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MathGroup Archive 2005

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Re: Converting a mapping into a well-defined function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56536] Re: Converting a mapping into a well-defined function
  • From: Peter Pein <petsie at arcor.de>
  • Date: Thu, 28 Apr 2005 02:40:35 -0400 (EDT)
  • References: <d4msni$257$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Gilmar wrote:
> Dear Mathematica User Friends:
> 
> I need to build a module that takes a non-empty set A
> (whose elements are 2-tuples), and appends (or not) those 2-tuples
> to a set B  according to the following criteria:
> 
> (1.) if the abscissa of a 2-tuple in A is unique (among the abscissas
> of all the other 2-tuples in A), then the module appends that 2-tuple
> to the set B.
> 
> (2.) if there are n 2-tuples in A that share the same abscissa, then
> the module appends the first of those 2-tuples to B ("first" here means
> the first of such n 2-tuple encountered, as you read A from left to
> right),but does not append the rest to B.
> 
> (3.) the module does not attempt to sort the set B, after the appending
> process is completed.
> 
> Example:
> 
> If A = {{a,5},{b,27},{a,14},{c,4},{e,94},{b,6},{d,9},{e,4}}
> 
> then
> 
> B=Module[A]={{a,5},{b,27},{c,4},{d,9},{e,94}}.
> 
> Another way to re-phrase the above example;
> 
> if A defines a mapping:
> 
> A(a)=5, A(b)=27, A(a)=14, A(c)=4, A(e)=94, A(b)=6, A(d)=9, A(e)=4
> 
> them the module converts the mapping A into a well-defined function:
> 
> A(a)=5, A(b)=27, A(c)=4, A(d)=9, A(e)=94.
> 
> Thank you for your help!
> 
I've got another one (about 10 times as fast as my other suggestion):

map2fun2[lst_]:=
 Cases[lst,{#,_},1,1]&/@
  Module[{f},f[y_]:=(f[y]=Sequence[];y);f/@First/@lst]

-- 
Peter Pein
Berlin


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