From: Stefan Lang <perfectly.normal.hacker@...>
Date: 2008-08-01T21:57:10+09:00
Subject: Re: Using array.select with grep

2008/8/1 Milo Thurston <knirirr@gmail.com>:
> Using irb I set up the following arrays:
>
>>> arr1
> => ["one", "two", "three"]
>>> arr2
> => ["two", "three", "four"]
>
> I would expect to be able to collect the elements of arr1 that are also
> in arr2 using select and grep, i.e.
>
>>> arr1.select { |y| arr2.grep(y) }
> => ["two", "three"]
>
> But what I actually get is:
>
>>> arr1.select { |y| arr2.grep(y) }
> => ["one", "two", "three"]
>
> What is my error or misunderstanding here?

Because grep returns an empty array when it can't find the
given string. And an empty array (everything except false and nil,
actually) is true in Ruby. Use #include? instead:

  arr1.select { |y| arr2.include?(y) }

Stefan