From: Jim Weirich <jim@...>
Date: 2005-05-22T13:37:59+09:00
Subject: Re: join not in Enumerable

On Saturday 21 May 2005 10:05 pm, David A. Black wrote:
> Hi --
>
> On Sun, 22 May 2005, Logan Capaldo wrote:
> > Just a few minutes ago I was playing with irb as I am wont to do, and
> > typed this:
> >
> > ('a'..'z').join(' ')
> >
> > Lo and behold it protested at me with a NoMethodError. I said to my
> > self, self there is no reason that has to be Array only functionality.
> > Why isn't it in Enumerable? So I said:
> >
> > module Enumerable
> >       def join(sep = '')
> >            inject do |a, b|
> >                     "#{a}#{sep}#{b}"
> >            end
> >        end
> > end
> >
> > And then I said ('a'..'z').join(' ') and got:
> > => "a b c d e f g h i j k l m n o p q r s t u v w x y z"
> >
> > #inject has to be the most dangerously effective method ever. But I
> > digress:
>
> You can speed it up a lot if you do this:

[... elided version using to_a ...]

The reason the non-to_a version is slow is because it creates a series of 
increasingly larger strings.  A faster version (without resorting to to_a) 
would build up a single string gradually.  Here is another version:

  def join(sep='')
    inject(nil) { |a, b|
      a ? (a << sep << b.to_s) : "#{b}"
    }
  end

Here are the timings I got ...

                  user     system      total        real
to_a:         0.580000   0.000000   0.580000 (  0.583975)
inject slow: 10.520000   0.210000  10.730000 ( 11.998484)
inject fast:  0.590000   0.020000   0.610000 (  0.651972)

-- 
-- Jim Weirich    jim@weirichhouse.org     http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)