From: Joseph McDonald <joe@...>
Date: 2002-07-07T00:02:58+09:00
Subject: Re: fast "set difference" on arrays


>> when I want to take the "set difference", arr3 = arr1-arr2, meaning "take
>> all elements from arr1 which dont appear in arr2" this takes forever - I
>> don't even know how long because I stopped early ;)

DT> The following runs in about .5s on my pokey old box:

DT>    s1 = {}
DT>    File.foreach(ARGV[1]) {|line| s1[line] = 1}
DT>    File.foreach(ARGV[0]) {|line| puts(line) unless s1[line]}

DT> Note that it's doing String comparisons, not integer ones, but if both
DT> of your files are generated the same way that won't be a problem.

Do you think Array#- should do the hash trick internally instead of a
complete scan x*y times ?  I do.

thanks,
-joe