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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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