problems with FindRoot: what worked with 4.2 does not work with 5.0

• To: mathgroup at smc.vnet.net
• Subject: [mg47356] problems with FindRoot: what worked with 4.2 does not work with 5.0
• From: "Edgar Dachs" <edgar.dachs at sbg.ac.at>
• Date: Tue, 6 Apr 2004 06:36:54 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```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.