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MathGroup Archive 2003

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Re: Don't understand Plot behavior

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39993] Re: Don't understand Plot behavior
  • From: bobhanlon at aol.com (Bob Hanlon)
  • Date: Fri, 14 Mar 2003 04:45:28 -0500 (EST)
  • References: <b4pf6u$bu4$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

f[t_]:= D[t  , t];

The Evaluate is needed to override Plot's attribute of HoldAll.  If you bury
the Evaluate inside a List then the first argument is not evaluated.  You need
to Evaluate the the first argument, that is, the List.

Plot[Evaluate[{f[t],0}],{t,0,1}];

or define f with

f[t_]:=Evaluate[D[t,t]];

Plot[{f[t],0},{t,0,1}];


Bob Hanlon
In article <b4pf6u$bu4$1 at smc.vnet.net>, Alain Cochard <al2 at noos.fr> wrote:

<< 
Subject:	Don't understand Plot behavior
From:		Alain Cochard <al2 at noos.fr>
To: mathgroup at smc.vnet.net
Date:		Thu, 13 Mar 2003 08:23:58 +0000 (UTC)


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.

[...]

General::stop: Further output of General::ivar
     will be suppressed during this calculation.

Plot::plnr: Evaluate[f[t]] is not a machine-size real number at t = 
              -8
    4.16667 10  .

[...]

General::stop: Further output of Plot::plnr
     will be suppressed during this calculation.

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]

In[5]:= $Version

Out[5]= 4.1 for Sun Solaris (November 8, 2000)

Is this behavior normal?  Thx in advance.

Alain Cochard
al2 at noos.fr

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