Boost logo

Boost Users :

From: John Eddy (johneddy101_at_[hidden])
Date: 2008-03-06 11:40:33


John Maddock <john <at> johnmaddock.co.uk> writes:

>
> johneddy101 <at> comcast.net wrote:
> >> Hello,
> >> I realize that this library is not yet part of the boost
> >> distribution but I am using it and have come accross an issue. I am
> >> concerned that the logic in the various check functions may be
> >> flawed. Take for example the check_uniform function. The logic is
> >> as follows:
> >> if lower Ok && upper Ok && lower < upper
> >> return true
> >> else
> >> raise error stating that lower must be less than upper
> >> return false
> >> end if
> >>
> >> The functions used to determine if lower and upper are ok will raise
> >> errors if appropriate. If the policy is such that raising an error
> >> does not cause a throw, then control will return to the
> >> check_uniform method with an indication that lower or upper is not
> >> ok and we will enter the else block. Herein lies the problem.
> >> Consider the following scenario: creating a uniform distribution
> >> with:
> >> lower = 0.0
> >> upper = postitive infinity
> >> Check upper will fail and send a message about the infinity. Enter
> >> the else block.
> >> A message will be raised that states that the lower parameter is not
> >> less than the upper.
> >> This is my concern. I believe the second message is unnecessary and
> >> potentially incorrect/misleading. Perhaps logic like shown below
> >> would be more appropriate:
> >> if lower not Ok || upper not Ok then
> >> return false
> >> else if lower >= upper
> >> raise error stating that lower must be less than upper
> >> return false
> >> else
> >> return true
> >> end if
>
> Yep, will change as suggested: note however you can only actually observe
> the domain_error handler being called twice (with different messages) if you
> are using a user-defined error handler *and* that handler doesn't throw an
> exception.
>
> Paul, I think this one was one of yours? Are we likely to need to give the
> other distros a once-over to check we don't do this elsewhere as well?
>
> Thanks for spotting this, John.
>

I think giving the others a look is a very good idea. There are some other
places where I noticed possible flaws.

- In the check_scale function in common_error_handling, the error reported
is "scale must be greater than 0" regardless of the problem. If scale is
positive infinity for example, this is a bit misleading. Perhaps separate
blocks would be appropriate. A block for the infinite test could report the
value as infinite and a block for the <= 0 test would give the current report.

Note that the above occurs in many places including check_weibull_shape,
check_weibull_x, check_alpha and check_beta (beta distribution), etc.

- For the lognormal distribution, the location parameter is the mean correct?
If that is the case, should there not be a check to see that it is >= 0? At
the very least, should there not at least be an isfinite check (using
check_location for example)?

- For the Bernoulli distribution, is it not the case that 0 < p < 1? I
believe you allow 0 <= p <= 1.

- For the exponential distribution, there is no check that lambda is finite.
Should there be?

- For the extreme value distribution, there is no check that the scale is
finite or that the location is finite. Should there be?

I think that similar audits of the other distributions will turn up similar
issues.

Thanks,
John


Boost-users list run by williamkempf at hotmail.com, kalb at libertysoft.com, bjorn.karlsson at readsoft.com, gregod at cs.rpi.edu, wekempf at cox.net