freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/data-structures/perform-a-difference-on-two-sets-of-data.english.md

---
id: 587d8254367417b2b2512c6e
title: Perform a Difference on Two Sets of Data
challengeType: 1
forumTopicId: 301706
---

## Description
<section id='description'>
In this exercise we are going to perform a difference on 2 sets of data. We will create a method on our <code>Set</code> data structure called <code>difference</code>. A difference of sets should compare two sets and return the items present in the first set that are absent in the second. This method should take another <code>Set</code> as an argument and return the <code>difference</code> of the two sets.
For example, if <code>setA = ['a','b','c']</code> and <code>setB = ['a','b','d','e']</code>, then the difference of setA and setB is: <code>setA.difference(setB) = ['c']</code>.
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: Your <code>Set</code> class should have a <code>difference</code> method.
    testString: assert((function(){var test = new Set(); return (typeof test.difference === 'function')})());
  - text: Your <code>difference</code> method should return the proper collection.
    testString: assert((function(){var setA = new Set(); var setB = new Set(); setA.add('a'); setA.add('b'); setA.add('c'); setB.add('c'); setB.add('d'); var differenceSetAB = setA.difference(setB); return (differenceSetAB.size() === 2) && DeepEqual(differenceSetAB.values(), [ 'a', 'b' ])})());

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
class Set {
  constructor() {
    // This will hold the set
    this.dictionary = {};
    this.length = 0;
  }
  // this method will check for the presence of an element and return true or false
  has(element) {
    return this.dictionary[element] !== undefined;
  }
  // this method will return all the values in the set
  values() {
    return Object.keys(this.dictionary);
  }
  // this method will add an element to the set
  add(element) {
    if (!this.has(element)) {
      this.dictionary[element] = true;
      this.length++;
      return true;
    }

    return false;
  }
  // this method will remove an element from a set
  remove(element) {
    if (this.has(element)) {
      delete this.dictionary[element];
      this.length--;
      return true;
    }

    return false;
  }
  // this method will return the size of the set
  size() {
    return this.length;
  }
  // This is our union method from that lesson
  union(set) {
    const newSet = new Set();
    this.values().forEach(value => {
      newSet.add(value);
    })
    set.values().forEach(value => {
      newSet.add(value);
    })

    return newSet;
  }
  // This is our intersection method from that lesson
  intersection(set) {
    const newSet = new Set();

    let largeSet;
    let smallSet;
    if (this.dictionary.length > set.length) {
      largeSet = this;
      smallSet = set;
    } else {
      largeSet = set;
      smallSet = this;
    }

    smallSet.values().forEach(value => {
      if (largeSet.dictionary[value]) {
        newSet.add(value);
      }
    })

    return newSet;
  }
  // change code below this line
  // change code above this line
}
```

</div>
</section>

## Solution
<section id='solution'>

```js
class Set {
  constructor() {
    this.dictionary = {};
    this.length = 0;
  }

  has(element) {
    return this.dictionary[element] !== undefined;
  }

  values() {
    return Object.keys(this.dictionary);
  }

  add(element) {
    if (!this.has(element)) {
      this.dictionary[element] = true;
      this.length++;
      return true;
    }

    return false;
  }

  remove(element) {
    if (this.has(element)) {
      delete this.dictionary[element];
      this.length--;
      return true;
    }

    return false;
  }

  size() {
    return this.length;
  }

  union(set) {
    const newSet = new Set();
    this.values().forEach(value => {
      newSet.add(value);
    })
    set.values().forEach(value => {
      newSet.add(value);
    })

    return newSet;
  }

  intersection(set) {
    const newSet = new Set();

    let largeSet;
    let smallSet;
    if (this.dictionary.length > set.length) {
      largeSet = this;
      smallSet = set;
    } else {
      largeSet = set;
      smallSet = this;
    }

    smallSet.values().forEach(value => {
      if (largeSet.dictionary[value]) {
        newSet.add(value);
      }
    })

    return newSet;
  }

  difference(set) {
    const newSet = new Set();

    this.values().forEach(value => {
      if (!set.dictionary[value]) {
        newSet.add(value);
      }
    })

    return newSet;
  }
}
```

</section>