为什么gquiz1的上界值小于键值

Question

// CPP Program to demonstrate the
// implementation of multiset
#include <iostream>
#include <iterator>
#include <set>

using namespace std;

int main()
{
    // empty multiset container
    multiset<int, greater<int> > gquiz1;

    // insert elements in random order
    gquiz1.insert(40);
    gquiz1.insert(30);
    gquiz1.insert(60);
    gquiz1.insert(20);
    gquiz1.insert(50);

    // 50 will be added again to
    // the multiset unlike set
    gquiz1.insert(50);
    gquiz1.insert(10);

    // printing multiset gquiz1
    multiset<int, greater<int> >::iterator itr;
    cout << "\nThe multiset gquiz1 is : \n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << *itr << " ";
    }
    cout << endl;

    // assigning the elements from gquiz1 to gquiz2
    multiset<int> gquiz2(gquiz1.begin(), gquiz1.end());

    // print all elements of the multiset gquiz2
    cout << "\nThe multiset gquiz2 \n"
            "after assign from gquiz1 is : \n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }
    cout << endl;

    // remove all elements up to element
    // with value 30 in gquiz2
    cout << "\ngquiz2 after removal \n"
            "of elements less than 30 : \n";
    gquiz2.erase(gquiz2.begin(), gquiz2.find(30));
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }

    // remove all elements with value 50 in gquiz2
    int num;
    num = gquiz2.erase(50);
    cout << "\ngquiz2.erase(50) : \n";
    cout << num << " removed \n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }

    cout << endl;

    // lower bound and upper bound for multiset gquiz1
    cout << "\ngquiz1.lower_bound(40) : \n"
        << *gquiz1.lower_bound(40) << endl;
    cout << "gquiz1.upper_bound(40) : \n"
        << *gquiz1.upper_bound(40) << endl;

    // lower bound and upper bound for multiset gquiz2
    cout << "gquiz2.lower_bound(40) : \n"
        << *gquiz2.lower_bound(40) << endl;
    cout << "gquiz2.upper_bound(40) : \n"
        << *gquiz2.upper_bound(40) << endl;

    return 0;
}

gquiz1 的上限打印 30，key 为 40。gquiz2 上限似乎只给出更高的值。我认为这与 multiset gquiz1 中的多个相似元素有关，但是在所有擦除函数之后在 gquiz2 中插入 50 两次时，gquiz2 上限仍然给出比键更高的值，而 gquiz1 给出更低的值。请帮助。

修改两次插入50的代码。 我试过插入 50 两次代码：

// CPP Program to demonstrate the
// implementation of multiset
#include <iostream>
#include <iterator>
#include <set>

using namespace std;

int main()
{
    // empty multiset container
    multiset<int, greater<int> > gquiz1;

    // insert elements in random order
    gquiz1.insert(40);
    gquiz1.insert(30);
    gquiz1.insert(60);
    gquiz1.insert(20);
    gquiz1.insert(50);

    // 50 will be added again to
    // the multiset unlike set
    gquiz1.insert(50);
    gquiz1.insert(10);

    // printing multiset gquiz1
    multiset<int, greater<int> >::iterator itr;
    cout << "\nThe multiset gquiz1 is : \n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << *itr << " ";
    }
    cout << endl;

    // assigning the elements from gquiz1 to gquiz2
    multiset<int> gquiz2(gquiz1.begin(), gquiz1.end());

    // print all elements of the multiset gquiz2
    cout << "\nThe multiset gquiz2 \n"
            "after assign from gquiz1 is : \n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }
    cout << endl;

    // remove all elements up to element
    // with value 30 in gquiz2
    cout << "\ngquiz2 after removal \n"
            "of elements less than 30 : \n";
    gquiz2.erase(gquiz2.begin(), gquiz2.find(30));
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }

    // remove all elements with value 50 in gquiz2
    int num;
    num = gquiz2.erase(50);
    cout << "\ngquiz2.erase(50) : \n";
    cout << num << " removed \n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << *itr << " ";
    }

   gquiz2.insert(50);
   gquiz2.insert(50);

    cout << endl;

    // lower bound and upper bound for multiset gquiz1
    cout << "\ngquiz1.lower_bound(40) : \n"
        << *gquiz1.lower_bound(40) << endl;
    cout << "gquiz1.upper_bound(40) : \n"
        << *gquiz1.upper_bound(40) << endl;

    // lower bound and upper bound for multiset gquiz2
    cout << "gquiz2.lower_bound(40) : \n"
        << *gquiz2.lower_bound(40) << endl;
    cout << "gquiz2.upper_bound(40) : \n"
        << *gquiz2.upper_bound(40) << endl;

    return 0;
}

除外：

The multiset gquiz1 is : 
60 50 50 40 30 20 10 

The multiset gquiz2 
after assign from gquiz1 is : 
10 20 30 40 50 50 60 

gquiz2 after removal 
of elements less than 30 : 
30 40 50 50 60 
gquiz2.erase(50) : 
2 removed 
30 40 60 

gquiz1.lower_bound(40) : 
40
gquiz1.upper_bound(40) : 
30
gquiz2.lower_bound(40) : 
40
gquiz2.upper_bound(40) : 
30

实际的：

The multiset gquiz1 is : 
60 50 50 40 30 20 10 

The multiset gquiz2 
after assign from gquiz1 is : 
10 20 30 40 50 50 60 

gquiz2 after removal 
of elements less than 30 : 
30 40 50 50 60 
gquiz2.erase(50) : 
2 removed 
30 40 60 

gquiz1.lower_bound(40) : 
40
gquiz1.upper_bound(40) : 
30
gquiz2.lower_bound(40) : 
40
gquiz2.upper_bound(40) : 
50

Answer 1

你的问题在这里

multiset<int, greater<int> > gquiz1;

因为您使用的是 greater<int>，所以 upper_bound 和 lower_bound 的通常含义已更改。

upper_bound 将返回值为的第一个元素少于键，lower_bound 将返回值为的第一个元素大于或等于到关键。

upper_bound 的精确定义是，对于 map<T,C> 和给定的键，它返回 C(key, x) 为真的第一个元素 x。如果 C 是一个大于操作，那么这个定义意味着你将获得第一个小于键的元素。换句话说，upper_bound的通常含义是相反的，

如果你改成这个

multiset<int> gquiz1;

您会看到 upper_bound 和 lower_bound 的通常含义返回。