Why is upper bound value of gquiz1 less than the key value

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;
}

The upper bound of gquiz1 prints 30 with key of 40. gquiz2 upper bound seems to give a higher value only. I thought it would have something to do with multiple similar elements in the multiset gquiz1 but on inserting 50 twice in the gquiz2 after all erase functions, the gquiz2 upper bound still gave higher value than the key whereas gquiz1 gave lower value. Pls help.

Modified code of inserting 50 twice. I tried this inserting 50 twice code:

// 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;
}

excepted:

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

actual:

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

john · Answer 1 · 2022-11-27T06:46:49.853

Your problem is here

multiset<int, greater<int> > gquiz1;

Because you are using greater<int> the usual meaning of upper_bound and lower_bound are changed. Using upper_bound will return the first element whose value is less than the key, and lower_bound will return the first element whose value is greater than or equal to the key.

The precise definition of upper_bound is that for map<T,C> and a given key, it returns the first element x for which C(key, x) is true. If C is a greater than operation, then this definition means you are going to get the first element which is less than the key. In other words the usual meaning of upper_bound is reversed,

If you change to this

multiset<int> gquiz1;

you will see that the usual meanings of upper_bound and lower_bound return.

Why is upper bound value of gquiz1 less than the key value

1 Answers1