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Re: Difference between two arrays

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Ben Bacarisse wrote:

> Andrew Poulos <ap_prog@hotmail.com> writes:
>> If I have two "simple" arrays and I need to create a third array of
>> elements that are only in one of the arrays. I found this
>>
>> Array.prototype.difference = function (a) {
>>   return this.filter(function (i) {
>>     return !(a.indexOf(i) > -1);
>>   });
>> };
>
> […]
> Anyway, the call constructs an array the includes only those elements of
> "this" that can't be found in "a".
> 
> Depending on lots of unknowns, it may be faster to invert the arrays.
> I.e. to show the presence of element x by setting a[x] to some specific
> value, rather than have some index i at which a[i] === x.

You can not simply invert arrays in ECMAScript implementations.  Arrays are 
objects, therefore keys are properties of an object.  Values whose string 
representation is the same as a property of the object can overwrite built-
in properties.
 
> This makes testing for membership fast, but at the expense of other
> things.  In particular, unless the implementation is clever, a very wide
> range of values can result is very large inverted arrays.

If the values can be unambiguously converted to String, using Object 
instances with empty prototype chain is recommended.  The property name 
would be the string representation of the value.  Otherwise, a Map 
implementation such as those in JSX:map.js could/should be used.
 
> The symmetric difference then looks like this:
> 
> Array.prototype.symDiff = function (a) {
>     var count = [];
>     this.forEach(function (e) { count[e] = 1; });
>     a.forEach(function (e) { count[e] = (count[e] || 0) | 2; });
>     var result = [];
>     count.forEach(function (e, i) { if (e !== 3) result.push(i); });

In most cases, you want to use a “for … in …” loop instead of 
Array.prototype.forEach(), particularly where speed is of the essence.  Not 
only can you save several function calls this way, you can also avoid 
unnecessary iterations with sparse arrays.  It should be tested before 
whether array indexes are enumerable (per ES and in recent implementations 
they are), and potentially augmented prototypes have to be considered.

Also, forEach() operates *only* on array indexes.  Therefore, the approach 
above will fail if values are not array indexes.

<http://stackoverflow.com/a/17000264/855543>

-- 
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Difference between two arrays Andrew Poulos <ap_prog@hotmail.com> - 2014-05-27 11:55 +1000
  Re: Difference between two arrays Denis McMahon <denismfmcmahon@gmail.com> - 2014-05-27 05:58 +0000
  Re: Difference between two arrays John C <rescattered@gmail.com> - 2014-05-27 03:34 -0700
    Re: Difference between two arrays Spamless <Spamless@Nil.nil> - 2014-05-31 06:35 -0500
      Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-31 14:57 +0200
        Re: Difference between two arrays Spamless <Spamless@Nil.nil> - 2014-06-01 03:43 -0500
          Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-06-01 14:01 +0200
            Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-06-01 16:53 +0200
  Re: Difference between two arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-05-27 13:41 +0100
    Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-27 17:41 +0200
    Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-27 17:49 +0200
      Re: Difference between two arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-05-27 17:34 +0100
        Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-27 19:33 +0200
          Re: Difference between two arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-05-27 20:03 +0100
    Re: Difference between two arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-05-27 17:16 +0100
      Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-27 19:29 +0200
  Re: Difference between two arrays Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-05-27 17:21 +0200
  Re: Difference between two arrays Dr J R Stockton <reply1400@merlyn.demon.co.uk.invalid> - 2014-05-28 18:22 +0100
    Re: Difference between two arrays Dr J R Stockton <reply1400@merlyn.demon.co.uk.invalid> - 2014-05-30 22:23 +0100
    Re: Difference between two arrays "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2014-05-31 14:59 -0700
      Re: Difference between two arrays "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-06-01 00:50 +0200
        Re: Difference between two arrays "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2014-05-31 16:17 -0700
          Re: Difference between two arrays "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-06-01 10:49 +0200
            Re: Difference between two arrays "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2014-06-01 13:37 -0700
      Re: Difference between two arrays Dr J R Stockton <reply1400@merlyn.demon.co.uk.invalid> - 2014-06-01 18:56 +0100

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