[Boost-users] iterate over key_type of map

From: gast128 (gast128_at_[hidden])
Date: 2006-07-24 06:04:52

Dear all,

in our company we use the STL map frequently. However one often wants to 
iterate over the key_type only (or mapped type, or make desicions for the key 
type, and take action on its corresponded map type).

With the transform_iterator this is possible, but the resulting code is not an 
improvement over writing the loop explicitly. For example I want to search for 
the maximum string length:

typedef std::map<std::string, double> StringDoubleMap;
typedef boost::function<StringDoubleMap::key_type (const 
StringDoubleMap::value_type&)> Function;
typedef boost::transform_iterator<Function, StringDoubleMap::const_iterator> 
TransformIterator;

StringDoubleMap mp = boost::assign::map_list_of("George", 0.0)
                                               ("Bill",   0.0)
                                               ("Ronald", 0.0)
                                               ("Jimmy",  0.0);
Function fc = boost::bind(&StringDoubleMap::value_type::first, _1);

TransformIterator itBegin(mp.begin(), fc);
TransformIterator itEnd(mp.end(), fc);

TransformIterator it = std::max_element(itBegin, itEnd, boost::bind
(&std::string::length, _1) < boost::bind(&std::string::length, _2));
if (it != itEnd)
{
   std::cout << it->c_str() << ' ' << it->length() << std::endl;
}

A select1st or select2nd would already a great help.

Wkr, 
me