C++ 使用 std::sort 对集合进行排序
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Sorting Sets using std::sort
提问by M.A
I would like to know if we can sort a pre created set. When I first create the set s_p2, I sort using a different element point.getLength(). but after user input i would like to sort the items according to the x value point.getX(). How i do this ?
我想知道我们是否可以对预先创建的集合进行排序。当我第一次创建集合 s_p2 时，我使用不同的元素 point.getLength() 进行排序。但在用户输入后，我想根据 x 值 point.getX() 对项目进行排序。我怎么做？
It seems like set container does not have a sort function. And i am advised to use vector. But sets are able to store unique elements only.
似乎设置容器没有排序功能。我被建议使用矢量。但是集合只能存储唯一元素。
Q1: How can i sort a set depending on the criteria
Q1：如何根据条件对集合进行排序
Q2: If set is unable to do this than which STL container is the best choice and how can i sort the elements in the container.
Q2：如果 set 无法做到这一点，那么哪个 STL 容器是最佳选择，我该如何对容器中的元素进行排序。
回答by Yakk - Adam Nevraumont
You cannot resort a set, how it sorts is part of the type of the particular set. A given sethas a fixed set order that cannot be changed.
你不能求助于 a set，它如何排序是特定类型的一部分set。给set定的集合顺序是固定的，无法更改。
You could create a new setwith the same data relatively easily. Just create a new setthat sorts based on the new criteria.
您可以set相对轻松地使用相同的数据创建一个新的。只需set根据新标准创建一个新的排序。
If you want to use the two sets in the same code, you'll have to abstract the access to the underlying set.
如果要set在同一代码中使用这两个s，则必须抽象对底层set.
Now, if you are doing rare reads and modifications, using a vectorthat you sort manually is often a better idea. You can remove duplicates by using the std::unique-eraseidiom.
现在，如果您正在执行罕见的读取和修改，则使用vector手动排序的 a 通常是一个更好的主意。您可以使用std::unique-erase习语删除重复项。
回答by Rob?
std::setstores its members in a sorted fashion. If you walk through the set from .begin()to .end(), you will have a sorted list of items.
std::set以排序的方式存储其成员。如果您从.begin()到遍历集合.end()，您将获得一个排序的项目列表。
If you don't like the default sort criteria, you may supply a 2nd template parameter to std::set<>
如果你不喜欢默认的排序标准，你可以提供第二个模板参数 std::set<>
回答by edW
You can have two sets and keep them in sync or copy one to the other.
您可以拥有两组并保持同步或将一组复制到另一组。
#include <iostream>
#include <set>
using namespace std;
struct AB
{
AB(int a,int b) : _a(a),_b(b) {}
int _a;
int _b;
};
struct byA
{
bool operator () (const AB& lhs, const AB& rhs)
{
return lhs._a <= rhs._a;
}
};
struct byB
{
bool operator () (const AB& lhs, const AB& rhs)
{
return lhs._b <= rhs._b;
}
};
typedef set<AB,byA> ByA;
typedef set<AB,byB> ByB;
typedef ByA::const_iterator ByAIt;
typedef ByB::const_iterator ByBIt;
void getByB(const ByA &sA,ByB &sB)
{
for(ByAIt iter=sA.begin(); iter!=sA.end();++iter) {
const AB &ab=*iter;
sB.insert(ab);
}
}
int main(int argc, const char **argv)
{
ByA sA;
sA.insert(AB(3,6));
sA.insert(AB(1,8));
sA.insert(AB(2,7));
ByB sB;
getByB(sA,sB);
cout << "ByA:" << endl;
for(ByAIt iter=sA.begin(); iter!=sA.end();++iter) {
const AB &ab=*iter;
cout << ab._a << "," << ab._b << " ";
}
cout << endl << endl;
cout << "ByB:" << endl;
for(ByBIt iter=sB.begin(); iter!=sB.end();++iter) {
const AB &ab=*iter;
cout << ab._a << "," << ab._b << " ";
}
cout << endl;
return 0;
}
program returns: ByA 1,8 2,7 3,6
程序返回：ByA 1,8 2,7 3,6
ByB: 3,6 2,7 1,8
乙：3,6 2,7 1,8
