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Re: Evaluation of functions inside Plot
*To*: mathgroup at smc.vnet.net
*Subject*: [mg22029] Re: [mg21993] Evaluation of functions inside Plot
*From*: Hartmut Wolf <hwolf at debis.com>
*Date*: Fri, 11 Feb 2000 02:38:11 -0500 (EST)
*Organization*: debis Systemhaus
*References*: <200002100725.CAA05549@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
Hans-Peter Kunzle schrieb:
>
> 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):
>
> Plot3D[
> { Abs[x+y I], Hue[ N[(Pi+Arg[x + yI]) /(2 Pi)]]},
> {x,-2,2},{y,-2,2}],
> AxesLabel->{"Re","Im","Abs[z]"},PlotPoints->15
> ];
>
> This produces several error messages
>
> Plot3D::plnc:
> {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.
>
Dear Hans-Peter,
if you did respect literally the error message you got, you will cut
your expression out of plot and check:
In[19]:=
{Abs[x + y I], Hue[N[(Pi + Arg[x + yI])/(2 Pi)]]} /. {x -> -2., y ->
-2.}
Out[19]=
{2.82843, Hue[0.159155 (3.14159\[InvisibleSpace] + Arg[-2. + yI])]}
...and see that you made a typo. A space between yI will repair
everything!
Kind regards, Hartmut
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