From: "David A. Black" <dblack@...>
Date: 2008-08-01T22:01:01+09:00
Subject: Re: Using array.select with grep

Hi --

On Fri, 1 Aug 2008, Stefan Lang wrote:

> 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) }

No, that's not the same.

>> array = %w{ one two three }
=> ["one", "two", "three"]
>> array.grep(/e/)
=> ["one", "three"]
>> array.include?("e")
=> false


David

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