How to get values from one array based on another array?

Question

So, I am trying to figure out a situation where I would populate an array (b[]) with the index numbers from another array (a[]) whose elements meet a certain criteria (array b would be index numbers based on array a which is an array of images, and would populate b when width is greater than height).

So, I came up with a hypothetical function in hopes of getting an array output where I would get a listing from a[] that align with values from b[]. Needless to say, neither attempt came up with anything of value.

var a = ['ab', 'bc', 'cd', 'de', 'ef', 'fg', 'gh', 'hi'];
var b = [2, 4, 5];


var d = function(a, b) {
  var func1 = [];
  var func2 = [];

  for(i = 0; i > b.length; i++) {
    func1.push(a[b[i]]);
  }

  console.log('func1 = ' + func1); // 'func1 = []'

  for(i=0; i > a.length; i++) {
    if(b.indexOf(a.indexOf(a[i])) > -1) {
      func2.push(a[i])
    }
  }
  console.log('func2 = ' + func2); // 'func2 = []'
}


d(a,b) // Ideally outputs ['cd', 'ef', 'fg']

Is this a matter of function scope, or am I missing the point of .push?

use "<" instead of ">" in your for loops

Superdrac
– Superdrac

2016-08-04 17:11:55 +00:00
Commented Aug 4, 2016 at 17:11 — Superdrac
– Superdrac, Commented Aug 4, 2016 at 17:11
That was awfully derpy of me. Thanks!

ms_nitrogen
– ms_nitrogen

2016-08-04 17:14:21 +00:00
Commented Aug 4, 2016 at 17:14 — ms_nitrogen
– ms_nitrogen, Commented Aug 4, 2016 at 17:14

Runner · Accepted Answer · 2016-08-04 17:17:45Z

1

The comparisons in your for loops are backwards. They should be like this:

for(i = 0; i < b.length; i++) {
    func1.push(a[b[i]]);
  }

  console.log('func1 = ' + func1); // 'func1 = []'

  for(i=0; i < a.length; i++) {
    if(b.index(a.indexOf(a[i])) > 1) {
      func2.push(a[i])
    }
  }

Also, b.index is not a function. I assume you meant indexOf:

var a = ['ab', 'bc', 'cd', 'de', 'ef', 'fg', 'gh', 'hi'];
var b = [2, 4, 5];


var d = function(a, b) {
  var func1 = [];
  var func2 = [];

  for(i = 0; i < b.length; i++) {
    func1.push(a[b[i]]);
  }

  console.log('func1 = ' + func1); // 'func1 = []'

  for(i=0; i < a.length; i++) {
    if(b.indexOf(a.indexOf(a[i])) > 1) {
      func2.push(a[i])
    }
  }
  console.log('func2 = ' + func2); // 'func2 = []'
}


d(a,b) // Ideally outputs ['cd', 'ef', 'fg']

This outputs:

func1 = cd,ef,fg
func2 = fg

answered Aug 4, 2016 at 17:17

Runner

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1 Comment

ms_nitrogen Over a year ago

It was meant to be if(b.indexOf(a.indexOf(a[i])) > -1). In terms of efficiency, is func1 better than func2 since it has less loops to go through?

ms_nitrogen · Accepted Answer · 2016-08-04 17:16:50Z

0

use for(i=0; i < a.length; i++) instead! Proper syntax is the key to success! Thanks @Superdrac

answered Aug 4, 2016 at 17:16

ms_nitrogen

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