Re: [Boost-users] Using lambda for sort

From: Andrej van der Zee (mavdzee_at_[hidden])
Date: 2007-08-08 20:57:50

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 ==========
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> Boost-users_at_[hidden]
>
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> 

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