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MathGroup Archive 1998

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Re: Plotting vector-valued functions



Malcolm Boshier wrote:
> 
>     I have a problem which is related to the recent thread about
> plotting lists of functions.  In the case when a vector-valued function
> is expensive or impossible to Evaluate before plotting, Plot apparently
> forces you to evaluate the function repeatedly at each value of the
> independent parameter.  This can be very inefficient.
>     As an example, suppose that f[z] returns the eigenvalues of a 5 x 5
> matrix which is a function of z.  In general this function cannot be
> evaluated without a value for z, so
> Plot[ Evaluate[f[z]], {z, zmin, zmax}] doesn't work.
>     The only way around this that I have found is something like:
> 
> Plot[{f[z][[1]],  f[z][[2]], f[z][[3]], f[z][[4]], f[z][[5]]}, {z, zmin,
> zmax}]
> 
> which of course requires 5 evaluations of f[z] for each value of z.
>     It seems that unless the head of the first argument to Plot is List,
> Plot assumes that it will evaluate to a real number and returns with an
> error if it later finds that it doesn't.  Why can't Plot trust the user
> long enough to discover that the function will evaluate to a list?
> Thanks for any solutions or explanations, Malcolm


I tend to use the "function that remembers its values" construct.  For
example:

f[x_]:=f[x]=Module[{},Return[{f1,f2,f3}]]
g[x_]:=Module[{},Return[{g1,g2,g3}]]

This function returns a vector.  Suppose I want to graph all three
components on the same graph

Plot[{f[x][[1]],f[x][[2]],f[x][[3]]},{x,low,high}]

The f function executes once for every point.  The second and third
values are just look-ups.  On the other hand

Plot[{g[x][[1]],g[x][[2]],g[x][[3]]},{x,low,high}]

The g function executes 3 times for every point.

If memory is a problem, then you can Clear it after the graph.

-- 
Remove the _nospam_ in the return address to respond.



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