Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2002
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2002

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Problem with user defined functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg37365] Re: [mg37340] Problem with user defined functions
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Fri, 25 Oct 2002 02:47:07 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

There are a number of problems with what you are trying to do. I think 
rather than explaining what is wrong it may be more useful to indicate 
one approach that works.

You can define your function as you did originally (assuming that was 
really what you wanted to do!)

test[x_] := p /. FindRoot[Sin[p] == x, {p, 1}][[1]]

Create a plot (contrary to the usual practice in such cases do not wrap 
Evaluate around the function to be plotted!)

g = Plot[test[x], {x, 0, 1}]

Now create an interpolating function from the graph:

h = Interpolation[Cases[g, Line[{x__}] -> x, Infinity]]

You can now plot the derivative:

Plot[h'[t], {t, 0, 1}]



On Thursday, October 24, 2002, at 03:55 PM, Farhat Habib wrote:

> Hello all,
>
> I am trying to evalute a function which looks like this. This is an
> analogous problem not the actual problem.
>
> test[x_] := p /. FindRoot[Sin[p] == x, {p, 1}][[1]]
> data[x_] := D[test[x], x]
> Plot[data[x], {x, 0, 1}]
>
> Now, when trying to evaluate the derivative mathematica replaces both 
> the
> x's with the number at which it is trying to calculate the datapoint. 
> This
> gives an error of 'xxxx is not a valid variable'. Replacing D[test[x]]
> with test'[x] doesn't help either though it results in a different set 
> of
> errors. Is there any of way of getting around this problem?
>
> Thanks in advance,
>
> Farhat
>
>
>
>


Andrzej Kozlowski
Yokohama, Japan
http://www.mimuw.edu.pl/~akoz/
http://platon.c.u-tokyo.ac.jp/andrzej/



  • Prev by Date: Re: To plot solutions, FindRoot as a function
  • Next by Date: real valued function from complex
  • Previous by thread: Re: Problem with user defined functions
  • Next by thread: RE: Problem with user defined functions