Re: problems with FindRoot: what worked with 4.2 does not work with 5.0
- To: mathgroup at smc.vnet.net
- Subject: [mg47380] Re: problems with FindRoot: what worked with 4.2 does not work with 5.0
- From: "Gareth J. Russell" <gjr2008 at columbia.edu>
- Date: Wed, 7 Apr 2004 03:16:40 -0400 (EDT)
- Organization: Columbia University
- References: <c4u3gg$9l6$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
In <c4u3gg$9l6$1 at smc.vnet.net> Edgar Dachs wrote:
>
> I have written a Mathematica program called PET (Petrological
> elementary tools) for Mathematica (Dachs, 1998, Computers &
> Geosciences, 24/3: 219-235) und worked out a new version that I
> tested with Mathematica version 4.2. With 5.0, I now find problems
> that several of my functions do not work any more, obviously do to
> changes that have been employed between 4.2 and 5.0 concerning
> FindRoot. Below is a little test-function that illustrates the problem:
> the return-value of the function depends on the value of x, as in the
> much more complex petrological functions that I have written, where
> calculations depend e.g. on the value of the temperature.
>
> test[x_] := Module[{y},
> If[x > 0, y = x + 1];
> If[x <= 0, y = x - 1];
> Return[y]
>]
> With 4.2 FindRoot (using two startin values for x) could have been
> used to solve for a special value, e.g.
>
> FindRoot[test[x] == 5, {x, {8, 9}}]
>
> with the obvious result: {x -> 4.}
>
> With 5.0 the call
>
> FindRoot[test[x] == 5, {x, 9}]
>
> (only one starting value can be specified with 5.0) yields the error
> message: FindRoot::nlnum "the function value {..} is not a list of
> numbers with dimension {1} at {x} = {9}
>
> I would be grateful if someone could help me solving this problem, at
> the moment I don't know what to do, but it looks like a compatibility
> problem between Mathematica versions 4.2 and 5.0.
>
> Thanks in advance,
>
> Dr. Edgar Dachs
> Department of Mineralogy
> University Salzburg
> Austria
Edgar,
I'm not sure why it worked in v4 and failed in v5, but your coding of
the If statement is a bit unorthodox. If you rewrite thus:
test[x_] := Module[{y},
y = If[x > 0, x + 1, x - 1];
Return[y]
] ]
It works.
In[34]:=
FindRoot[temp[x]==5,{x,8}]
Out[34]=
{x->4.}
This particular piece of code could, of course, be written
test[x_] := If[x > 0, x + 1, x - 1]
Gareth Russell
Columbia University