Path: csiph.com!feeder.erje.net!1.eu.feeder.erje.net!newsfeed0.kamp.net!newsfeed.kamp.net!fu-berlin.de!uni-berlin.de!not-for-mail
From: Ben Finney <ben+python@benfinney.id.au>
Newsgroups: comp.lang.python
Subject: Re: how to convert code that uses cmp to python3
Date: Sat, 09 Apr 2016 21:49:22 +1000
Lines: 45
Message-ID: <mailman.115.1460202579.2253.python-list@python.org>
References: <57064D0D.1030701@rece.vub.ac.be> <CAPTjJmrGiQS9uhFU-+TQ4nUWQ2p4gmJoQitMotS0Ob--qm1iWg@mail.gmail.com> <5706C961.2000009@rece.vub.ac.be> <CAPTjJmrbL870mV1kU8nHar=bPyKZRmhKP-8iUs_tpYVo5vhhOA@mail.gmail.com> <57075F43.7060004@rece.vub.ac.be> <85fuuw5ypl.fsf@benfinney.id.au> <5707B2CE.1010407@rece.vub.ac.be> <CAPTjJmrMv5O5vwrbfHJJCdiHi87naPOanhPmk4Ba_SLRgu1m4Q@mail.gmail.com> <5707BE18.1050805@rece.vub.ac.be> <CAPTjJmoMuqwg0Q_fp=qitC3wWkXYHtdqBNVGzO+3ToAmsgit8Q@mail.gmail.com> <5708C8B0.6050904@rece.vub.ac.be> <85twjb3su5.fsf@benfinney.id.au>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Cancel-Lock: sha1:x3ppFc+g2co4liSJNqO6YZo7lzs=
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux)
Precedence: list
Xref: csiph.com comp.lang.python:106725

Antoon Pardon <antoon.pardon@rece.vub.ac.be> writes:

> You don't seem to understand. I only do two comparisons and no the
> equality is not necesarrily cheaper.
>
> I am talking about the difference between the following two:
>
>     if arg.key < node.key:   # possible expensive computation
>         go_left()
>     elif arg.key == node.key # possible expensive computation
>         found_node()
>     else:
>         go_right()
>
> and
>
>     delta = cmp(arg.key, node.key) # possible expensive computation
>     if delta < 0:                  # cheap computation
>         go_left()
>     elif delta == 0:               # cheap computation
> 	found_node()
>     else:
>         go_right()

I find that a dubious claim.

The ‘cmp’ implementation must decide *at least* between three
conditions: less-than, equal-to, greater-than. That is *at least* two
inflection points.

The implementation of ‘__lt__’ and the implementation of ‘__eq__’ each
only need to decide two conditions (true, false). So that is *at most*
two inflection points.

If you're saying the latter implementation is somehow *more* expensive
than the former, I think the implementations are suspect and likely can
be improved: at least parity should be possible, to the point of
statistical insignificance.

-- 
 \        “Perchance you who pronounce my sentence are in greater fear |
  `\   than I who receive it.” —Giordano Bruno, burned at the stake by |
_o__)  the Catholic church for the heresy of heliocentrism, 1600-02-16 |
Ben Finney