Re: Re: Re: how to delete duplicate items in the same list
- To: mathgroup at smc.vnet.net
- Subject: [mg45633] Re: [mg45592] Re: [mg45576] Re: how to delete duplicate items in the same list
- From: "Sseziwa Mukasa,,(978) 536-2359" <mukasa at jeol.com>
- Date: Fri, 16 Jan 2004 19:57:14 -0500 (EST)
- References: <bto1uf$2ka$1@smc.vnet.net> <bu0cjl$ah9$1@smc.vnet.net> <200401140626.BAA19761@smc.vnet.net> <200401161105.GAA10620@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On Jan 16, 2004, at 6:05 AM, Andrzej Kozlowski wrote:
> The possibility of using Union for testing the presence or absence of
> duplicates is a side effect rather than the main purpose of Union.
>
Rather than arguing the behavior of Union[a] as being a side effect or
a short hand for Union[a,a] why not consider it a shorthand for
Union[a,{}] in which case there is no confusion. I suppose there is no
confusion in the case of Union[a,a] but it's hard to imagine why
someone would care about that expression in the first place. I am not
mathematically sophisticated though, so forgive me if I don't
understand the import of Union[a,a].
There is still the issue of the representation of a set in Mathematica
especially due to the unfortunate choice of {} as list delimiters.
From a programmer's point of view the mathematical object set is
probably best represented by a hash function which hashes all possible
patterns of bits, since ultimately a set can contain anything a
computer can represent, and a membership function which can tell
whether the hash value for a specific bit pattern is occupied for a
specific set. That's unwieldy in practice, and a sorted list is
probably sufficient, however since a hash only has the concept of
membership or nonmembership there is no idea of repetition of elements.
Thus the representation of sets as a list of unique elements
representing the members, sorting just helps with efficiency. It's not
a rigorous explanation, I'd be happy to hear what a proper
mathematician/computer scientist has to say on the subject, but I think
it explains the behavior of Union[a].
Regards,
Ssezi
- References:
- Re: how to delete duplicate items in the same list
- From: AES/newspost <siegman@stanford.edu>
- Re: Re: how to delete duplicate items in the same list
- From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
- Re: how to delete duplicate items in the same list