Re: [boost] [accumulators] Help understanding implementation of weighted accumulators

Sorry to revive an old thread, but I'd still be interested in the answers to these questions. -Gabe On Wed, May 1, 2013 at 11:06 PM, Gabriel Redner wrote:

Hi Eric,

Thanks very much for your response. That does help, and I have some follow-ups.

First, let me make sure I am understanding correctly. If my Sample is some FancyType whose default constructor does not do the right thing (or perhaps does not exist), I should do:

accumulator_set > acc(sample = some_initial_fancytype_value);

Is that right? If so, I would say that: - This usage is not mentioned in the docs anywhere, and - It seems strange to overload the meaning of 'sample' in this way. Wouldn't it be clearer and accomplish the same goal to use a different name? "initialValue" perhaps? - Do all accumulators support this feature? I can imagine some that might not (indeed I am implementing one). This is not mentioned in the docs either.

Second, I have a sort-of-question-but-mostly-a-kvetch about defining keywords for accumulators that need initialization parameters (e.g. tail). The library itself uses the macro BOOST_PARAMETER_NESTED_KEYWORD for this, which is not documented. I do not understand what this macro does - why the name, why the alias, etc? My own accumulators need initialization parameters, and I have just been aping what the library itself does - using the macro although it's not documented, and hoping for the best. It seems to be working, but I'd like to understand if this is the right approach, whether I can rely on this undocumented macro, and what it's doing. Can you shed any light?

Thanks, -Gabe

On Wed, May 1, 2013 at 8:23 PM, Eric Niebler wrote:

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On 5/1/2013 1:13 PM, Gabriel Redner wrote:

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Hi folks,

I am trying to implement a weighted accumulator, but I'm having trouble understanding a few things about the existing ones.

Looking at the implementation of weighted_sum, I see (weighted_sum.hpp:37):

template<typename Args> weighted_sum_impl(Args const &args) : weighted_sum_( args[parameter::keyword<Tag>::get() | Sample()] * numeric::one<Weight>::value ) { }

My questions are:

- It's taking the first sample from the args and multiplying it by a weight of 1. Isn't this wrong - ignoring the real weight of this sample? Shouldn't this produce incorrect results?

It's not taking the first sample. It's (optionally) taking an element representing zero. In most cases, a default-constructed Sample() is satisfactory for a zero element, but in some cases it's not. Consider what happens if your Sample type is std::vector<int>, and all your samples are going to be 3-element vectors. Then your zero is a std::vector<int> with 3 zeros. Any other starting value would cause all future computation to fail.

And we multiply by one to make the type promotion come out right.

...

- I've never understood anyways how there can be an initial argument during construction of the accumulator. I haven't found any explanation in the docs beyond "Maybe there is an initial value in the argument pack". Maybe? How?

I hope that clears it up.

-- Eric Niebler Boost.org http://www.boost.org

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