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MathGroup Archive 2003

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RE: Don't understand Plot behavior

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39992] RE: [mg39971] Don't understand Plot behavior
  • From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
  • Date: Fri, 14 Mar 2003 04:45:24 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

>-----Original Message-----
>From: Alain Cochard [mailto:al2 at noos.fr]
To: mathgroup at smc.vnet.net
>Sent: Thursday, March 13, 2003 9:04 AM
>To: mathgroup at smc.vnet.net
>Subject: [mg39992] [mg39971] Don't understand Plot behavior
>
>
>
>In[1]:= f[t_]:= D[t  , t] 
>
>In[2]:= Plot[Evaluate[f[t]] , {t, 0, 1}]
>
>Out[2]= -Graphics-    [The plot is fine]
>
>In[3]:= Plot[{Evaluate[f[t]],0} , {t, 0, 1}]
>
>                         -8
>General::ivar: 4.16667 10   is not a valid variable.
>
>[...]
[...]

>
>Out[3]= -Graphics- [The plot just shows the axes]
>
>In[4]:= harhar=Evaluate[f[t]]; Plot[{harhar,0} , {t, 0, 1}]
>
>Out[4]= -Graphics- [The plot is fine]
>
[...]
>
>Is this behavior normal?  Thx in advance.
>
>Alain Cochard
>al2 at noos.fr
>
>

Alain,

Absolutely normal, yes.

Plot[Evaluate[{f[t], 0}], {t, 0, 1}, PlotStyle -> Hue[.7]]

is fine (and is effectively the same as your In[4] example.



Now, what's wrong with Plot[{Evaluate[f[t]],0} , {t, 0, 1}]?

First, as Evaluate is not met at level 1, {Evaluate[f[t]],0} will be
untouched (because of HoldAll attribute of Plot), and the machinery of Plot
begins to start with first value t = 4.16667*^-8.

What happens then? Quite the same as in

In[100]:= Block[{t = 4.16667*^-8}, f[t]]
>From In[100]:=
General::"ivar": "4.16667`*^-8 is not a valid variable."
Out[100]= \[PartialD]\_4.16667`*^-8 4.16667`*^-8


--
Hartmut Wolf



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