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Re: Get list of function variables?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83534] Re: Get list of function variables?
  • From: Jason Quinn <jason.lee.quinn at gmail.com>
  • Date: Thu, 22 Nov 2007 04:51:12 -0500 (EST)
  • References: <fhjs5v$4t3$1@smc.vnet.net> <fhu7j5$7b1$1@smc.vnet.net>

On Nov 21, 6:09 am, Szabolcs <szhor... at gmail.com> wrote:
> It seems that you were not convinced by the previous posts that the
> original question does not really make sense in Mathematica (because
> there are no "function variables", or "functions", only patterns and
> transformation rules, and the pattern names don't carry any
> information by themselves).  So here is an example of a mathematical
> function, implemented in Mathematica, on which getVariables[] does not
> work:
>
> factorial[0] = 1
> factorial[n_] := n factorial[n-1]
>
> May I ask what you intend to use getVariables[] for?

I am aware that the "solutions" I posted do not hold in general. My
basic desire for the getVariables function was to help generate a new
function with those variables (and another set of the same size). I
wanted a notebook that could give the the error function for ANY
function the user inputed without having to do any work. Maybe there's
a better way than how I did it (but I have a working notebook so I'm
better off than I was).

In other words give some function, say, the price to carpet an "x" by
"y" by "z" room at "w" per square unit area:
f(w,x,y,z)=w(x*y*z)
I want the sigma error on f given sigma errors on the variables. So a
new function "ef"
ef(x,y,z,ex,ey,ez)
where ef is defined by the usual error propagation equation a sum over
partial derivatives squared. The "e" prefix denotes the sigma error on
the corresponding variable. Note the new function recognizes w as a
parameter.

If I went about this in some ass-backwards fashion by grabbing the
variables from f directly, I'm a little bit embarrassed but would be
interested in knowing a better way.

Jason










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