Filter array by array of indexes

Question

I have a first array of values and a second array of indexes. I would like to filter the first array and return only values that have indexes in the second array.

So, given:

arr1 = [4775453877338112, 5901353784180736, 6605041225957376]
arr2 = [0,2]

I would like to return:

output = [4775453877338112, 6605041225957376]

Thanks. For bonus points, why doesn't the following work?

var output = arr1.filter( (item) => arr1.indexOf(item) in arr2 === true )

Apologies if this simple question is a duplicate of this: Filter array based on an array of index, but the underscore is throwing me off.

var output = arr1.filter( (item) => arr2.indexOf(arr1.indexOf(item)) >= 0 ) will work Seems most Effective — Vishal Sharma
– Vishal Sharma, Commented Feb 6, 2018 at 13:10

gurvinder372 · Accepted Answer · 2018-02-06 13:07:56Z

11

For bonus points, why doesn't the following work?

Because you are checking if the index is present in arr2, not the item itself.

Just simply use a map

var output = arr2.map( s => arr1[s] );

Demo

var arr1 = [4775453877338112, 5901353784180736, 6605041225957376];
var arr2 = [0,2];
var output = arr2.map( s => arr1[s] );
console.log( output );

answered Feb 6, 2018 at 13:07

gurvinder372

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2 Comments

rvictordelta Over a year ago

Ah, the index, not the integer representing the index... Explains the weird results I was seeing. Thank you

Patrick Roberts Over a year ago

If it's helpful to anyone, .map() accepts a second argument for the context of the callback, so the solution could be rewritten like this: var output = arr2.map(function (s) { return this[s]; }, arr1); if you want to define a reusable function rather than an anonymous function expression.

Yury Tarabanko · Accepted Answer · 2018-02-06 13:08:18Z

4

var output = arr1.filter( (item) => arr1.indexOf(item) in arr2 === true ) indexOf returns index of element not a boolean. You could use new Array.prototype.includes or test for value to be -1

Also if order is the same you could loop over index array instead.

const arr1 = [4775453877338112, 5901353784180736, 6605041225957376]
const arr2 = [0,2]

const output = arr2.map(i => arr1[i])

console.log(output)

answered Feb 6, 2018 at 13:08

Yury Tarabanko

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Comments

Manuel Mannhardt · Accepted Answer · 2018-02-06 13:09:43Z

3

A simple loop might be all you need:

arr1 = [4775453877338112, 5901353784180736, 6605041225957376];
arr2 = [0,2];
arr3 = [];

for (i = 0; i < arr2.length; i++) {
    arr3.push(arr1[arr2[i]]);
}

Result:

Array [ 4775453877338112, 6605041225957376 ]

edited Feb 6, 2018 at 13:09

answered Feb 6, 2018 at 13:07

Manuel Mannhardt

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2 Comments

flx Over a year ago

well. guess we had the same intention

Manuel Mannhardt Over a year ago

Sure, its the most simple and intentional approach ^^

Boris Lobanov · Accepted Answer · 2018-02-06 13:12:46Z

2

Here's a solution without using an extra empty array and without nesting of the for loop. Also works in O(n) rather than O(n^2) suggested by some people here.

var arr1 = [4775453877338112, 5901353784180736, 6605041225957376];
var arr2 = [0,2];

var result = arr2.map(i => arr1[i]);

answered Feb 6, 2018 at 13:12

Boris Lobanov

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Comments

flx · Accepted Answer · 2018-02-06 13:07:35Z

1

Your task is easier when writing a readable code instead of complex code:

var arr1 = [4775453877338112, 5901353784180736, 6605041225957376];
var arr2 = [0,2];
var output = [];

for (var i = 0; i < arr2.length; i++) {
  output.push(arr1[ arr2[i] ]);
}

console.log(output);

answered Feb 6, 2018 at 13:07

flx

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Comments

curlyBraces · Accepted Answer · 2018-02-06 13:07:17Z

0

Try the following: `

let arr1 = [4775453877338112, 5901353784180736, 6605041225957376]
let arr2 = [0,2]

let result = arr1.filter(i => arr2.includes(arr1.indexOf(i)))
console.log(result);

`

answered Feb 6, 2018 at 13:07

curlyBraces

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1 Comment

STEEL Over a year ago

I like it :D (nice answer there)

Paritosh Mahale · Accepted Answer · 2018-02-06 13:10:37Z

0

Try this

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
<script src="https://code.jquery.com/jquery-3.1.0.js"></script>
<script>
  $("document").ready(function(){
    arr1 = [4775453877338112, 5901353784180736, 6605041225957376]
arr2 = [0,2]
$.each( arr2, function( key, value ) {
  alert( arr1[value]);
});
    
  })
</script>  
</body>
</html>

answered Feb 6, 2018 at 13:10

Paritosh Mahale

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1 Comment

flx Over a year ago

having to use a whole libary for that task might be a little bit overload

Tirth Shah · Accepted Answer · 2018-02-06 13:11:49Z

0

Without using lodash this is a way to solve it. Do not use arr.forEach, it is slower than for.

const arr1 = [4775453877338112, 5901353784180736, 6605041225957376]
const arr2 = [0,2]
const finalArray = [];
let j = 0;
for (let i = 0, len = arr1.length; i < len; i++) {
  if(i==arr2[j]) {
    j++;
    finalArray.push(arr1[i]);
  }
}

answered Feb 6, 2018 at 13:11

Tirth Shah

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1 Comment

Manuel Mannhardt Over a year ago

You are overcomplicating it, just loop over the array with the requested indizies ^^

mickl · Accepted Answer · 2018-02-06 16:59:23Z

0

Modify the output variable as follows:

var output = arr1.filter((item) => arr2.includes(arr1.indexOf(item)));

edited Feb 6, 2018 at 16:59

mickl

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answered Feb 6, 2018 at 13:18

Mohini Borhade

1

Comments

user9322204 · Accepted Answer · 2018-02-06 13:40:45Z

-1

To be more specific irrespective of type of value, use reflection concept.

import java.lang.reflect.Array; . . .

.

. int[] arr1 = {1, 3, 7}; int[] arr2 = {0,2};

for (var i = 0; i < arr2.length; i++) { Object o=Array.get(arr1, arr2[i]);

System.out.println(o);

}

Output : 1 7

answered Feb 6, 2018 at 13:40

user9322204

1

2 Comments

Claudia Over a year ago

Welcome to Stack Overflow! Take a moment to read through the editing help in the help center. Formatting on Stack Overflow is different than other sites.

Patrick Roberts Over a year ago

This question is for JavaScript, not Java.

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