Re: binomial distribution setter function

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2020-01-20 08:35:17

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.

Regards, Alex