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>