C++ multimap equal_range()
C++ multimap equal_range()
C++ multimap equal_range()函数用于返回包含容器中所有关键元素的范围边界等于x。
如果x与容器中的任何键都不匹配，则返回值范围为长度0，并且两个迭代器都指向大于x的最接近值。否则，如果x大于容器中的所有元素，则指向结尾。
语法
pair<const_iterator,const_iterator> equal_range (const key_type& k) const;
pair<iterator,iterator> equal_range (const key_type& k);
范围由两个迭代器定义。它返回一个范围的边界，该范围包括容器中所有具有与k等效的键的元素。
参数
k : 要在 multimap 容器中搜索的键。
返回值
此函数返回对。其中对::第一位于范围的下边界，具有与lower_bound(x)返回的值相同的值，而对::第二与该上边界(x)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
已访问容器(没有const非非const版本都不能修改容器)。
没有访问映射值: 同时访问和修改元素是安全的。
异常安全性
如果引发异常，则容器中没有任何更改。
示例1
让我们看一个简单的示例:
#include <iostream>
#include <map>
using namespace std;
int main(void) {
map<char, int> m = {
{'a', 1},
{'b', 2},
{'c', 3},
{'c', 4},
{'e', 5},
};
auto ret = m.equal_range('b');
cout << "Lower bound of b is: " << ret.first->first <<
" = " << ret.first->second << endl;
cout << "Upper bound of b is: " << ret.second->first <<
" = " << ret.second->second << endl;
return 0;
}
输出:
Lower bound of b is: b = 2
Upper bound of b is: c = 3
在上面的示例中，b的下限是b，b的上限是c。
示例2
我们来看一个简单的示例:
#include <iostream>
#include <multimap>
using namespace std;
int main()
{
// initialize container
multimap<int, int> mp;
// insert elements in random order
mp.insert({ 4, 30 });
mp.insert({ 1, 40 });
mp.insert({ 6, 60 });
pair<multimap<int, int>::iterator, multimap<int, int>::iterator> it;
// iterator of pairs
it = mp.equal_range(10);
cout << "The lower bound is " <<
it.first->first << ":" << it.first->second;
cout << "\nThe upper bound is " <<
it.second->first << ":" << it.second->second;
return 0;
}
输出:
The lower bound is 3:0
The upper bound is 3:0
在上面的示例中，equal_range()函数返回0，因为它试图查找10(不是 multimap mp的键)。
示例3
让我们看一个简单的例子:
#include <map>
#include <iostream>
int main( )
{
using namespace std;
typedef multimap <int, int, less<int> > IntMMap;
IntMMap m1;
multimap <int, int> :: const_iterator m1_RcIter;
typedef pair <int, int> Int_Pair;
m1.insert ( Int_Pair ( 1, 10 ) );
m1.insert ( Int_Pair ( 2, 20 ) );
m1.insert ( Int_Pair ( 3, 30 ) );
pair <IntMMap::const_iterator, IntMMap::const_iterator> p1, p2;
p1 = m1.equal_range( 2 );
cout << "The lower bound of the element with "
<< "a key of 2 in the multimap m1 is: "
<< p1.first-> second << "." << endl;
cout << "The upper bound of the element with "
<< "a key of 2 in the multimap m1 is: "
<< p1.second-> second << "." << endl;
// Compare the upper_bound called directly
m1_RcIter = m1.upper_bound( 2 );
cout << "A direct call of upper_bound( 2 ) gives "
<< m1_RcIter-> second << "," << endl
<< " matching the 2nd element of the pair"
<< " returned by equal_range( 2 )." << endl;
p2 = m1.equal_range( 4 );
// if no match is found for the key,
// both elements of the pair return end( )
if ( ( p2.first == m1.end( ) ) && ( p2.second == m1.end( ) ) )
cout << "The multimap m1 doesn't have an element "
<< "with a key less than 4." << endl;
else
cout << "The element of multimap m1 with a key >= 40 is: "
<< p1.first-> first << "." << endl;
}
输出:
The lower bound of the element with a key of 2 in the multimap m1 is: 20.
The upper bound of the element with a key of 2 in the multimap m1 is: 30.
A direct call of upper_bound( 2 ) gives 30,
matching the 2nd element of the pair returned by equal_range( 2 ).
The multimap m1 doesn't have an element with a key less than 4.
示例4
让我们看一个简单的示例:
#include <iostream>
#include <string>
#include <map>
int main()
{
std::multimap<std::string, int> m = {
{"A", 3},
{"B", 1},
{"A", 4},
{"D", 5}
};
using iterator = decltype(m)::iterator;
std::pair<iterator, iterator> ret = m.equal_range("B");
for (iterator it = ret.first; it != ret.second; ++it) {
std::cout << it->first << "," << it->second << std::endl;
}
}
输出:
