Re: [Boost-users] Using lambda for sort

From: chun ping wang (cablepuff_at_[hidden])
Date: 2007-08-08 21:26:23

sorry, my bad.

you should use std::pair<int, int> country_kv;

replace countries_t in the lambda code with country_kv;

this should work.

On 8/8/07, Andrej van der Zee <mavdzee_at_[hidden]> wrote:
>
> If you need multiple indexes over your data structures
> you could also consider to use boost::multi_index
> containers. In that case you can put all your data
> members (key and value) in a struct and define two (or
> more) indexes over it.
> Greets,
> Andrej
>
>
> --- 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
> >
>
>
>
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