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Re: Tagged list processing

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86922] Re: [mg86891] Tagged list processing
  • From: János <janos.lobb at yale.edu>
  • Date: Wed, 26 Mar 2008 04:54:09 -0500 (EST)
  • References: <200803250617.BAA10618@smc.vnet.net>

On Mar 25, 2008, at 2:17 AM, carlos at Colorado.EDU wrote:
> This is related to a recent question. Best stated through an example.
> I have two flat lists of equal length:
>
> tag={0,0,1,0,0,  -3,0,0,0,0,          2,4,1,0,0};
> val={0,0,4,5,6,  P,1/2,0,-3.0,a+4, 8,16.4,0,Sin[ang],Cos[ang]};
>
> tag contains only integers whereas val can have numbers or
> expressions that will (eventually) evaluate to a number.
>
> Task. Generate a pointer list to all nz values in tag, and a
> corresponding list of values:
>
> nztag={3,6,11,12,13};   (* may be empty *)
> nzval={4,P,8,16.4,0};   (* may be empty *)
>
> and also build the complementary lists
>
> ztag={1,2,4,5,7,8,9,10,14,15};
> zval={0,0,5,6,1/2,0,-3.0,a+4, Sin[ang],Cos[ang]};
>
> Obviously ztag=Position[tag,0]; but which
> form argument do I use for nztag in Position[tag,form]?
> Can I build zval  and nzval with Table for speed ?
> (lists may contain 10^4 to 10^6  items)
>
> Constraint: must work in 4.0 thru 6.0 since some of my
> remote students have legacy academic versions. Thanks.


Here are some newbie approaches:

First I would create a combined list with Inner:

In[4]:=tagval = Inner[List, tag,    val, List]

Then, you can get nztag for example with Condition:

> In[13]:=
> Flatten[Position[tagval,
>    {a_, b_} /; a != 0]]
> Out[13]=
> {3, 6, 11, 12, 13}

With the best,

J=E1nos



--------------------------------------------------------
The more money has to be spent to fix a problem, the more likely it 
is that the people with the money will agree to fix it.
/Alex Satrapa/





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