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MathGroup Archive 1999

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vector pattern?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20258] vector pattern?
  • From: Susan Hovde <hovde at al.noaa.gov>
  • Date: Sun, 10 Oct 1999 00:04:08 -0400
  • Organization: NOAA Aeronomy Lab
  • Sender: owner-wri-mathgroup at wolfram.com

I'd like to define a pattern for a 2D vector v such that

v = {x,y}  and

v[t] = {x[t],y[t]}.

I tried

v[t___] := If[Count[{t},s_]==0,{x,y},{x[t],y[t]}]

and this gets me _almost_ what I want.  I get v[t] = {x[t], y[t]} and
v[] = {x,y}, but when I ask for v, I just get v back.

I then tried adding

v := {x,y}

but then when I ask for v[t] I get {x,y}[t] instead of {x[t],y[t]}.  (I
also tried v := v[] with the same result.)  I have tried various Hold
functions to try to get v not to be evluated too early, to no avail.
Can someone help me out?

Susan Hovde
NOAA Aeronomy Lab
Boulder, Colorado
hovde at al.noaa.gov



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