Question

Is there any way to get at the actual messages generated during the evaluation of an expression in Mathematica? Say I m numerically solving an ODE and it blows up, like so

In[1] := sol = NDSolve[{x [t] == -15 x[t], x[0] == 1}, x, {t, 0, 1}, 
                       Method -> "ExplicitEuler"];

In this case, I ll get the NDSolve::mxst error, telling me the maximum number of 10000 steps was reached at t == 0.08671962566152185. Now, if I look at the $MessageList variable, I only receive the message name; in particular, the information about the value of t where NDSolve decided to quit has been lost.

Now, I can always get that information from sol using the InterpolatingFunctionDomain function from one of the standard add-on packages, but if I can somehow pull it out of the message, it would be quite helpful.

Answer 1

You might be able to use $MessagePrePrint to set up a function which would store away each of the messages for later retrieval.

Answer 2

I don t know if this will work, but if the only thing you want to know are the values of specific parameters at the point of error then a kludgy way of getting them would be to define those variables with dummy values globally. This works with loop counters, but I don t know if it works from within NDSolve. Another kludge would be to make t Dynamic and have an evaluated cell with t.

A more elegant (and probably the correct) approach would be to use Reap and Sow.

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