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Cover image for 1 line of code: How to split an Array in half
Martin Krause
Martin Krause

Posted on • Edited on

1 line of code: How to split an Array in half

#javascript #webdev #performance #codequality 
const splitInHalf = arr => [arr.slice(0, Math.ceil(arr.length / 2)), arr.slice(Math.ceil(arr.length / 2))];
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Returns an new array where the first entry is the "first" and the second entry is the "second" half of the original array.

The repository & npm package

You can find the all the utility functions from this series at github.com/martinkr/onelinecode
The library is also published to npm as @onelinecode for your convenience.

The code and the npm package will be updated every time I publish a new article.

Follow me on Twitter: @martinkr and consider to buy me a coffee

Photo by zoo_monkey on Unsplash

Top comments (17)

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jonrandy profile image
Jon Randy 🎖️ •

Probably slower, but:

const splitInHalf = s=>s.match(new RegExp('.{1,'+~~((s.length+1)/2)+ '}','g'))
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martinkr profile image
Martin Krause •

Thank you for your contribution. I was going test the performance and compare both code snippets but upon closer inspection I think yours is only working on strings not array. Can you take a closer look?

Thank you

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jonrandy profile image
Jon Randy 🎖️ • • Edited

Here's an array one:

const splitInHalf = a=>[a,a.splice(~~((a.length+1)/2))]
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martinkr profile image
Martin Krause •

Checked the benchmark (jsbench.github.io/#44d4426b04ed292...) and yours is way faster.
I updated the code and the article!
Thank you

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jonrandy profile image
Jon Randy 🎖️ •

Careful with this one though - mutation

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jackmellis profile image
Jack •

Good spot. Definitely avoid this method unless you love bugs and impure functions 😅

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jonrandy profile image
Jon Randy 🎖️ • • Edited

Non-mutating version - slower though:

const splitInHalf = (a,b=[])=>[b=[...a],b.splice(b.length+1>>1)]
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Speed comparision of all 3 methods.

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martinkr profile image
Martin Krause •

I will stick with the original version.
If you find a faster one without mutations we can use it.
Cheers!

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jonrandy profile image
Jon Randy 🎖️ •

Ah, I misread - thought you were doing strings

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martinkr profile image
Martin Krause •

No Problem, strings will be next month ;)
Cheers!

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frankwisniewski profile image
Frank Wisniewski •

I don't see any use case

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jamesthomson profile image
James Thomson •

You have view with dynamic columns. You want to display all the data within the array evenly between these columns. As columns are added/removed you need to split the array.

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frankwisniewski profile image
Frank Wisniewski • • Edited

In my opinion, the alignment of columns is done in CSS.

<!DOCTYPE html>
<html lang="de">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    .myList{
     column-count: 2; 
     column-width: 50%;
     column-rule-style:dotted; 
     column-rule-width:1px; 
    }
    .myList p{
      display:inline-block;
      width:100%;
    }
  </style>
</head>
<body>
<div class="myList" id=myContainer></div>
<script>
    let myArray=[1,1,5,3,4,5,6,7,8,1,12,13,15,16]
    let myHTML = ''
    myArray.map(el => myHTML += '<p>'+el+'</p>')
    myContainer.innerHTML=myHTML         
</script>
</body>
</html>
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jamesthomson profile image
James Thomson •

Yes, you can definitely use css columns, but sometimes you just need things or flexible. Maybe it's not a column, but a grid. Or maybe you start with columns, but can divide them into grids (like in VSCode).

Another use case could be splitting up players into teams. If you have a group of players and you need to make equal teams, then you'll need to split that array.

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frankwisniewski profile image
Frank Wisniewski •

far fetched...but OK, enough words for such an simple function....

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jamesthomson profile image
James Thomson •

You said you didn't see any use cases, I'm merely presenting them to you. Do what you will with it.

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darkwiiplayer profile image
𒎏Wii 🏳️‍⚧️ • • Edited 
const split = num => ([...arr]) => Array(num).fill(0)
   .map((_,i)=>Math.floor(arr.length/num)+(i<arr.length%num))
   .map(n => arr.splice(0, n))
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