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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: [mg76673] Re: V6 evaluation inside Table and plot
  • From: dh <dh at metrohm.ch>
  • Date: Fri, 25 May 2007 06:46:00 -0400 (EDT)
  • References: <f33onq$l4v$1@smc.vnet.net>


Hi Gerry,

this is an ugly trap you encoutered. Consider: Attributes[Table] and you 

will see that Table has the Attribute HoldAll. That means f'' is not 

evaluated before beeing fed to Table. Now Table seems to have a small 

brain damage, because it does the derivative wrong. You can see this from:

Table[f''[y], {y, 3, 3, 1}] // Trace

why this happens I can not tell, but I assume there is some confusion 

between variables local to Function and Table. Please inform Wolfram 

about this bug and let me know the answer.

hope this helps,Daniel



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

> 




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