Simple STL question - using STL algorithm on STL map

Indrawati

Hi If I'm not wrong, an element in STL map is a pair of . Given a map of a number of elements, how can I use STL algorithms like for_each(), accumulate(), etc on just the data, ignoring the keys? Thanks!

Michael Dunn

The parameter of the for_each predicate is the result of dereferencing an iterator, which in the case of a map is a map<key,value>::value_type, or pair<const key,value>. So the predicate can look at the second member of the pair:

#include <map>
#include <algorithm>
typedef map<char,int> mymap;

struct TotalIt
{
void operator()( mymap::value_type elt ) // same as "pair<const char,int> elt"
{
m_total += elt.second;
}

int m\_total;
TotalIt() : m\_total(0) { }

};

int main()
{
mymap M;

M\['a'\] = 1;
M\['b'\] = 2;
M\['c'\] = 3;
M\['d'\] = 4;
M\['e'\] = 5;

for\_each ( M.begin(), M.end(), TotalIt() );

}

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Paul Ranson

How about this?

#include <iostream>
#include <map>
#include <iterator>
#include <numeric>

template <typename I> class Pair2ndIterator : public I
{
public :
Pair2ndIterator ( I i ) : I ( i )
{
}
typename I::value_type::second_type& operator* ()
{
return (I::operator*()).second ;
}
} ;

template <typename I> Pair2ndIterator<I> MakePair2ndIterator ( I it )
{
return Pair2ndIterator<I> ( it ) ;
}

int main()
{
std::map<char, int> mymap ;
mymap.insert ( std::make_pair ( 'a', 10 )) ;
mymap.insert ( std::make_pair ( 'b', 11 )) ;
mymap.insert ( std::make_pair ( 'c', 12 )) ;
mymap.insert ( std::make_pair ( 'd', 13 )) ;
mymap.insert ( std::make_pair ( 'e', 14 )) ;

std::cout << "Accumulate ( mymap ) = " <<
    std::accumulate ( MakePair2ndIterator ( mymap.begin ()),
    MakePair2ndIterator ( mymap.end ()), 0 ) ;

return 0;

}

Paul