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

Subject: [Boost-users] boost lambda for processing std::vector
From: CR Koudella (ckoudell_at_[hidden])
Date: 2009-11-28 08:40:24

Hi, 

I am trying to understand if boost.lambda is well suited for the quick processing of an std::vector<double>.
I must apologize for my sketchy understanding of the library and this is the reason why I turn to you for 
your help. 

Imagine that we are given an std::vector<double>, say, x', of some size and that we need to extend x' 
somehow to x in order to use it as an argument for some function f.

So, say x' = (3,3,3) and x can be obtained by some map as x = (3,1,3,1,3). 

Ideally, this problem would fit into function composition using bind or lambda::bind. Define g(x) = f[x(x')].
The composition operation is standard. What is not for me is how to write the function x(x') using 
boost.lambda.

It is always possible to write a quick C++ functor that will encode in its state the map defining how to
generate x from x'. This would typically proceed by constructing a return std::vector<double> to be 
populated according to the map. But, can this be done easily with boost.lambda also? I am not sure. I 
seem to be finding it difficult to formulate expressions that return types such as std::vectors, while 
operating on the vector elements. I can see that the library is well suited to write small functors on the fly 
that will operate on container items (as is documented), but when it comes to full containers, I am kind of 
stuck. Any attempt seems to be leading to complicated script and dead ends, which makes me think that 
I am better off relegating the case to a small C++ functor written elsewhere in the file.

Not sure if you see where I am coming from, but in a functional programming language or a higher level scripting 
language such as python, this kind of operation is quite easy to do on the fly. What I am trying to figure out 
is how much of the 'in-place definition of code in c++' should reasonably be handled with boost.lambda. Again, 
I can clearly see the power of boost.lambda when applied to std::for_each or std::transform, etc..., but I am 
not sure how to leverage it beyond that and I haven't seen many examples out there trying to do that either.

Insights and help greatly appreciated, 

Many thanks, 
ChrisK