📜  C++ STL-Multiset.equal_range()函数

📅  最后修改于: 2020-10-20 01:18:40             🧑  作者: Mango

C++ STL Multiset.equal_range()

C++ Multiset equal_range()函数用于返回包含容器中等于val的所有元素的范围的边界。

如果val与容器中的任何值都不匹配,则返回值范围将为长度0,并且两个迭代器均指向大于val的最近值。否则,如果val大于容器中的所有元素,则它指向end。

句法

pair equal_range (const value_type& val) const;
pair                    equal_range (const value_type& val);

该范围由两个迭代器定义,一个指向不小于val的第一个元素,另一个指向大于val的第一个元素。

参数

val:要在Multiset容器中搜索的值。

返回值

该函数返回pair。其中pair :: first位于范围的下边界,具有与lower_bound(val)将返回的值相同的值,pair :: second是与upper_bound(val)将返回的值相同,它对应的范围的上限。

复杂度

大小为对数。

迭代器有效性

没有变化。

数据竞争

容器被访问(const和非const版本都不能修改容器)。

同时访问Multiset的元素是安全的。

异常安全

如果引发异常,则容器中没有任何更改。

例子1

让我们看一个简单的例子:

#include 
#include 

using namespace std;

int main(void) {
   multiset m = {'a','b','c','a'};

   auto ret = m.equal_range('b');

   cout << "Lower bound of b is: " << *ret.first<< endl;

   cout << "Upper bound of b is: " << *ret.second<< endl;

   return 0;
}

输出:

Lower bound of b is: b
Upper bound of b is: c

在上面的示例中,b的下限为b,b的上限为c。

例子2

让我们看一个简单的例子:

#include 
#include 

using namespace std;
 
int main()
{
     // initialize container
    multiset mp;
 
    // insert elements in random order
    mp.insert( 4 );
    mp.insert( 1 );
    mp.insert( 4 );
 
    pair::const_iterator,multiset::const_iterator> ret;
 
    ret = mp.equal_range(10);
    cout << "The lower bound is: " << *ret.first;
    cout << "\nThe upper bound is: " << *ret.second;
 
    return 0;
}

输出:

The lower bound is 3
The upper bound is 3

在上面的示例中,equal_range()函数返回到end(),即3,因为它试图查找Multisetmp中不存在的10,因此,它返回到结尾。

例子3

让我们看一个简单的例子:

#include   
#include   
  
int main( )  
{  
   using namespace std;  
   typedef multiset > IntMultiset;  
   IntMultiset s1;  
   multiset  > :: const_iterator s1_RcIter;  
  
   s1.insert( 10 );  
   s1.insert( 20 );  
   s1.insert( 30 );  
  
   pair  p1, p2;  
   p1 = s1.equal_range( 20 );  
  
   cout << "The upper bound of the element with "  
        << "a key of 20 in the multiset s1 is: "  
        << *(p1.second) << "." << endl;  
  
   cout << "The lower bound of the element with "  
        << "a key of 20 in the multiset s1 is: "  
        << *(p1.first) << "." << endl;  
  
   // Compare the upper_bound called directly   
   s1_RcIter = s1.upper_bound( 20 );  
   cout << "A direct call of upper_bound( 20 ) gives "  
        << *s1_RcIter << "," << endl  
        << "matching the 2nd element of the pair"  
        << " returned by equal_range( 20 )." << endl;  
  
   p2 = s1.equal_range( 40 );  
  
   // If no match is found for the key,  
   // both elements of the pair return end( )  
   if ( ( p2.first == s1.end( ) ) && ( p2.second == s1.end( ) ) )  
      cout << "The multiset s1 doesn't have an element "  
           << "with a key less than 40." << endl;  
   else  
      cout << "The element of multiset s1 with a key >= 40 is: "  
           << *(p1.first) << "." << endl; 
           
           return 0;
}  

输出:

The upper bound of the element with a key of 20 in the multiset s1 is: 30.
The lower bound of the element with a key of 20 in the multiset s1 is: 20.
A direct call of upper_bound( 20 ) gives 30,
matching the 2nd element of the pair returned by equal_range( 20 ).
The multiset s1 doesn't have an element with a key less than 40.

例子4

让我们看一个简单的例子:

#include 
#include 

using namespace std;

int main ()
{
  std::multiset mymultiset;

  for (int i=1; i<=5; i++) mymultiset.insert(i*10);   // mymultiset: 10 20 30 40 50

  pair::const_iterator,multiset::const_iterator> ret;
  ret = mymultiset.equal_range(30);

  cout << "the lower bound points to: " << *ret.first << '\n';
  cout << "the upper bound points to: " << *ret.second << '\n';

  return 0;
}

输出:

the lower bound points to: 30
the upper bound points to: 40