stl map中的lower_bound和 upper

stl map中的lower_bound和 upper_bound

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发布时间：2019-06-21

本文共 3397 字，大约阅读时间需要 11 分钟。

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  4 #include 
      
        5  6 int main( ) 7 { 8    using namespace std; 9    map 
       
         m1;10    map 
        
          :: const_iterator m1_AcIter, m1_RcIter;11    typedef pair 
         
           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2的元素的迭代器，当然是<3,30>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 returned24    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    else31       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 location36    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 thanthat of the initial element of m1 is: 20.

1 // map_lower_bound.cpp 2 // compile with: /EHsc 3 #include  4 #include 
       
         5  6 int main( ) 7 { 8    using namespace std;    9    map 
        
          m1;10    map 
         
           :: const_iterator m1_AcIter, m1_RcIter;11    typedef pair 
          
            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值大于等于2的是<2,20>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 returned22    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    else28       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 location33    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 }