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Evaluation of functions inside Plot

  • To: mathgroup at
  • Subject: [mg21993] Evaluation of functions inside Plot
  • From: Hans-Peter Kunzle <HP.Kunzle at>
  • Date: Thu, 10 Feb 2000 02:25:36 -0500 (EST)
  • Organization: University of Alberta
  • Sender: owner-wri-mathgroup at

I still do not understand well enough how expressions get
evaluated inside a Plot or similar functions (NDSolve etc.).

This time I copied the code directly from the (quite old)
book by T.W. Gray and J. Glynn, Exploring Mathematics with
Mathematica (p.142):

  { Abs[x+y I], Hue[ N[(Pi+Arg[x + yI]) /(2 Pi)]]},

This produces several error messages

    {Abs[x + y I], Hue[(N[([Pi] + Arg[x + yI])\(2 Pi)]]} is neither a 
    machine-size real number at {x,y}={-2.,-2.} nor a list of a real number 
    and a \alid color directive.

This book was written for Mathematica 2, of course, and presumably
the code must have worked then. Has anything changed since then?

I have tried to wrap 'Evaluate' around some of the expressions, but
this had no effect. Since Abs and Arg only work for numerical
arguments I also tried to replace them by explicit expressions in
x and y. But this did not help either.

Any help is appreciated.

H.P. Künzle                    | Office: (780)492-3798,492-3396 
Dept. of Mathematical Sciences | Fax:    (780)492-6826
University of Alberta          | E-mail: HP.Kunzle at 
Edmonton, Canada T6G 2G1 | WWW:

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