Re: Numerical calculation of derivative functions
- To: mathgroup at smc.vnet.net
- Subject: [mg17033] Re: [mg17000] Numerical calculation of derivative functions
- From: Maarten.vanderBurgt at icos.be
- Date: Wed, 14 Apr 1999 02:11:50 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Lars,
You should use the Evaluate[ ] command.
Have a look at the difference between the functions g and f below.
In[1]:= g[x_]:=D[Sin[x],x]
In[2]:= ?g
"Global`g"
g[x_] := D[Sin[x], x]
In[3]:= g[1.]
General::"ivar": 1.`is not a valid variable.
In[4]:= f[x_]:=Evaluate[D[Sin[x],x]]
In[5]:= ?f
"Global`f"
f[x_] := Cos[x]
In[6]:= f[1.]
Out[6]= 0.540302
In[7]:= f[1]//N
Out[7]= 0.540302
Maarten
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Maarten van der Burgt
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ICOS Vision Systems
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tel. + 32 16 398220 direct + 32 16 398316 fax. + 32 16
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e-mail: maarten.vanderburgt at icos.be
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L B Venema <lvenema at kvi.nl> on 10-04-99 08:13:35 AM
Subject: [mg17033] [mg17000] Numerical calculation of derivative functions
Probably this question is very trivial, but can someone inform me
how I can force mathematica to calculate the numerical result from a
previously evaluated formal solution, like
f[x_]:=D[sin[x],x];
f[x]/.x->1
Also N[%] is not more specific than D[sin[1]].
I think the problem is related that I cannot force Mathematica to first
evaluate the differential and subsequently substitutes the x by 1 in the
cos. (My real problem involves unknown dimensions of arrays in a non-linear
optimization problem).
Thank you for your help,
Lars Venema