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Re: Don't understand Plot behavior

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.


or define f with



Bob Hanlon
In article <b4pf6u$bu4$1 at>, Alain Cochard <al2 at> wrote:

Subject:	Don't understand Plot behavior
From:		Alain Cochard <al2 at>
To: mathgroup at
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}]

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 = 
    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


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