CopySecond is a container code snippets to copy the second element of the std::pairs in a std::vector.
CopySecond using the
STL only
#include <vector> ///CopySecond copies the second std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopySecond.htm template <class T, class U> const std::vector<U> CopySecond(const std::vector<std::pair<T,U> >& v) { std::vector<U> w; const int size = static_cast<int>(v.size()); for (int i=0; i!=size; ++i) { w.push_back(v[i].second); } return w; }
CopySecond using
Boost
#include <algorithm> #include <vector> #include <boost/lambda/bind.hpp> #include <boost/lambda/lambda.hpp> ///CopySecond copies the second std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopySecond.htm template <class T, class U> const std::vector<U> CopySecond(const std::vector<std::pair<T,U> >& v) { std::vector<U> w; std::transform( v.begin(), v.end(), std::back_inserter(w), boost::lambda::bind(&std::pair<T,U>::second, boost::lambda::_1) ); return w; }
CopySecond test
#include <vector> ///CopyFirst copies the first std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopyFirst.htm template <class T, class U> const std::vector<T> CopyFirstFor(const std::vector<std::pair<T,U> >& v) { std::vector<T> w; const int size = static_cast<int>(v.size()); for (int i=0; i!=size; ++i) { w.push_back(v[i].first); } return w; } ///CopySecond copies the second std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopySecond.htm template <class T, class U> const std::vector<U> CopySecondFor(const std::vector<std::pair<T,U> >& v) { std::vector<U> w; const int size = static_cast<int>(v.size()); for (int i=0; i!=size; ++i) { w.push_back(v[i].second); } return w; } #include <algorithm> #include <vector> #include <boost/lambda/bind.hpp> #include <boost/lambda/lambda.hpp> ///CopyFirst copies the first std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopyFirst.htm template <class T, class U> const std::vector<T> CopyFirst(const std::vector<std::pair<T,U> >& v) { std::vector<T> w; std::transform( v.begin(), v.end(), std::back_inserter(w), boost::lambda::bind(&std::pair<T,U>::first, boost::lambda::_1) ); return w; } ///CopySecond copies the second std::pair elements from a std::vector of std::pairs //From http://www.richelbilderbeek.nl/CppCopySecond.htm template <class T, class U> const std::vector<U> CopySecond(const std::vector<std::pair<T,U> >& v) { std::vector<U> w; std::transform( v.begin(), v.end(), std::back_inserter(w), boost::lambda::bind(&std::pair<T,U>::second, boost::lambda::_1) ); return w; } #include <cassert> #include <string> int main() { //C++11 initializer list const std::vector<std::pair<int,std::string> > v = { {0,"Zero"}, {1,"One"}, {2,"Two"}, {3,"Three"}, {4,"Four"} }; { const std::vector<int> w1 = CopyFirstFor(v); const std::vector<int> w2 = CopyFirst(v); const std::vector<int> expected = { 0,1,2,3,4 }; assert(w1 == expected); assert(w2 == expected); } { const std::vector<std::string> w1 = CopySecondFor(v); const std::vector<std::string> w2 = CopySecond(v); const std::vector<std::string> expected = { "Zero", "One", "Two", "Three", "Four" }; assert(w1 == expected); assert(w2 == expected); } }