Re: [Boost-users] Using lambda for sort

From: chun ping wang (cablepuff_at_[hidden])
Date: 2007-08-08 12:01:35

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));

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
> ==========
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