Re: Tagged list processing

*To*: mathgroup at smc.vnet.net*Subject*: [mg86935] Re: [mg86891] Tagged list processing*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Wed, 26 Mar 2008 04:56:37 -0500 (EST)*References*: <200803250617.BAA10618@smc.vnet.net>

On 25 Mar 2008, at 07:17, 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. This should work in all versions of Mathematica: 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]}; nztag = Position[tag, z_ /; z != 0] {{3}, {6}, {11}, {12}, {13}} nzval = Extract[val, nztag] {4, P, 8, 16.4, 0} etc. Andrzej Kozlowski > >

**References**:**Tagged list processing***From:*carlos@colorado.edu