Underflow

[Chris Sharman]

Mikael A Olesen wrote:
> Hello
> 
> I have a qtp program that sometimes crash with the following error:
> 
>  *E* Floating point underflow occurred (number too small).
>  *Fatal Error* *1370* Notify Cognos Customer Support
> 
> The program runs on just about the same amount of data every time and most
> times it goes okay.

This is an arithmetic issue.
You don't mention your platform, so I'll give a VMS example. The same 
arguments apply to all platforms though.

F float (float*4) supports around 7 digits precision, in the range 
10^-38 to 10^38.
D float (alternate float*8) supports 16 digits precision, same range 
(10^-38 to 10^38).
G float (float*8) supports 15 digits precision, range 10^-308 to 10^308.

When the exponent goes beyond the upper bound (ie number too large to 
represent) that's a floating point overflow.
When it goes beyond the lower bound, that's either a floating point 
underflow, or rounded to zero, depending on achitecture and 
configuration. (If you're just subtotalling numbers, rounding to zero is 
probably the desired behaviour - if you're multiplying, dividing, or 
doing anything more complex, you probably want an error.

Look out for calculations that can have very small intermediate results, 
and use wider-ranging floats or rearrange your calculations (with 
parentheses) to ensure that intermediate results are OK.
Eg.
def a float*4 = 10^-20
def b float*4 = a*a*10^10 ; this will underflow (or round) on the a*a
def b float*4 = a*(a*10^10) ; this will be ok

Also be aware that defines are evaluated before selects, so calculations 
may be carried out on garbage, causing arithmetic errors, before record 
complexes are rejected.

The example is for VMS, but the same argument applies to all 
architectures - it's just the names, sizes, precisions, and ranges of 
the floating point formats that may change - consult your manuals.

Chris



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