compare two arrays and return duplicate values

Question

How may I retrieve an element that exists in two different arrays of the same document.

For example. In Posts collection, document has the fields 'interestbycreator' and 'interestbyreader.' Each field contain user Ids.

'interestbycreator':  //an array of ids here. IdA, idB, IdC, IdD, IdE,
'interestbyreader':  //an array of ids here. IdB, idE, iDF

Basically I wish to find all the ids that exist in both arrays, so that should be IdB and IdE.

I am able to pluck all the values from an array with underscore and store them in a variable. Can they be compared to each other this way and return duplicates? Or can someone shed some light on another solution.

Example to retrieve all Ids from 'interestbyreader

var interestbypostcreater = Posts.find({_id: Meteor.user().profile.postcreated[0]}, {fields: {interestbyreader: 1}}).fetch();
var interestedReaderIds = _.chain(interestbypostcreator).pluck('interestbyreader').flatten().value();

Assume I have the other array 'interestbycreator' stored in a variable called interestIdcreator, can they be compared to find duplicates and return these duplicates?

Have you considered using _.intersection ? underscorejs.org/#intersection — saimeunt
– saimeunt, Commented Oct 13, 2014 at 15:16

Community · Accepted Answer · 2017-05-23 12:09:38Z

19

As saimeunt said in the comments when you have access to underscore use intersection but you can also do it with plain javascript:

var x = ['IdA', 'idB', 'IdC', 'IdD', 'IdE'];
var y = ['idB', 'IdE', 'IdF'];

var z = x.filter(function(val) {
  return y.indexOf(val) != -1;
});

console.log(z);

The array z contains the double entries then.

Credits to https://stackoverflow.com/a/14930567/441907

edited May 23, 2017 at 12:09

CommunityBot

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answered Oct 13, 2014 at 15:24

Nick Russler

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3 Comments

meteorBuzz Over a year ago

How can I get the 'return' results inside another variable? I want to preserve them as an array and not convert them so I can use the variable to query the database.Thank you for your help

Nick Russler Over a year ago

You have to deep copy the source array e.g. with slice() jsfiddle.net/94mjfhyh

Nick Russler Over a year ago

In exchange for memory this can be sped up by using a set for lookups.

meteorBuzz · Accepted Answer · 2014-10-29 16:15:17Z

11

As Saimeunt pointed out, it can be done as

var x = ['IdA', 'idB', 'IdC', 'IdD', 'IdE'];
var y = ['idB', 'IdE', 'IdF'];

var z = _.intersection(x, y);

answered Oct 29, 2014 at 16:15

meteorBuzz

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