return the first non repeating character in a string in javascript

Question

So I tried looking for this in the search but the closest I could come is a similar answer in several different languages, I would like to use Javascript to do it.

T

Answer 1

Here's an O(n) solution with 2 ES6 Sets, one tracking all characters that have appeared and one tracking only chars that have appeared once. This solution takes advantage of the insertion order preserved by Set.

const firstNonRepeating = str => {
  const set = new Set();
  const finalSet = new Set();
  str.split('').forEach(char => {
    if (set.has(char)) finalSet.delete(char);
    else {
      set.add(char);
      finalSet.add(char);
    }
  })
  const iter = finalSet.values();
  return iter.next().value;
}

Answer 2

let str='aabcbd'
let ans=''
for (let i=0;i<str.length;i++){
    if(str.indexOf(str.charAt(i))===str.lastIndexOf(str.charAt(i))){
    ans+=str.charAt(i)
    break
    }
}
console.log(ans)

Answer 3

First of all, start your loop at 1, not 0. There is no point in checking the first character to see if its repeating, obviously it can't be.

Now, within your loop, you have someString[i] and someString[i - 1]. They are the current and previous characters.

if someString[i] === someString[i - 1] then the characters are repeating, if someString[i] !== someString[i - 1] then they are not repeating, so you return someString[i]

I won't write the whole thing out for you, but hopefully the thought process behind this will help

Answer 4

I came accross this while facing similar problem. Let me add my 2 lines. What I did is a similar to the Guffa's answer. But using both indexOf method and lastIndexOf.

My mehod:

function nonRepeated(str) {
   for(let i = 0; i < str.length; i++) {
      let j = str.charAt(i)
      if (str.indexOf(j) == str.lastIndexOf(j)) {
        return j;
      }
   }
   return null;
}

nonRepeated("aabcbd"); //c

Simply, indexOf() gets first occurrence of a character & lastIndexOf() gets the last occurrence. So when the first occurrence is also == the last occurence, it means there's just one the character.

Answer 5

> var firstNonRepeatedCharacter = function (str){   
> for(i=0;i<str.length;i++){
>     if(str.indexOf(str.charAt(i)) === str.lastIndexOf(str.charAt(i))){
>       console.log(str.charAt(i));
>       break;
>     }   } }
> 
> firstNonRepeatedCharacter ("areerak");

you can check below link

https://codepen.io/t3veni/pen/wvvxJzm

Answer 6

I used object to keep track of characters count in a string then return the char that has the fa value of 1. Here is a demo:

function firstNotRepeatingCharacter(s) {

// initialize an empty object to store chars

    let seen = {};
    let letter = '';

  // iterate over each char in a string
  // if it is already there increase value by one
  // else set the value to 1

    for(let char of s){
            if (seen[char]){
                    seen[char] +=1;
            } else {
                    seen[char] = 1;
            }
    }

  // iterate over the new constructed object
  // if the value is 1 and the output variable is empty
  // return the associated key to the value 1
  // else return '_'

    for(let v in seen){
        while(seen[v] == 1 && letter === ''){
            letter += v;
            return letter;
        } 
    }
return('_');
}

console.log(firstNotRepeatingCharacter("abacabad"));
console.log(firstNotRepeatingCharacter("cbc"));
console.log(firstNotRepeatingCharacter("bcccccccccccccyb"));
console.log(firstNotRepeatingCharacter("aaa"));