Re: To retrieve Keys or values of a Map
Saeed Amrollahi wrote:
Dear all
I have a question. Is there any generic algorithm or solution to
access the keys or values
of a map? I frequently face to such problem and I usually write a
function to copy
the keys/values of a map into a container let say a vector.
Many thanks, in advance, for your help.
I've written a utility roughly like this:
#include <string>
#include <algorithm>
#include <iostream>
#include <list>
#include <map>
#include <iterator>
namespace {
template <typename T1, typename T2>
const T1& take_first(const typename std::map<T1,T2>::value_type& pair) {
return pair.first;
}
}
template <typename T1, typename T2>
std::list<T1> keys(const std::map<T1,T2>& in) {
std::list<T1> klist(in.size());
std::transform(in.begin(), in.end(), klist.begin(), take_first<T1,T2>);
return klist;
}
int main() {
std::map<std::string, int> map;
map["test"] = 0;
map["test2"] = 0;
const std::list<std::string>& keys = ::keys(map);
std::copy(keys.begin(), keys.end(),
std::ostream_iterator<std::string>(std::cout, "\n"));
return 0;
}
Although this isn't exactly ideal, and would be a nice place to use a
lambda function in C++0x too.
There's a few annoying things about this though, the function take_first
has to live inside the anonymous namespace because it's not legal to
write the following, which given the absence of lambda functions would
be cleaner in my view:
template <typename T1, typename T2>
std::list<T1> keys(const std::map<T1,T2>& in) {
struct {
const T1& operator()(const typename std::map<T1,T2>::value_type&
pair) {
return pair.first;
}
} take_first;
std::list<T1> klist(in.size());
std::transform(in.begin(), in.end(), klist.begin(), take_first);
return klist;
}
I also never got type deduction for the function pointer argument to
std::transform to work, which I kind of expected it would, although
that's not exactly a problem.
Alan