C++ Multiset lower_bound()
C++ multiset lower_bound()
C++ multiset lower_bound()函数用于返回指向等效于 multiset 容器中键的迭代器val传入参数。
如果 multiset 容器中不存在val，它将返回一个迭代器，该迭代器指向刚好大于val的直接下一个元素。
语法
iterator lower_bound (const value_type& val) const; //until C++ 11
iterator lower_bound (const value_type& val); //since C++ 11
const_iterator lower_bound (const value_type& val) const; //since C++ 11
参数
val : 要在 multiset 容器中搜索的值。
返回值
它返回一个迭代器，该迭代器指向 multiset 容器中的值，该值等效于参数中传递的val。如果没有这样的元素，则返回end()。
复杂度
大小为对数。
迭代器有效性
没有更改。
数据争用
访问了容器(const和非const版本都不能修改 multiset )。
同时访问容器的元素是安全的。
异常安全性
如果引发异常，则 multiset 合中没有任何更改。
示例1
让我们看一下获取给定键下限的简单示例:
#include <iostream>
#include <set>
using namespace std;
int main(void) {
multiset<char> m = {'a','b','c','a','c'};
auto it = m.lower_bound('c');
cout << "Lower bound(=) of c is: " << *it;
return 0;
}
输出:
Lower bound(=) of c is: c
在上面的示例中，c的下限是c。
示例2
让我们看一个简单的示例，将下位的multiset元素删除绑定到上限:
#include <iostream>
#include <set>
using namespace std;
int main ()
{
multiset<int> mymultiset;
multiset<int>::iterator itlow,itup;
for (int i=1; i<10; i++) mymultiset.insert(i*10); // 10 20 30 40 50 60 70 80 90
itlow=mymultiset.lower_bound (30); // ^
itup=mymultiset.upper_bound (60); // ^
mymultiset.erase(itlow,itup); // 10 20 70 80 90
std::cout << "mymultiset contains:";
for (multiset<int>::iterator it=mymultiset.begin(); it!=mymultiset.end(); ++it)
cout << ' ' << *it;
cout << '\n';
return 0;
}
输出:
mymultiset contains: 10 20 70 80 90
在上面的示例中，delete()函数将multiset的元素从下限(=)擦除到上限(>)，并打印其余内容。
示例3
让我们看一个简单的示例:
#include <set>
#include <iostream>
using namespace std;
int main( )
{
using namespace std;
multiset <int> s1;
multiset <int> :: const_iterator s1_AcIter, s1_RcIter;
s1.insert( 10 );
s1.insert( 20 );
s1.insert( 30 );
s1_RcIter = s1.lower_bound( 20 );
cout << "The element of multiset s1 with a key of 20 is: "
<< *s1_RcIter << "." << endl;
s1_RcIter = s1.lower_bound( 40 );
// if no match is found for the key, end( ) is returned
if ( s1_RcIter == s1.end( ) )
cout << "The multiset s1 doesn't have an element "
<< "with a key of 40." << endl;
else
cout << "The element of multiset s1 with a key of 40 is: "
<< *s1_RcIter << "." << endl;
// The element at a specific location in the multiset can be found
// by using a dereferenced iterator that addresses the location
s1_AcIter = s1.end( );
s1_AcIter--;
s1_RcIter = s1.lower_bound( *s1_AcIter );
cout << "The element of s1 with a key matching "
<< "that of the last element is: "
<< *s1_RcIter << "." << endl;
return 0;
}
输出:
The element of multiset s1 with a key of 20 is: 20.
The multiset s1 doesn't have an element with a key of 40.
The element of s1 with a key matching that of the last element is: 30.
示例4
让我们看一个简单的示例:
#include<set>
#include<iostream>
using namespace std;
int main()
{
multiset<int> mp;
// insert elements in random order
mp.insert( 2 );
mp.insert( 1 );
mp.insert( 5 );
mp.insert( 4 );
cout<<"Elements are: \n";
for (auto it = mp.begin(); it != mp.end(); it++) {
cout << (*it)<< endl;
}
// when 2 is present
auto it = mp.lower_bound(2);
cout << "The lower bound of key 2 is ";
cout << (*it)<< endl;
// when 3 is not present
// points to next greater after 3
it = mp.lower_bound(3);
cout << "The lower bound of key 3 is ";
cout << (*it)<< endl;
// when 6 exceeds
it = mp.lower_bound(6);
cout << "The lower bound of key 6 is ";
cout << (*it);
return 0;
}
输出:
Elements are:
1
2
4
5
The lower bound of key 2 is 2
The lower bound of key 3 is 4
The lower bound of key 6 is 4
在上面的示例中，当我们尝试查找超出容器的值的下限时，或者可以说 multiset 容器中不存在该值时，它将返回到末尾。
