From: "David A. Black" <dblack@...>
Date: 2010-10-11T19:15:00+09:00
Subject: Re: Testing for subarrays

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Hi --

On Mon, 11 Oct 2010, Jes�s Gabriel y Gal�n wrote:

> On Mon, Oct 11, 2010 at 11:27 AM, Michel Demazure <michel@demazure.com> wrote:
>> I want to check whether a given array contains a given subarray and
>> return the corresponding range.
>> Is there a builtin method ? If not, do you have better than this :
>>
>> class Array
>> �def look_up(sub_array)
>> � �len = sub_array.size
>> � �self.each_cons(len).with_index do |cons, index|
>> � � �return Range.new(index, index + len - 1) if cons == sub_array
>> � �end
>> � �nil
>> �end
>> end
>
> Not necessarily better:
>
> irb(main):001:0> a = [1,2,3,4,5,6,7,8]
> => [1, 2, 3, 4, 5, 6, 7, 8]
> irb(main):002:0> b = [4,5,6]
> => [4, 5, 6]
> irb(main):004:0> a.each_cons(b.size).to_a.index(b)
> => 3
> irb(main):005:0> a.each_cons(b.size).to_a.index([5,1,2])
> => nil

Here's another way that is more verbose but that doesn't create the
whole array (which probably doesn't matter anyway, unless it's a huge
array and you're cycle-shaving). I also prefer extending individual
objects, rather than adding to Array.

module SubArrayFinder
   def look_up(sub_array)
     sub_size = sub_array.size
     index = find_index.with_index do |e,i|
       self[i, sub_size] == sub_array
     end
     return(index..index+sub_size-1) if index
   end
end


David

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

   The                   Ruby training with Black/Brown/McAnally
   Compleat              Philadelphia, PA, October 1-2, 2010
   Rubyist               http://www.compleatrubyist.com
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