Re: [Boost-users] Using lambda for sort

From: Olaf van der Spek (olafvdspek_at_[hidden])
Date: 2007-08-08 12:17:52

On 8/8/07, chun ping wang <cablepuff_at_[hidden]> wrote:
> use bind.
> try using bind.
> namespace bll = boost::lambda;
>
> std::sort(countries1.begin(), countries1.end(),
> bll::bind(&countries_t::second, _1) < bll::bind(&countries_t::second, _2));

Does that compile for you?
It doesn't for me.
Note that second is not a function.

------ Build started: Project: Lambda Test, Configuration: Debug Win32 ------
Compiling...
Lambda Test.cpp
c:\vc\temp\lambda test\lambda test.cpp(26) : error C2039: 'second' :
is not a member of 'std::map<_Kty,_Ty>'
        with
        [
            _Kty=int,
            _Ty=int
        ]
c:\vc\temp\lambda test\lambda test.cpp(26) : error C2065: 'second' :
undeclared identifier
c:\vc\temp\lambda test\lambda test.cpp(26) : error C2039: 'second' :
is not a member of 'std::map<_Kty,_Ty>'
        with
        [
            _Kty=int,
            _Ty=int
        ]
c:\vc\temp\lambda test\lambda test.cpp(26) : error C2780: 'void
std::sort(_RanIt,_RanIt)' : expects 2 arguments - 3 provided
        c:\program files\microsoft visual studio
8\vc\include\algorithm(3109) : see declaration of 'std::sort'
Lambda Test - 4 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

>  On 8/8/07, Olaf van der Spek <olafvdspek_at_[hidden]> wrote:
> >
> > Hi,
> >
> > I'm trying to sort a map based on second, but I get all kinds of
> > compiler errors and I have no idea what's wrong. A simple _1 > _2
> > works fine, but that compares the maps themselves.
> >
> > Also, is there a better way to write the copy from map to vector or to
> > sort the map in general?
> >
> > Note that I'm using a map because I'm counting how often an integer or
> > string occurs.
> >
> > #include <boost/lambda/lambda.hpp>
> > #include <boost/lambda/bind.hpp>
> > #include <algorithm>
> > #include <map>
> > #include <vector>
> >
> > using namespace std;
> > using namespace boost::lambda;
> >
> > int _tmain(int argc, _TCHAR* argv[])
> > {
> >         typedef std::map<int, int> countries_t;
> >         countries_t countries;
> >         countries[0] = 5;
> >         countries[1] = 3;
> >         countries[2] = 1;
> >         typedef vector<countries_t::const_iterator>
> countries1_t;
> >         countries1_t countries1;
> >         for (countries_t::const_iterator i = countries.begin(); i !=
> > countries.end(); i++)
> >                 countries1.push_back(i);
> >         sort(countries1.begin(), countries1.end(), _1->second >
> _2->second);
> >         return 0;
> > }
> >
> > ------ Build started: Project: Lambda Test, Configuration: Debug Win32
> ------
> > Compiling...
> > Lambda Test.cpp
> > c:\vc\temp\lambda test\lambda test.cpp(23) : error C2819: type
> > 'boost::lambda::lambda_functor<T>' does not have an
> overloaded member
> > 'operator ->'
> >         with
> >         [
> >             T=boost::lambda::placeholder<1>
> >         ]
> >         did you intend to use '.' instead?
> > c:\vc\temp\lambda test\lambda test.cpp(23) : error C2039: 'second' :
> > is not a member of 'boost::lambda::lambda_functor<T>'
> >         with
> >         [
> >             T=boost::lambda::placeholder<1>
> >         ]
> > c:\vc\temp\lambda test\lambda test.cpp(23) : error C2819: type
> > 'boost::lambda::lambda_functor<T>' does not have an
> overloaded member
> > 'operator ->'
> >         with
> >         [
> >             T=boost::lambda::placeholder<2>
> >         ]
> >         did you intend to use '.' instead?
> > c:\vc\temp\lambda test\lambda test.cpp(23) : error C2039: 'second' :
> > is not a member of 'boost::lambda::lambda_functor<T>'
> >         with
> >         [
> >             T=boost::lambda::placeholder<2>
> >         ]
> > c:\vc\temp\lambda test\lambda test.cpp(23) : error C2780: 'void
> > std::sort(_RanIt,_RanIt)' : expects 2 arguments - 3 provided
> >         c:\program files\microsoft visual studio
> > 8\vc\include\algorithm(3109) : see declaration of 'std::sort'
> > Lambda Test - 5 error(s), 0 warning(s)
> > ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped
> ==========
> > _______________________________________________
> > Boost-users mailing list
> > Boost-users_at_[hidden]
> > http://lists.boost.org/mailman/listinfo.cgi/boost-users
> >
>
>
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
>