheck = result.concat(stringArr.slice(0, i)).join('');

Secondly, once you've built the rotation-check, you need to compare it with rotatedString, not string. So:

if (rotationCheck === rotatedString){

score 0 · Answer 6 · answered Feb 12 '22 at 10:50

I might be late for this, but here is my solution and it include all edge cases.

function checkRotation(str1,str2){
  const char0 = str1[0];
  let ind = str2.indexOf(char0);
  while(ind>-1){
    const start = str2.substring(0,ind);
    const end = str2.substring(ind,str2.length);
    if(end+start === str1){
      return true;
    }
    ind = str2.indexOf(char0,ind+1)
  }
  return false
}
console.log(checkRotation("assads","adsass"))

score -1 · Answer 7 · answered Apr 25 '18 at 19:52

-1

Instead of trying to break it in random pieces and do different checks, you could instead do a count of the letters. This would be much simpler:

const countLetters = a => Array.prototype.reduce.call(a, (r, l) => Object.assign(r, { [l]: (r[l] || 0) + 1 }), {});

function checkRotatedString(a, b) {
  const countA = countLetters(a);
  const countB = countLetters(b);
  
  return Object.keys(countA).length === Object.keys(countB).length 
    && Object.entries(countA).every(([key, value]) => value === countB[key]);
}

// Tests
[['waterbottle', 'lewaterbott'], ['waterbottle', 'bottlewater'], ['waterbottle', 'erbottlewat'], ['waterbottle', 'lewaterbottx']]
  .forEach(a => console.log(a, checkRotatedString.apply(null, a)));

answered Apr 25 '18 at 19:52

samanime

25,408
15
90
139

2

No: `checkRotatedString('watterbotle', 'waterbottle')` – Scott Sauyet Apr 25 '18 at 19:58
Is that even a valid comparison though. None of his examples mentioned having more/fewer letters than the original (except the invalid case). – samanime Apr 25 '18 at 20:00
3

It's the same exact letters, just arranged differently. – Scott Sauyet Apr 25 '18 at 20:02

Check if second string is rotation of another string

7 Answers7