Find first recurring character using javascript

Question

Input: array=[2,5,1,2,3,5,1,2,4]

Output return 2

Input: array=[2,1,1,2,3,5,1,2,4]

Output return 1

Input array=[2,3,4,5]

Output return undefined

I run my code on repl.it and it always return undefined.

Is there any improve ways to let undefined disappear?

Also how to return undefined in third input?

What i try in my JS:

   
function FirstRecurringCharacter(arr){
for (i = 0; i < arr.length; i++) {
  if (arr.indexOf(arr[i]) !== i) {
    console.log(arr[i]);
    break;
  }
}
};
   
 FirstRecurringCharacter([2,5,1,2,3,5,1,2,4])//should return 2
   
 FirstRecurringCharacter([2,1,1,2,3,5,1,2,4])//should return 1
   
 FirstRecurringCharacter([2,3,4,5])//should rerturn undefined

When you want to return something, you should use a return statement. — str
– str, Commented Mar 4, 2019 at 13:22

Nina Scholz · Accepted Answer · 2019-03-04 13:23:32Z

5

You could take a Set and use Array#find.

function find(array) {
    var s = new Set;
    return array.find(v => s.has(v) || !s.add(v));
}

console.log(find([2, 5, 1, 2, 3, 5, 1, 2, 4])); // 2
console.log(find([2, 1, 1, 2, 3, 5, 1, 2, 4])); // 1
console.log(find([2, 3, 4, 5]));                // undefined

answered Mar 4, 2019 at 13:23

Nina Scholz

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5 Comments

Jacky Over a year ago

OK..What are .add and .has mean?

Eklavya Over a year ago

If set already has the element then add else return ans because now you got same element again

Nina Scholz Over a year ago

add is to add a value to the set and has test the set, if the value is in the set.

Todd Austin Over a year ago

why the !s.add instead of just s.add? trying to wrap my brain around that...

Nina Scholz Over a year ago

@ToddAustin s.add(v) is to add the value to the set. this returns always the instance of the used set. the second expression is now always truthy, but not wanted, because only the first part should decide and if a value is not in the set, it should return a falsy value.

Onur Arı · Accepted Answer · 2019-03-04 13:23:10Z

3

You should return the value, not log to console.

   
function FirstRecurringCharacter(arr){
  for (i = 0; i < arr.length; i++) {
    if (arr.indexOf(arr[i]) !== i) {
      return arr[i];
    }
  }
  return undefined;
};
   
 console.log(FirstRecurringCharacter([2,5,1,2,3,5,1,2,4]))//should return 2
   
 console.log(FirstRecurringCharacter([2,1,1,2,3,5,1,2,4]))//should return 1
   
 console.log(FirstRecurringCharacter([2,3,4,5]))//should rerturn undefined

answered Mar 4, 2019 at 13:23

Onur Arı

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2 Comments

Onur Arı Over a year ago

I only modified your code, @Nina Scholz's answer is better, you should refer to it.

Evik Ghazarian Over a year ago

Ari Can you please explain why there is no arr.indexOf(arr[i]) > -1 in the condition?

kulotjunior07 · Accepted Answer · 2020-01-08 16:02:06Z

0

function firstReccuringChar(arr) {
  let map = {};
  for (let i = 0; i < arr.length; i++) {
    if (map[arr[i]] !== undefined) {
      return arr[i];
    } else {
      map[arr[i]] = i;
    }
    console.log(map);
  }
  return undefined;
}

console.log(firstReccuringChar([2, 5, 1, 2, 3, 5, 1, 2, 4]));

// Another example using Hash Tables or Objects.

edited Jan 8, 2020 at 16:02

answered Jan 8, 2020 at 15:56

kulotjunior07

11 bronze badge

1 Comment

Priyank Over a year ago

Hey, thanks for answering! Your answer will be more helpful if you described the code that you have written.

Elugens · Accepted Answer · 2020-10-09 16:58:55Z

0

function firstRecurringCharacter(input) {
  let result = {};
  let len = input.length;

  for (let i = 0; i < len; i++) {
    if (result[input[i]] !== undefined) {
      return input[i];
    } else {
      result[input[i]] = i;
    }
  }

  return undefined;
}

answered Oct 9, 2020 at 16:58

Elugens

223 bronze badges

3 Comments

Elugens Over a year ago

This is the correct answer to the problem. As stated, in the exact same answer above. This is obviously a question in a classroom or online course regarding hash tables. It should have been obvious by the posters question.

camille Over a year ago

I don't understand your comment. You said it's the exact same answer as another answer. Which one, and why post a new answer if they're the same? It would help for you to include some explanation as to how this solves the problem in ways that haven't been posted already

Darlan Dieterich Over a year ago

needed answer with explanation

the-tunwebdev · Accepted Answer · 2022-06-13 15:24:18Z

0

const firstReccuringChar = (arr) => {
    for (let i = 0; i < arr.length; i++) {
       for (let j = i + 1; j < array.length; j++) {
          if (arr[i] === arr[j]) {
            return arr[i];
         }
        }
     }
  return undefined;
  };

this is another solution but it's not the best one because it's O(n^2)

answered Jun 13, 2022 at 15:24

the-tunwebdev

1

Comments

EaglePrem · Accepted Answer · 2024-03-06 20:51:12Z

Since you want to log to console and also seems like you are attempting a naive solution, I presented my solution in the same manner. Time Complexity is O(n^2).

function firstRecurringCharacter(input) {
  let index = Infinity;
  let firstRecurringValue = 0;
  for (let i = 0; i < input.length; i++) {
    for (let j = i + 1; j < input.length; j++) {
      if (input[i] === input[j]) {
        // Edge Case: Selects lowest index repeated while traversing through the nested loop
        if (j < index) {
          index = j;
          firstRecurringValue = input[j];
        }
      }
    }
  }
  return firstRecurringValue !== 0
    ? console.log(firstRecurringValue)
    : console.log(undefined);
}
firstRecurringCharacter([2, 5, 1, 2, 3, 5, 1, 2, 4]); // returns 2
firstRecurringCharacter([2, 1, 1, 2, 3, 5, 1, 2, 4]); // returns 1
firstRecurringCharacter([2, 3, 4, 5]); // returns undefined

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