C++如何将地图复制到向量
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C++ how to copy a map to a vector
提问by Hyman BeNimble
What's the best way in C++ to copy a pair from a map to vector? I'm doing this so I can subsequently sort the vector.
C++ 中将一对从映射复制到向量的最佳方法是什么？我这样做是为了随后可以对向量进行排序。
回答by wilhelmtell
vector<pair<K,V> > v(m.begin(), m.end());
or
或者
vector<pair<K,V> > v(m.size());
copy(m.begin(), m.end(), v.begin());
copy()is in <algorithm>.
copy()在<algorithm>.
回答by CTT
This should do what you want:
这应该做你想做的：
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <iterator>
using namespace std;
bool cmp(const pair<int, int> &p1, const pair<int, int> &p2)
{
return p1.second < p2.second;
}
int main()
{
map<int, int> m;
for(int i = 0; i < 10; ++i)
m[i] = i * -i;
vector<pair<int, int> > v;
copy(m.begin(), m.end(), back_inserter(v));
sort(v.begin(), v.end(), cmp);
for(int i = 0; i < v.size(); ++i)
cout << v[i].first << " : " << v[i].second << endl;
return 0;
}
回答by Spire
If you're using a std::map, it's already sorted by the key. Just create an iterator and iterate over the map from begin() to end() and you're done.
如果您使用的是 std::map，则它已按键排序。只需创建一个迭代器并从 begin() 到 end() 遍历地图，就完成了。
If you'd like to sort by something other than the map key, you can use the same iterator and push a copy of each element onto your vector as you iterate over the map.
如果您想按地图键以外的其他内容排序，您可以使用相同的迭代器，并在遍历地图时将每个元素的副本推送到您的向量上。
回答by Michael Kohne
If your purpose is just to sort by the type instead of the key, you might want to look at Boost::Bimap. It lets you access both parts of the map pair as keys. Presumably you could iterate over it in order of the second key just as easily as the first.
如果您的目的只是按类型而不是键排序，您可能需要查看Boost::Bimap。它允许您将映射对的两个部分作为键访问。据推测，您可以像第一个键一样轻松地按照第二个键的顺序对其进行迭代。
回答by dirkgently
A mapstores a pair -- a key and a value. Which part do you want to copy? Or, do you want to copy both to two distinct vectors?
Amap存储一对——一个键和一个值。您要复制哪个部分？或者，您想将两者都复制到两个不同的vectors 吗？
I want to copy both. Once that's done, I need to figure out how to sort the vector by the secondvalue in the pair.
我想复制两个。完成后，我需要弄清楚如何按对中的第二个值对向量进行排序。
template <class V>
struct sort_by_val {
bool operator()(V const& l, V const& r) {
return // ...
}
};
vector<pair<K, V> > outv(map.begin(), map.end());
sort(outv.begin(), outv.end(), sort_by_val());
回答by Andrew Shepherd
Assuming you want to copy the key and the value:
假设您要复制键和值：
std::map<Foo, Bar> m;
// Map gets populated
// (...)
// Copying it to a new vector via the constructor
std::vector<std::pair<Foo, Bar>> v(m.begin(), m.end());
// Copying it to an existing vector, erasing the contents
v.assign(m.begin(), m.end());
// Copying it to the back of an existing vector
v.insert(v.end(), m.begin(), m.end());
回答by Richard Corden
You can use a different map (or set) and use transform to do the sorting as you insert:
您可以使用不同的地图（或集合）并在插入时使用变换进行排序：
#include <map>
#include <algorithm>
typedef std::map<unsigned int, signed char> MapType1;
typedef std::map<MapType1::mapped_type, MapType1::key_type> MapType2;
struct SwapPair
{
MapType2::value_type operator()(MapType1::value_type const & v)
{
return std::make_pair (v.second, v.first);
}
};
int main ()
{
MapType1 m1;
for(int i = 0; i < 10; ++i)
m1[i] = i * -i;
MapType2 m2;
std::transform (m1.begin ()
, m1.end ()
, std::inserter (m2, m2.end ())
, SwapPair ());
}
I forgot to add that if you need to do this a lot then it might be better just to use a boost multi-indexcontainer.
我忘了补充一点，如果您需要经常这样做，那么使用 boost多索引容器可能会更好。
