# Array comparison.. why does this work **URL:** \ **Category:** Ruby\ **Created:** [March 1, 2007, 3:45am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244 "2007-03-01T03:45:40Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 3:45am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/1 "2007-03-01T03:45:40Z")
How does Ruby do Array comparision? ```auto old_inventory = ['a', 'b', 'd'] new_inventory = ['a','e', 'c'] puts "The following files have been added: " puts new_inventory - old_inventory puts "" puts "The following files have been deleted: " puts old_inventory - new_inventory ``` Notice the output is correct so it’s not a case of element 0’s being compared in each array. So… Does Ruby sort arrays in memory first? or is a scan done 1 element at a time? Thanks. ---
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 3:58am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/2 "2007-03-01T03:58:05Z")
Hi – On Thu, 1 Mar 2007, Jean N. wrote: > How does Ruby do Array comparision? One element at a time. > Notice the output is correct so it’s not a case of element 0’s being > compared in each array. > > So… > > Does Ruby sort arrays in memory first? or is a scan done 1 element at a > time? You’re not doing comparison, though; you’re doing subtraction. Subtraction doesn’t depend on order. You can think of it as “without”: [1,2,3] - [1] =\> [2,3] # [1,2,3] without 1 [1,2,3,1] - [1] =\> [2,3] # [1,2,3,1] without 1 [1,2,3] - [2] =\> [1,3] # [1,2,3] without 2 etc. David ---
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 5:21am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/3 "2007-03-01T05:21:18Z")
> You’re not doing comparison, though; you’re doing subtraction. > Subtraction doesn’t depend on order. You can think of it as > “without”: > > [1,2,3] - [1] =\> [2,3] # [1,2,3] without 1 > [1,2,3,1] - [1] =\> [2,3] # [1,2,3,1] without 1 > [1,2,3] - [2] =\> [1,3] # [1,2,3] without 2 > > etc. > > David Sorry, when I mentioned comparison I meant “Under the hood”. (I realise ‘-’ is subtractions ), but the interpreter needs to perform comparisoin to see if an element exists somewhere. I was curious if it just interates through one array looking for that element in the second, or does it (internally) sort the array, iterate each array performing comparision and if a \> b skipping the equality determination to save CPU cycles and doing a ‘next’. I guess what I"m saying (Makes more sense when you see pseudo code) is how does it work in the interpreter. ---
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 6:34am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/4 "2007-03-01T06:34:16Z")
> I was curious if it just interates through one array looking for that > element in the second, or does it (internally) sort the > array, iterate > each array performing comparision and if a \> b skipping the equality > determination to save CPU cycles and doing a ‘next’. I would guess that it would just do an include? call on the second array and delete the element if it is true. Doing the sort first would probably not be faster - a sort operation is not a cheap operation, and there is no guarantee that the members of an array are mutually sortable anyway ([“a”, 1, Object.new] for example) - there is no way in which the ‘-’ method would be able to rely on calling the ‘\<=\>’ method on its contents. Dan. ---
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 7:04am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/5 "2007-03-01T07:04:19Z")
Daniel S. wrote: > Doing the sort first would probably not be faster - a sort operation is > not a cheap operation, and there is no guarantee that the members of an > array are mutually sortable anyway ([“a”, 1, Object.new] for example) - > there is no way in which the ‘-’ method would be able to rely on calling > the ‘\<=\>’ method on its contents. > > Dan. Don’t forget, you can always just look at the source[1]. It looks to me like it converts the second array to a hash, then iterates through the first array, checking if each element is in the newly created hash. If it is, it skips it. If not, the value goes in the new array. -Justin [1] [http://ruby-doc.org/core/classes/Array.src/M002261.html](http://ruby-doc.org/core/classes/Array.src/M002261.html) ---
**Author:** ![jeannibee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/85e7bf/32.png) [@jeannibee](https://www.ruby-forum.com/u/jeannibee)\ **Post date:** [March 1, 2007, 7:30am UTC](https://www.ruby-forum.com/t/array-comparison-why-does-this-work/87244/6 "2007-03-01T07:30:32Z")
> Don’t forget, you can always just look at the source[1]. > It looks to me like it converts the second array to a hash, then > iterates through the first array, checking if each element is in the > newly created hash. > If it is, it skips it. If not, the value goes in the new array. > > -Justin > > [1] [http://ruby-doc.org/core/classes/Array.src/M002261.html](http://ruby-doc.org/core/classes/Array.src/M002261.html) Awesome I didn’t now where to find the sources to read them. Thanks!