From: "Guy N. Hurst" <gnhurst@...>
Date: 2001-05-11T16:58:31+09:00
Subject: [ruby-talk:14983] Re: Array search idiom


"James B. Crigler" wrote:
> 
> Here's a question about a ruby idiom.  I have an array that contains
> lines with mammal (classification. animal) pairs like the one found
> in the code snippet.  I need to find the first contiguous list of
> primates.  Assuming the array is called "a", here's how I solved the
> problem.  This does what I want, but is there a better way?
> 


Hmm. What if you would like the second contiguous list of primates?
Or perhaps the Nth list? What about for the others, like insectivora?

class Array
  def foo(re)
     inject([[]]){|x,i| if i=~re  then x[-1]<<i; x  else x[-1]==[] or x<<[]; x  end}
  end
end

a.foo(/^primates\./)[0]
#=> ["primates. tree shrews", "primates. lemurs", "primates. monkeys"]


Idea
====
This looks to me like it could be a problem of finding the Nth 
contiguous list meeting any condition -- and allowing a parameter
to set a threshhold for the list size, since a single item might not
count as consecutive in some cases (e.g. primes).

For a name, I think 'comb' is descriptive.


Ruby Code
=========
class Array
  def comb(min=2,&b)
    q=inject([[]]){|x,i| if b.call(i)  then x[-1]<<i; x;  else x<<[]; x  end}
    q.select{|i| i.size>=min }
  end
end

a.comb{|i| i=~/^primates\./}[0]	
#=> ["primates. tree shrews", "primates. lemurs", "primates. monkeys"]

a.comb{|i| i=~"^insectivora"}[0]
#=> ["insectivora. shrews", "insectivora. moles", "insectivora. hedgehogs"]


Explanation
===========
I also have a few other versions, but this seems like the most powerful and concise.
This particular one starts with an empty group, and fills it up with
consecutive matches. As soon as there is no match, a second empty group
is added to the container. Future matches are always added to the last group
in the container. (Then all groups containing at least the minimum number of items
are selected and returned.)

I make use of the Array#inject method as shown on page 45 of the pickaxe book:

class Array 
  def inject(n)
    each{|v| n = yield(n,v)}
    n
  end
end 


Other applications
==================
For the math enthusiasts, this can be used to find 'consecutive' primes:

odds=[3,5,7,9,11,13,15,17,19,21,23,25,27,29]

odds.comb(2){|i| i.prime? }
#=> [[3, 5, 7], [11, 13], [17, 19]]

odds.comb(3){|i| i.prime? }
#=> [[3, 5, 7]]


  
Guy N. Hurst

-- 
HurstLinks Web Development    http://www.hurstlinks.com/
Norfolk, VA  23510            (757)623-9688 FAX 623-0433
PHP/MySQL - Ruby/Perl - HTML/Javascript