From: Austin Ziegler <austin@...>
Date: 2003-09-16T04:38:35+09:00
Subject: Re: performance and style advice requested

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On Tue, 16 Sep 2003 02:07:53 +0900, Alex Martelli wrote:
> Ben Giddings wrote:
>> Some style advice:
>> On Sunday, September 14, 2003, at 12:44 PM, Alex Martelli wrote:
>>> # factorial, memoized via an Array
>>> $_fact_memo=3D[1,1,2,6,24,120,720]
>> Ugh. This is what really bothers me about python programs. Ugly
>> underscores.
> Heh -- matter of taste, I guess; what _I_ personally find ugly is
> that leading $, and I left the underscores in to try to ameliorate
> that. Of course, nothing stops you from moving to a camelCaseish
> $factMemo -- however, not only do I personally find that less
> readable, but I'd also like to leave a hint that the memo is an
> internal implementation detail, not a feature meant for external
> consumption, and I like the convention of a leading _ for that.
> (Of course, I _could_ wrap up the fact function and its memo in a
> single package in any of various ways, I'm sure, so the "hint"
> aspect is quite secondary).

Why not use a constant? In my unrolled implementation, that's what I
do. It's is a bit slower (but, as far as I can tell, not in the
unrolled implementations), but doesn't take advantage of the
algorithmic shortcut hinted at in earlier messages.

  FACT_MEMO =3D [1, 1, 2, 6, 24, 120, 720]

  def fact!(x)
    return 0 if x < 0
    result =3D FACT_MEMO[x]

    unless result
      ((FACT_MEMO.size)..x).each do |i|
        FACT_MEMO[i] =3D i * FACT_MEMO[i - 1]
      end
      result =3D FACT_MEMO[x]
    end

    result
  end

I'm sure there's other improvements that can be done; I went for
Ruby-idiom readability.

If it's not meant to be public at all, then since I'm using a class
here, I can use an instance variable (e.g., @fact_memo or
@__fact_memo__).

I'd much rather use a constant (FACT_MEMO) than a global
($_fact_memo). (This does point out a problem with the VIM syntax
highlighting for Ruby, in that it doesn't properly highlight
$_fact_memo because it finds $_.)

I don't think that I've used a single global variable in my Ruby
programming.

I'm attaching my version to this message.

-austin
--
austin ziegler    * austin@halostatue.ca * Toronto, ON, Canada
software designer * pragmatic programmer * 2003.09.15
                                         * 15.28.27

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