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MathGroup Archive 2007

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Re: V6 evaluation inside Table and plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg76650] Re: V6 evaluation inside Table and plot
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Fri, 25 May 2007 06:34:06 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f33onq$l4v$1@smc.vnet.net>

Gerry Flanagan wrote:
> Define a simple function
> 
> In[7]:= f = Function[{x}, x^3]
> 
> Out[7]= Function[{x}, x^3]
> 
> Taking two derivatives works as it should
> 
> In[2]:= f''[x]
> 
> I was converting some packages to work with V6, and immediately ran into 
> some evaluation issues I don't understand. It looks like forms like 
> f''[x] (derivative) are evaluated differently inside of Table and Plot 
> than outside.
> 
> Out[2]= 6 x
> 
> But this does not?
> 
> In[20]:= Table[f''[x], {x, 0, 1, .2}]
> 
> Out[20]= {0, 0, 0, 0, 0, 0}
> 
> Strangely, this works
> 
> In[4]:= Table[f'[x], {x, 0, 1, .2}]
> 
> Out[4]= {0., 0.12, 0.48, 1.08, 1.92, 3.}
> 
> Determining the derivative outside the Table makes it work again.
> 
> In[12]:= g = f'';
> 
> In[21]:= tmp = Table[g[x], {x, 0, 1, .2}]
> 
> Out[21]= {0., 1.2, 2.4, 3.6, 4.8, 6.}
> 
> Gerry Flanagan
> 

Note that the second differentiation will take place as expected if you 
wrapped your function(s) within Evaluate (or set the option Evaluated to 
True in the Plot command).

Clear[f]
f = Function[{x}, x^3];
Table[Evaluate[{f'[x], f''[x]}], {x, 0, 1, .2}]
Plot[Evaluate[{f'[x], f''[x]}], {x, 0, 1, .2}]
Plot[{f'[x], f''[x]}, {x, 0, 1, .2}, Evaluated -> True]

Regards,
Jean-Marc


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