Re: Locate an underflow

[thompson(at)orpheus.nascom.nasa.gov (William Thompson)]

m.hadfield@niwa.cri.nz ("Mark Hadfield") writes:

>From: "Francesco" <francesco.spada@jrc.it>
>> I've got this error
>> 
>> % Program caused arithmetic error: Floating underflow
>> 
>> Is it possible to know where it appens without putting a million of
>> stop or breakpoint in my program?

>Set the !EXCEPT system variable to 2.

>From IDL 5.4 documentation:

>    !EXCEPT

>    An integer variable that controls when IDL checks for invalid
>    mathematical computations (exceptions), such as division by zero.
>    The three allowed values are:

>    Value Description

>    0 Never report exceptions.

>    1 Report exceptions when the interpreter is returning to an
>    interactive prompt (the default).

>    2 Report exceptions at the end of each IDL statement. Note that
>    this slows IDL by roughly 5% compared to setting !EXCEPT=1.

>    For more information on invalid mathematical computations and
>    error reporting, see Math Errors. The value of !EXCEPT is used by
>    the CHECK_MATH function to determine when to return errors. See
>    CHECK_MATH for details.

>---
>Mark Hadfield
>m.hadfield@niwa.cri.nz  http://katipo.niwa.cri.nz/~hadfield
>National Institute for Water and Atmospheric Research


What I would really like is something that would differentiate between
underflow errors and all other exceptions.  I think that most users don't know
what an underflow "error" is, and their initial reaction is that something bad
is going on.  I see that the IDL help now contains the following statement:

    A Note on Floating-Point Underflow Errors

    Floating-point underflow errors occur when a non-zero result is so
    close to zero that it cannot be expressed as a normalized
    floating-point number. In the vast majority of cases, floating-point
    underflow errors are harmless and can be ignored. For more information
    on floating-point numbers, see Accuracy & Floating-Point Operations.

In the old days, IDL didn't even bother to report underflow errors.

Even when you do know that underflow "errors" are (almost always) harmless,
continuously getting these messages is highly annoying.  I'd use !EXCEPT=0 to
suppress underflow messages, except that it would also suppress other
potentially more serious error messages as well.

How about something like the following for additional values for !EXCEPT:

Value	Description

 3	Suppress underflow errors, but report other exceptions when the
	interpreter is returning to an interactive prompt.

 4	Suppress underflow errors, but report other exceptions at the end of
	each IDL statement.

William Thompson