在C++ STL中设置lower_bound()函数

set :: lower_bound()是C++ STL中的内置函数，该函数返回一个迭代器，该迭代器指向容器中的元素，该迭代器等效于在参数中传递的k。如果set容器中不存在k，则该函数返回一个迭代器，该迭代器指向刚好大于k的紧邻的下一个元素。如果在参数中传递的键超过了容器中的最大值，则迭代器返回的指针将指向set.end()集合容器中最后一个元素之外的元素。
句法：

set_name.lower_bound(key)

参数：此函数接受单个强制性参数键，该键指定要返回其lower_bound的元素。
返回值：该函数返回一个迭代器，该迭代器指向容器中的元素，该迭代器等效于参数中传递的k。如果set容器中不存在k，则该函数返回一个迭代器，该迭代器指向刚好大于k的紧邻的下一个元素。如果在参数中传递的键超出了容器中的最大值，则返回的迭代器等效于s.end()(特殊的迭代器指向最后一个元素)。
下面的程序说明了上述函数：

CPP

// CPP program to demonstrate the
// set::lower_bound() function
#include 
using namespace std;
int main()
{
 
    set s;
 
    // Function to insert elements
    // in the set container
    s.insert(1);
    s.insert(4);
    s.insert(2);
    s.insert(5);
    s.insert(6);
 
    cout << "The set elements are: ";
    for (auto it = s.begin(); it != s.end(); it++)
        cout << *it << " ";
 
    // when 2 is present
    auto it = s.lower_bound(2);
    if (it != s.end()) {
        cout << "\nThe lower bound of key 2 is ";
        cout << (*it) << endl;
    }
    else
        cout << "The element entered is larger than the "
                "greatest element in the set"
             << endl;
    // when 3 is not present
    // points to next greater after 3
    it = s.lower_bound(3);
    if (it != s.end()) {
        cout << "The lower bound of key 3 is ";
        cout << (*it) << endl;
    }
    else
        cout << "The element entered is larger than the "
                "greatest element in the set"
             << endl;
 
    // when 8 exceeds the max element in set
    it = s.lower_bound(8);
    if (it != s.end()) {
        cout << "The lower bound of key 8 is ";
        cout << (*it) << endl;
    }
    else
        cout << "The element is larger than the greatest "
                "element in the set"
             << endl;
 
    return 0;
}

输出：

The set elements are: 1 2 4 5 6 
The lower bound of key 2 is 2
The lower bound of key 3 is 4
The element is larger than the greatest element in the set

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