# Comparing Arrays **URL:** \ **Category:** Ruby\ **Created:** [April 5, 2006, 6:44pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905 "2006-04-05T18:44:05Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 6:44pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/1 "2006-04-05T18:44:05Z")
I have two arrays: originalList and deleteList. I’m trying to iterate through original list and remove each item if it appears in the deleteList. Simple enough. Here’s my snippet for doing this: originalList.each do |item| if deleteList.include?(item) puts "Deleting " + item originalList.delete(item) end end Unfortunately, this code doesn’t seem to be doing what I think it should. It seems to skip some items in originalList. I can provide examples upon request. Is there anything wrong with my algorithm? ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 6:58pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/2 "2006-04-05T18:58:34Z")
I would think that trying to remove items from the list you are enumerating is generally a bad thing… I don’t know enough about Ruby internals to know whether that applies to Ruby, but I generally avoid it. Might you want this: originalList = originalList - deleteList ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 6:58pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/3 "2006-04-05T18:58:40Z")
On Apr 5, 2006, at 11:44 AM, Nathan O. wrote: > I have two arrays: originalList and deleteList. Just FYI, the Ruby naming convention is to make those original\_list and delete\_list. > I’m trying to iterate > through original list and remove each item if it appears in the > deleteList. Simple enough. Sure is, try: original\_list - delete\_list 😉 James Edward G. II ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 7:29pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/4 "2006-04-05T19:29:10Z")
On 4/5/06, Nathan O. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Unfortunately, this code doesn’t seem to be doing what I think it > should. It seems to skip some items in originalList. I can provide > examples upon request. Is there anything wrong with my algorithm? I got bit by something similar this past weekend. It turns out that Array#delete deletes based on == comparisons (generally means two objects are from the same class and have the same instance variable values), not eql? comparisons (means same object) as I had guessed. That can be a significant difference. For one thing it allows the delete method to delete more than one object from the array. If you want to delete a specific object based on its identity, as far as I know you need to do it like this. obj\_to\_delete = whatever my\_array.delete\_if { |obj| obj.eql?(obj\_to\_delete) } ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 10:51pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/5 "2006-04-05T22:51:29Z")
On Apr 5, 2006, at 3:40 PM, Nathan O. wrote: > As an aside, is\_this the ruby naming convention, orThis? All the > references I’ve seen on the web useThis. Either is fine, I’m just > wondering now that it’s been brought up. vars\_and\_methods\_like\_this ClassesAndModulesLikeThis CONSTANTS\_GENERALLY\_LIKE\_THIS # some people use the class style here too nothingLikeThisJavaStyle # 😉 James Edward G. II ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 10:40pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/6 "2006-04-05T22:40:37Z")
Mark V. wrote: > I got bit by something similar this past weekend. It turns out that > Array#delete deletes based on == comparisons (generally means two > objects are from the same class and have the same instance variable > values), not eql? comparisons (means same object) as I had guessed. > That can be a significant difference. For one thing it allows the > delete method to delete more than one object from the array. If you > want to delete a specific object based on its identity, as far as I > know you need to do it like this. > > obj\_to\_delete = whatever > my\_array.delete\_if { |obj| obj.eql?(obj\_to\_delete) } That sounds like something not many people would assume, and therefor maybe something that might be reviewed before 2.0… but anyways! The list1 - list2 way of doing this worked wonders. Funny, I used this two weeks ago, the last time I hacked on this script, and had completely forgotten about it. Many thanks to everyone! As an aside, is\_this the ruby naming convention, orThis? All the references I’ve seen on the web useThis. Either is fine, I’m just wondering now that it’s been brought up. ---
**Author:** ![nko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/91b2a8/32.png) [@nko](https://www.ruby-forum.com/u/nko)\ **Post date:** [April 5, 2006, 11:08pm UTC](https://www.ruby-forum.com/t/comparing-arrays/56905/7 "2006-04-05T23:08:03Z")
James G. wrote: > vars\_and\_methods\_like\_this > > ClassesAndModulesLikeThis > > CONSTANTS\_GENERALLY\_LIKE\_THIS # some people use the class style here > too > > nothingLikeThisJavaStyle # 😉 > > James Edward G. II Seems you’re right! I was looking at documentation for 1.6. Everything I’m finding for 1.8 is\_this\_way. Wonderful. I’ll try to remember that for my _next_ project!