Paul A. Bristow wrote:
-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org]
On
Behalf Of Thijs van den Berg Sent: 23 November 2008 20:11 To: boost@lists.boost.org Subject: Re: [boost] [math distributions] where to check for validity of
distribution
variables?
Paul A. Bristow wrote:
-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org]
On
Behalf Of Thijs van den Berg Sent: 22 November 2008 14:48 To: boost@lists.boost.org Subject: Re: [boost] [math distributions] where to check for validity of
distribution
variables?
John Maddock wrote:
Thijs van den Berg wrote:
>>>> What do you think? We might turn "having valid parameters" >>>> into a property of *all* distribution. As an alternative, we >>>> might add a non member function bool valid<distributionType... >>>> but that wouldn't allow for caching validation in e.g. a >>>> constructor >>>> >>>> >> Sounds fine to me. >> >> > thats great! What's your opinion on the fact that you can only set > parameter in the constructor? > E.g. the normal distribution does a parameter check in the > constructor, and those parameters can't change after that. > > That's what the existing distributions do. In fact we could omit most of the subsequent parameter checking code if we could figure out whether the error handlers will throw or not on error (in fact we *can* get this information at compile time and make the subsequent checks a no-op if we know that the constructor would have thrown on error... we just ran out of time on that refinement).
I don't understand this, it has to do with my lack of knowledge on
this...
If you ensure
that the parameters get checked in the constructor, why would that check
*not* throw
an error when needed?
Often you just want to return a NaN, infinity or a 'best guess'.
So John devised the rather complicated - but very useful - policies.
Most important they are needed to provide the C++ Standard library C-style error behaviour.
enum error_policy_type { throw_on_error = 0, // throw an exception. errno_on_error = 1, // set ::errno & return 0, NaN, infinity or best
guess..
ignore_error = 2, // return 0, NaN, infinity or best guess. user_error = 3 // call a user-defined error handler.
Hi Paul, thanks for the info! I'll have to delve into those concepts a bit more I see. Regarding the checking in non member functions for the validity of the distribution: would it be possible for the distributin contructor to fail
runtime (before the
distribution parameters can be validated)? Would it be safe for me to assume that * if a distribution validate its parameters in the constructor * if the constructor doesn't throw an error then * there is no need to check the distribution parameters anymore after
construction,
e.g. in a non-member function. if so, then I would send in new distributions with only checks in the
constructor
another option is to check parameters (and throw errors) in the
distribution parameter
access member funtions like "RealType location() const {return
m_location;}". A
possible drawback in that is that sometimes a *combination* of parameters
is valid or
not.
As I recall, because the chosen policy for the constructor might not cause it to throw, we decided on 'belt and braces' repeated checks, even if it proved redundant (because the check is cheap). If there are other combinations that might cause trouble, this means this is even more sensible.
ah! you're saying you van have one type of policy for the distribution (construtor) and another policy type in some non-member function like pdf. that explains the things I'm seeing! A final question regarding the error checking is this: Suppose a distribution has a couple of valid an invalid parameters. E.g. normal(2,0), whith has a valid mean=2 and invalid std=0. Formally that would make the distribution object invalid... There are at least two possible view on what to do with non-member fuctions. 1) Make *all* of them return NaN because the distribution in invalid. This is a mathematical interpretation or 2) (current implementation) try to give an answer when possible, this is a "can we calculate the result?" interpretation. In this case we can calculate the mean (it's 2), but we can't calculate the pdf because that would give an divide by zero. I'm asking this because I'd like to stick to you're approach with new code, and *not* because I want to discuss a preference for any of the two... :)
Compile time might be tricky depending on the complicity of the parameter
validation
code, but simple range check on the parameters could be done compile
time.
What
mechanism are your thinking about regarding compile checking, e.g. that
scale>0?
The complexity of policy options make it much simpler to do a run-time check.
You'd save a tiny bit on run-time - but probably pay in compile time?
Paul
I think the same about that. runtime is good enough, and even unavoidable
if you
would allow distribution parameters to be set runtime. Btw ,why isn't that
implemented
(allowing distribution paramters to be set riu-time)? Lack of
implementation time
(postponed to future versions), of is it a design choice?
As I recall, construction (and destruction) is cheap (compared to a cdf, pdf etc) , it is simplest to make users construct a new distribution.
yes, I agree, that's a good way to implement the problem I described with the current interface! Works fine!
Paul
--- Paul A. Bristow Prizet Farmhouse Kendal, UK LA8 8AB +44 1539 561830, mobile +44 7714330204 pbristow@hetp.u-net.com
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