DEV Community

/[Abejide Femi Jr]\s/
/[Abejide Femi Jr]\s/

Posted on

Finding a single item in an array

#javascript #algorithms

If we have an array, and we want to find the item(s) that appears once in the array.

const arr = [1,1,2,2,4,5,4]
Enter fullscreen mode Exit fullscreen mode

The single item in the array above is 5, how can we solve this using JavaScript? We can use a hashmap aka Hash table, we need to convert the array to an object with the item as the key and their number of occurrence as the value.

const hashMap = {};
for (i of arr) {
  if (hashMap[i]) {
    hashMap[i]++;
  } else {
    hashMap[i] = 1;
  }
}
console.log(hashMap)
//
Enter fullscreen mode Exit fullscreen mode

We declared an empty object, and loop through the array, our array item is passed in as the key, and we check to see if this item appears in the array more than once, if it does, we increment the value, if it appears once we set the value to 1.

We get the result below

{ '1': 2, '2': 2, '4': 2, '5': 1 }
Enter fullscreen mode Exit fullscreen mode

As we can see 5 is the only number that appears once in the array, with a value of 1.

We need to loop through the object and get the keys with the value of 1, then push it into an array.

const single = []
for (i in hashMap) {
  if (hashMap[i] === 1) {
    single.push(Number(i));
  }
}
console.log(single)
Enter fullscreen mode Exit fullscreen mode

The single item in the array is a string, it is converted to a number as we push it into the array.

We get the value below.

[ 5 ]
Enter fullscreen mode Exit fullscreen mode

Thank you for reading.

Top comments (5)

Collapse
 
gilfewster profile image
Gil Fewster •

Great example of using a hash table to tally and track data.

If you're interested, you could also use the Array sort() and filter() methods to quickly extract unique elements.

  1. use the sort method to place all identical elements in adjacent indexes of a new array.

  2. Use filter to generate another array which only includes elements which are different to the elements immediately before and after them in the sorted array (and are therefore unique).

const animals = ["panther","cat","dog","monkey","cat"]
const nums = [1,2,3,4,1,1,2,6,7,2,4,5,9];

const getUniqueElements = (arr) => {
  const sorted = arr.sort();
  return sorted.filter((item, index) => {
    return (item != sorted[index-1]) && (item != sorted[index+1])
  });
}


console.log(getUniqueElements(animals)); // ['dog' ,'monkey', 'panther']
console.log(getUniqueElements(nums)); // [3, 5, 6, 7, 9]
Enter fullscreen mode Exit fullscreen mode
Collapse
 
gilfewster profile image
Gil Fewster •

Postscript

Removing duplicate items from your array
EG convert [1,2,2,1,4,8,8,9] to [1,2,4,8,9]

Create a new set from your original array and then spread the set into a new array.

const animals = ["cat","dog","monkey","cat"]
const nums = [1,7,2,4,1,1,2,7,2,4];

const uniqueAnimals = [...new Set(animals)]; // ["cat","dog","monkey"]
const uniqueNums = [...new Set(nums)]; // [1,7,2,4]
Enter fullscreen mode Exit fullscreen mode
Collapse
 
shaileshcodes profile image
Shailesh Vasandani •

Nice post! The great benefit to using a HashMap in this way is that the entire operation takes O(n) time, whereas sorting would usually take something like O(n log n) time.

You could probably also condense it to a one-liner (or maybe two-liner) and keep the O(n) property by doing something like this:

const arr = [1,1,2,2,4,5,4];
const singles = [];
const map = {};

arr.forEach(el => map[el] ? map[el]++ : map[el] = 1);
Object.keys(map).forEach((k) => map[k] == 1 ? singles.push(k) : null);
Enter fullscreen mode Exit fullscreen mode

Thanks for sharing!

Collapse
 
sroehrl profile image
neoan •

I am usually not a fan of ugly one liners, but if you want it concise:

const arr = [1,1,2,2,4,5,4]
const singles = [];
arr.forEach(el => arr.filter(el2 => el == el2).length == 1) ? singles.push(el) : null)
Enter fullscreen mode Exit fullscreen mode
Collapse
 
farhajhussain44 profile image
farhajhussain44 •

const arr = [1,1,2,2,4,5,4];
let singles=[];
arr.forEach((el,index,array)=>{
let single=array.filter(element => element === el);
if(single.length === 1){
singles.push(single[0])
}
});

Read next

alfredosalzillo profile image

The React `useInsertionEffect` Hook

Alfredo Salzillo -

priyankdeep78 profile image

The 5 Best Design Tools in 2024⚡️

Priyank Deep Singh -

mana95 profile image

Understanding about React Outlet with proper routing

Manoj Prasanna 🚀 -

adaobilynda profile image

useReducer and how it is different from useState

Adaobi Okwuosa -