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

30 Nov 2009

      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@gmail.com
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] boost lambda for processing std::vector
On Sun, Nov 29, 2009 at 7:14 PM, CR Koudella  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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