stl map中的lower_bound和 upper_bound

Posted 2020-08-20 Boblim

map中的lower_bound和upper_bound的意思其实很简单，就两句话：

map::lower_bound(key):返回map中第一个大于或等于key的迭代器指针

map::upper_bound(key):返回map中第一个大于key的迭代器指针

所以，理解这两个函数请不要按照字面意义思考太复杂，因为仅仅是不小于（lower_bound）和大于（upper_bound）这么简单。

 

看两个msdn里的例子

 1 // map_upper_bound.cpp
 2 // compile with: /EHsc
 3 #include <map>
 4 #include <iostream>
 5 
 6 int main( )
 7 {
 8    using namespace std;
 9    map <int, int> m1;
10    map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
11    typedef pair <int, int> Int_Pair;
12 
13    m1.insert ( Int_Pair ( 1, 10 ) );
14    m1.insert ( Int_Pair ( 2, 20 ) );
15    m1.insert ( Int_Pair ( 3, 30 ) );
16 　
17 　// 返回m1中第一个key值大于或等于2的元素的迭代器，当然是<2,20>
18 　m1_RcIter = m1.upper_bound( 2 );
19    cout << "The first element of map m1 with a key "
20         << "greater than 2 is: "
21         << m1_RcIter -> second << "." << endl;
22 
23    // If no match is found for the key, end is returned
24    m1_RcIter = m1. upper_bound ( 4 );
25 
26    // m1中key值并没有大于或等于4的 
27    if ( m1_RcIter == m1.end( ) )
28       cout << "The map m1 doesn‘t have an element "
29            << "with a key greater than 4." << endl;
30    else
31       cout << "The element of map m1 with a key > 4 is: "
32            << m1_RcIter -> second << "." << endl;
33 
34    // The element at a specific location in the map can be found 
35    // using a dereferenced iterator addressing the location
36    m1_AcIter = m1.begin( );
37    m1_RcIter = m1. upper_bound ( m1_AcIter -> first );
38    cout << "The 1st element of m1 with a key greater than\n"
39         << "that of the initial element of m1 is: "
40         << m1_RcIter -> second << "." << endl;
41 }

 

The first element of map m1 with a key greater than 2 is: 30.
The map m1 doesn‘t have an element with a key greater than 4.
The 1st element of m1 with a key greater than
that of the initial element of m1 is: 20.

 1 // map_lower_bound.cpp
 2 // compile with: /EHsc
 3 #include <map>
 4 #include <iostream>
 5 
 6 int main( )
 7 {
 8    using namespace std;   
 9    map <int, int> m1;
10    map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
11    typedef pair <int, int> Int_Pair;
12 
13    m1.insert ( Int_Pair ( 1, 10 ) );
14    m1.insert ( Int_Pair ( 2, 20 ) );
15    m1.insert ( Int_Pair ( 3, 30 ) );
16 　　//key值大于2的是<3,30>
17    m1_RcIter = m1.lower_bound( 2 );
18    cout << "The first element of map m1 with a key of 2 is: "
19         << m1_RcIter -> second << "." << endl;
20 
21    // If no match is found for this key, end( ) is returned
22    m1_RcIter = m1. lower_bound ( 4 );
23 
24    if ( m1_RcIter == m1.end( ) )
25       cout << "The map m1 doesn‘t have an element "
26            << "with a key of 4." << endl;
27    else
28       cout << "The element of map m1 with a key of 4 is: "
29            << m1_RcIter -> second << "." << endl;
30 
31    // The element at a specific location in the map can be found 
32    // using a dereferenced iterator addressing the location
33    m1_AcIter = m1.end( );
34    m1_AcIter--;
35    m1_RcIter = m1. lower_bound ( m1_AcIter -> first );
36    cout << "The element of m1 with a key matching "
37         << "that of the last element is: "
38         << m1_RcIter -> second << "." << endl;
39 }

 

The first element of map m1 with a key of 2 is: 20.

The map m1 doesn‘t have an element with a key of 4.

The element of m1 with a key matching that of the last element is: 30.