From: Stefano Crocco <stefano.crocco@...>
Date: 2008-08-01T21:50:50+09:00
Subject: Re: Using array.select with grep

On Friday 01 August 2008, Milo Thurston wrote:
> 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?
>
> Before anyone asks, I'm not using  the array intersecttion (arr1 & arr2)
> because I am testing this expression prior to using it to find objects
> in an array (arr1) where an object parameter is found in an array of
> strings (arr2), e.g. to find all people from the array of people objects
> whose first name is in a list of names:
>
> arr1.select { |y| arr2.grep(y.string_of_interest) }
>
> Although if there's a better way of doing that I'd be interested to
> know.

Array#grep returns an array with the matches, which means it returns an empty 
array, which evaluates to true, if there's no match. You can do what you want 
with

arr1.select{|y| !arr2.grep(y).empty?}

I hope this helps

Stefano