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Tagged list processing
*To*: mathgroup at smc.vnet.net
*Subject*: [mg86891] Tagged list processing
*From*: carlos at colorado.edu
*Date*: Tue, 25 Mar 2008 01:17:08 -0500 (EST)
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.
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