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Re: Tagged list processing

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86916] Re: Tagged list processing
  • From: michael.p.croucher at googlemail.com
  • Date: Wed, 26 Mar 2008 04:52:56 -0500 (EST)
  • References: <fsa5fq$adp$1@smc.vnet.net>

On 25 Mar, 06:18, car... 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.

How about this?

nztag = Flatten[Position[tag, x_ /; Abs[x] > 0]]
nzval = val[[nztag]]
ztag = Flatten[Position[tag, 0]]
zval = val[[ztag]]

Hope this helps,
Mike



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