[Boost-Users] boost::lambda how to question

From: Jaakko Jarvi (jajarvi_at_[hidden])
Date: 2003-08-25 13:48:48

In our last exciting episode Eric Twietmeyer wrote:

It should be doable, but it's not the easiest thing to do.
Look at the section 5.11. on nesting STL algorithm invocations:

http://www.boost.org/libs/lambda/doc/ar01s05.html#sect:nested_stl_algorithms

  Jaakko

> 
> Hello,
> 
> I'm been having great fun getting to know and starting to use the lambda
> library over the past month or so.  I can't believe how well the optimizing
> compiler I have (Vc7.1) is able to convert these lambda expressions used in
> for_each or find_if into tight loops.
> 
> However, there I things I'm certain are possible, but I haven't been able to
> figure out.  More examples of using the library than are found in the doc
> would be extremely useful.
> 
> For instance, how do I do the equivalent of the following using a single
> for_each with suitable lambda expression as operator?
> 
> using namespace boost::lambda;
> std::vector<int> vec1;
> std::vector<int> vec2;
> 
> struct tOp
> {
>    std::vector<int>& m_vec;
>    tOp( std::vector<int>& vec ) : m_vec( vec ) {}
>    void operator()( int entry )
>    {
>       std::vector<int>::iterator it = std::find_if( m_vec.begin(),
> m_vec.end(), _1 == entry );
>       if ( it != m_vec.end() ) m_vec.erase( it );
>    }
> };
> 
> // How to use lambda expression here for third arg?
> std::for_each( vec1.begin(), vec1.end(), tOp( vec2 ));
> 
> Basically, I have two lists (vectors) that overlap.  I want to erase all
> elements from the one vector from the other vector, i.e. determine if the
> element from vec1 exists in vec2, and if so erase it from vec2.
> 
> If this is possible and relatively straight forward using a lambda
> expression, I would love to see an example.
> 
> Thanks in advance,
> 
> Eric Twietmeyer
> 
> PS It looks like something of this sort can be done with std::set_difference
> if one outputs into a third vector, but for argument's sake, let's assume I
> don't want to introduce a third vector.
> 
> 
>