Re: [Boost-users] boost lambda for processing std::vector

Subject: Re: [Boost-users] boost lambda for processing std::vector
From: CR Koudella (ckoudell_at_[hidden])
Date: 2009-11-30 08:52:54

Hi OvermindDL1, 

thanks again. You are bringing up yet another library here. I need to do some study. 
Will write again.

Thanks, 
ChrisK

> Date: Sun, 29 Nov 2009 19:47:49 -0700
> From: overminddl1_at_[hidden]
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] boost lambda for processing std::vector
> 
> On Sun, Nov 29, 2009 at 7:14 PM, CR Koudella <ckoudell_at_[hidden]> wrote:
> >
> > thanks, I have started to look at the doc, but I am not there yet.
> >
> > imagine we have something like this, without too much thought and excuse
> > the basic style laid out here:
> >
> > struct XMapVec
> > {
> > public:
> > XMapVec(const std::vector<double> & vY, const std::vector<unsigned> & vMap)
> > : _vY(vY), _vMap(vMap), _vBuffer(vMap)
> > {
> > }
> >
> > std::vector<double> operator()(const std::vector<double> & vX) const
> > {
> > unsigned nCnt = 0
> > for (int nI = 0; nI < _vBuffer.size(); ++nI)
> > {
> >     if (_vMap[nI])
> >     {
> >         _vBuffer[nI] = vX[nCnt];
> >         ++nCnt;
> >     }
> > }
> >
> > return _vBuffer;
> > }
> > private:
> >
> > // assume _vY same size as _vMap
> > // the state
> > std::vector<double> _vY;
> > // binary Map: if element = 1, then update corresponding entry in _vBuffer
> > std::vector<unsigned> _vMap;
> > // set initial state at construction and mutate relevant elements on () call
> > mutable std::vector<double> _vBuffer;
> > };
> >
> > this is not nicely written, but more importantly it is not nice to have this
> > in
> > a dedicated functor. the question is, how to rewrite this in terms of a
> > lambda
> > expression, which can then be used in a function composition operation?
> 
> Looks like an accumulator...
> Well a completely lazy version of that could be (using both phoenix and fusion):
> 
> 
> std::vector<double> vY;
> std::vector<unsigned> vMap;
> std::vector<double> vBuffer;
> 
> // Assuming vY is the same size as vMap and vBuffer
> auto XMapVec =
> 	let(_a=begin(vY),_b=begin(vMap),_c=begin(vBuffer),_d=end(vY),_e=(unsigned)0)
> 	[
> 		for_(nothing,_a!=_d,(++_a,++_b,++_c))
> 		[
> 			if_(*_b)
> 			[
> 				(*_c) = _1(_e),
> 				++_e
> 			]
> 		]
> 	];
> 
> This is not nicely written either, but demonstrates how it can be used anyway.
> 
> This is still completely untested, I am still at work.
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