From: Paul Brannan <paul@...>
Date: 2002-01-31T06:30:43+09:00
Subject: Re: test for presence in array

On Thu, Jan 31, 2002 at 05:44:29AM +0900, Kamil Kukura wrote:
> I ran into typical situation where I needed to test number x against a
> couple of values. What entered my mind is:
> 
> [10, 20, 100, 500].include?(x)
> 
> I like it but I think I would prefer to have x.in? [10, 20, 100, 500]
> I guess it could go into Comparable module. Or is there better /nicer/ way?

If you already have an array, then your solution works, but is an O(n)
operation.  If you are concerned about speed, then you can use a hash:

  if {1=>true, 2=>true, 3=>true, 4=>true}.include?(x) then
    # do stuff
  end

Not the nicest syntax in the world, but it works.  There are also a
number of nice Set classes out there that do the same thing with a
cleaner syntax.

As far as your question goes, I don't think it really makes sense for a
contained item to know about a container.  That is, a Fixnum should not
have to have any knowledge of the Array in which it is stored.  x.in?(y)
could call y.include?(x), but this assumes a specific interface for y
(that is, it assumes y has a method called include?).  In the end, both
are equally readable, and the solution Ruby uses now is slightly less
complex.

Paul