How can I split an array into two arrays with alternating elements

Question

I want to split an array of strings into two arrays. However, when I push the strings into the new arrays, it should be alternating. So, if the array is:

let alph = [a,b,c,d,e,f]

Then the new arrays would look like:

firstArr = [a,c,e]
secondArr = [b,d,f]

How can I do it so I'm not repeating myself? I have the following code, and it works, but I do not want to write two of the same filter functions (keep things DRY):

let firstArr = alph.filter((letter, index) => {
  return index % 2 === 0;
})

Nina Scholz · Accepted Answer · 2018-06-28 20:08:22Z

13

You could take an array of the both arrays and take the index as indicator for the wanted array for pushing.

let alph = ['a', 'b', 'c', 'd', 'e', 'f'],
    first = [],
    second = [],
    temp = [first, second];
    
alph.forEach((v, i) => temp[i % 2].push(v));

console.log(first);
console.log(second);

answered Jun 28, 2018 at 20:08

Nina Scholz

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Comments

Asons · Accepted Answer · 2018-06-28 20:16:49Z

2

Since filter creates one array, you need two, or use e.g. forEach

var arr = ["a","b","c","d","e","f"], firstArr = [], secondArr = [];

arr.forEach( (a,i) => {
  (i % 2 === 0) ? firstArr.push(a) : secondArr.push(a);
})

console.log(firstArr)
console.log(secondArr)

edited Jun 28, 2018 at 20:16

answered Jun 28, 2018 at 20:11

Asons

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Comments

Ashley · Accepted Answer · 2018-06-28 20:27:00Z

1

For better readability there's nothing wrong with having separate filter functions for these. To clean it up a little you could use arrow functions and make them 1 liners and then pass them in the filter function, like:

const alpha = ['a', 'b', 'c', 'd', 'e', 'f'];
const filterByEvens = (letter, index) => index % 2 === 0;
const filterByOdds = (letter, index) => index % 2 !== 0;
const evens = alpha.filter(filterByEvens);
const odds = alpha.filter(filterByOdds);

answered Jun 28, 2018 at 20:27

Ashley

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Comments

Taki · Accepted Answer · 2018-06-28 21:10:29Z

0

you can use reduce for this :

const alph = ['a', 'b', 'c', 'd', 'e', 'f'];

const result = alph.reduce((acc, letter, ndx) => {
  acc[ndx % 2] = acc[ndx % 2] || [];
  acc[ndx % 2].push(letter);
  return acc;
}, []);

const [firstArr, secondArr] = result;

console.log(firstArr, secondArr);

answered Jun 28, 2018 at 21:10

Taki

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