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Re: procedures

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96950] Re: [mg96937] procedures
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Sat, 28 Feb 2009 06:40:15 -0500 (EST)
  • References: <200902271116.GAA21679@smc.vnet.net> <7F1970D7-0EE8-42D6-BC3E-6095D648BBA6@jeol.com> <1235749530.4275.7.camel@linux-sslk.site>

On Feb 27, 2009, at 10:45 AM, Davide Venturelli wrote:

> Thank you for the answer!
>
>> What is the purpose of assigning its value to
>> itself?
>
> I am evaluating a recursive function.
> I want to store every computed value in such a way that next time the
> code calls it, it remembers if it has already evaluated it.
> So while I need := for definition of the recursive functions, I use  
> the
> Do loop to assign actual values to the array.
>
> Do you have a better idea?

You can implement functions with memory as

F[x_]:=f[x]=(*body goes here*)

This would obviate the need for Do loop, just evaluating D[x,n] would  
be sufficient.

Search for FunctionsThatRememberValuesTheyHaveFound in the  
Mathematica help documentation (for v7).

> I now have another problem, anyway!
>
> When I do FindRoot[f[x],{x,0}] it looks for all instances of x in the
> definition of the function, and THEN evaluate the function for the
> symbol X.

I think this occurs because FindRoot defaults to Newton's method and  
it needs to take the derivative of the expression.

> What I want is to override this symbolic method and call f[x] with the
> numerical value of x, since in the definition of f[x] I have recursive
> functions that would imply to evaluate a huge polynomial of x, and
> keeping track of all the terms is really lengthy.
> While if I substitute x=x0 at each recursive step, the computation is
> less costy.
>
> So is there a way to tell mathematica in FindRoot to call the function
> with numerical values, like it was a black box?

This comes up from time to time on this mailing list.  The usual  
suggestion is to make the function definition valid only for numeric  
arguments see

http://groups.google.com/group/comp.soft-sys.math.mathematica/ 
browse_thread/thread/a076ff6d9b72b462

for example.

Hope that helped,

Sseziwa Mukasa


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