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RE: How to operate on strictly numerical functions ?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg24118] RE: [mg24102] How to operate on strictly numerical functions ?
*From*: Wolf Hartmut <hwolf at debis.com>
*Date*: Wed, 28 Jun 2000 02:11:40 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
My answer see below
> -----Original Message-----
> From: Mark Harder [SMTP:harderm at ucs.orst.edu]
To: mathgroup at smc.vnet.net
> Sent: Tuesday, June 27, 2000 6:52 AM
> To: mathgroup at smc.vnet.net
> Subject: [mg24102] How to operate on strictly numerical functions ?
>
>
> Some of the Mathematica functions made for numerical procedures
> appear to actually evaluate by substitution of symbolic variables with
> replacement rules. An example of this, which has blocked me for more
> than a week now, is NonlinearRegress, which seems to evaluate the
> user-supplied model function symbolically, then evaluate the resulting
> expression through replacement of the independent variables and the
> current set of adjustable parameters through replacement rules that it
> constructs. My model function requires evaluation of the SVD (with
> SingularValues[] ) of a matrix computed from the independent variables
> and the parameters of the model, and so NonlinearRegress fails, since
> SingularValues[] can't accept a non-numeric matrix. A simpler example I
> have encountered is the numeric derivative function, which I'll use to
> illustrate my problem.
> First, construct a simple test function which, through an If[] test,
> won't evaluate for non-numeric arguments:
> In[804]:=
> ClearAll[x, tstFn1]
> tstFn1[x_] := If[NumericQ[x ], Return[x^2];, Print["NonNumeric x
> and/or y."];]
>
> In[808]:=tstFn1[2]
> Out[808]=4
> In[809]:=tstFn1[u]
>
> & try to use ND[] on this function:
>
> <<NumericalMath`NLimit`
>
> In[806]:=ClearAll[u, v]
> ND[tstFn1[u ], u, 1.]
> Out[807]= 0
>
> So, I'm looking for some means of modifying testFn1 to cause ND to
> evaluate it only after numeric substitution for u. Is there some way of
> Hold-ing evaluation until numeric values are assigned to arguments of a
> function? Is this sort of thing impossible in Mathematica? Do I have
> to write my own numeric routines for finding derivatives, gradients,
> Jacobians, least-squares fits, etc. by procedural routines ala FORTRAN?
> Thanks for any help you can offer.
>
> mark e. harder
>
> harderm at ucs.orst.edu
>
[Wolf Hartmut] Dear Mark,
perhaps this observation might help you:
In[20]:= << NumericalMath`NLimit`
In[23]:= Attributes[ND]
Out[23]= {Protected, ReadProtected}
so there are no Hold Attributes here, this will evaluate tstFn1[u] in first
place. However there might be more layers of evaluation lurking inside. So
let's test that:
In[44]:= uu = NestList[Unevaluated, Hold[tstFn1[u]], 5]
This contains up to five 'protection layers' agains evaluation.
In[51]:= Print[Block[{u = 1.}, ReleaseHold[#]]] & /@ uu
>From In[51]:= 1.
>From In[51]:= 1.
>From In[51]:= 1.
>From In[51]:= Unevaluated[tstFn1[u]]
>From In[51]:= Unevaluated[Unevaluated[tstFn1[u]]]
>From In[51]:= Unevaluated[Unevaluated[Unevaluated[tstFn1[u]]]]
Out[51]= {Null, Null, Null, Null, Null, Null}
This was for getting familiar with the test, now apply it:
In[47]:= (Print["---"]; ND[ReleaseHold[#], u, 1.]) & /@ uu
>From In[47]:= "---"
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "---"
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "---"
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "---"
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "---"
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "NonNumeric x and/or y."
>From In[47]:= "---"
Out[47]= {0, 0, 0, 0, 0, 2.}
The last result looks very pleasing, not so pleasing would be the next layer
(I avoided here).
So if you need to have your function unevaluated up to a certain --
generally unknown -- point in computation with supplied packages you
certainly live on the knife's edge.
However perhaps this might be a 'cheating device' against unwanted
evaluation:
In[59]:= tstFn2[x_] := If[NumericQ[x], x^2,
Unevaluated[Unevaluated[tstFn2[x]]]]
In[60]:= tstFn2[u]
Out[60]= Unevaluated[tstFn2[u]]
In[61]:= ND[tstFn2[u], u, 1.]
Out[61]= 2.
Kind regards, Hartmut
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