Re: binomial distribution setter function

From: Kemin Zhou (keminz_at_[hidden])
Date: 2020-03-17 21:26:11

Sorry for not doing the speed check.  My argument is purely based on
looking at the code and counting the number of executions.
The cost of constructing an object, and the code of updating only.
In this situation, I have a tight loop, with the input values already
validated thus can save the extra
check inside the constructor.

On Mon, Jan 20, 2020 at 2:15 AM Paul A Bristow via Boost <
boost_at_[hidden]> wrote:

>
>
> > -----Original Message-----
> > From: Boost <boost-bounces_at_[hidden]> On Behalf Of Alexander
> Grund via
> > Boost
> > Sent: 20 January 2020 08:35
> > To: boost_at_[hidden]
> > Cc: Alexander Grund <alexander.grund_at_[hidden]>
> > Subject: Re: [boost] binomial distribution setter function
> >
> > Am 18.01.20 um 07:57 schrieb Kemin Zhou via Boost:
> > > I am looking at the binomial.hpp file.
> > > I need to use the binomial object in a loop which requires to have
> > > high performance.
> > > I am thinking constructing the binomial object each time in the loop
> > > would be less efficient than if I construct a single object, the each
> > > time reset the p parameter and use the object.
> > >
> > > I am not sure why the setter method was not provided.
> > >
> > > 296       RealType success_fraction() const
> > > 297       { // Probability.
> > > 298         return m_p;
> > > 299       }
> > > 300       void set_success_fraction(RealType p) {
> > > 301          m_p=p;
> > > 302       }
> > > Line 296 is the getter method, I added the setter method at line 300.
> >
> > As usual: Did you measure before making assumptions about performance?
> > Next: Did you check what the ctor does? What is your reasoning for your
> > statement?
> >
> > This is not meant to sound harsh but rather spark usual scientific work
> practices.
> >
> > You'll see that the constructor does nothing but check invariants. Your
> setter does
> > not do so and hence is wrong (for some definition of wrong as usual) So
> the only
> > overhead can be due to check of valid parameters. Depending on how you
> pass in
> > the arguments this can even be removed, so try it first and measure
> where your
> > performance suffers or use e.g. godbolt to check the assembly to verify
> > assumptions.
> >
> > Then the better solution would be to provide a ctor that does not do
> verification,
> > probably the policy system can be used if it isn't already. Again it
> needs to be
> > argued why this would be required and how much benefit it brings.
>
> I concur with this assessment.
>
> I suspect that you presume that construction is expensive, when it is
> really very cheap, only carrying out some quick sanity checks, and a few
> assignments.
>
> You need to be quite certain that these handful of instructions are on
> your critical path before doing something that will expose you to risks
> from passing a bad parameter.
>
> Paul A. Bristow
>
> PS Reminder, it you don't put your stuff inside try'n'catch blocks, you
> won't get any error messages helping you see what went wrong 😊
>
> But of course that will slow things down a bit.  But useful to have the
> checks until you are certain that your code is correct?
>
>
>
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-- 
Kemin Zhou
858 366 8260