C++中std :: set :: lower_bound和std :: lower_bound之间的区别

先决条件： C++中的随机访问迭代器，C++中的双向迭代器。

C++中的std :: lower_bound：
C++中的lower_bound()方法用于返回一个迭代器，该迭代器指向[first，last)范围内的第一个元素，该元素的值不少于给定值。这意味着该函数返回的下一个最小数字的索引正好大于该数字。

C++中的std :: set :: lower_bound
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，该元素等于参数中传递的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，则返回的迭代器将指向设置容器中的最后一个元素。

以下是std :: lower_bound和std :: set :: lower_bound之间的区别：

S.No.	std::lower_bound()	std::set::lower_bound()
1	Syntax: std::lower_bound(ForwardIterator first, ForwardIterator last, const T& val).	Syntax: std::lower_bound(const value_type& val).
2	The std::lower_bound has Random Access Iterators and Non Random Access Iterators.	The std::set::lower_bound has Bidirectional Iterators.
3	This function optimizes the number of comparisons which is efficient for Random Access Iterators.	This function optimises the number of comparisons using Bidirectional Iterators
4	The running time complexity is O(log2N) for random-access iterators but for non random-access iterators it is O(N).	The running time complexity is always O(log2N) due to it’s Binary Search Tree implementation.

下面是带有CPU执行时间的程序，它将说明两个功能的运行时间。

用来说明std :: lower_bound()的程序：

// C++ program to illustrate
// std::set::lower_bound
#include 
#include 
using namespace std;
  
// Function whose time is to
// be measured
void fun()
{
    // Initialise the set
    set s;
  
    // Insert element in the set
    for (int i = 0; i < 10; i++) {
        s.insert(i);
    }
  
    // Use lower_bound() function
    // to find 5
    set::iterator it;
    it = lower_bound(s.begin(), s.end(), 5);
}
  
// Driver Code
int main()
{
    // Use function gettimeofday()
    // can get the time
    struct timeval start, end;
  
    // Start timer
    gettimeofday(&start, NULL);
  
    // unsync the I/O of C and C++.
    ios_base::sync_with_stdio(false);
  
    // Function Call
    fun();
  
    // Stop timer
    gettimeofday(&end, NULL);
  
    // Calculating total time taken
    // by the program.
    double time_taken;
  
    time_taken = (end.tv_sec
                  - start.tv_sec)
                 * 1e6;
  
    time_taken = (time_taken
                  + (end.tv_usec
                     - start.tv_usec))
                 * 1e-6;
  
    cout << "Time taken by program is : "
         << fixed
         << time_taken << setprecision(6);
    cout << " sec" << endl;
    return 0;
}

输出：

Time taken by program is : 0.000046 sec

用来说明std :: set :: lower_bound()的程序：

// C++ program to illustrate
// std::lower_bound
#include 
#include 
using namespace std;
  
// Function whose time is to
// be measured
void fun()
{
    // Initialise the set
    set s;
  
    // Insert element in the set
    for (int i = 0; i < 10; i++) {
        s.insert(i);
    }
  
    // Use set::lower_bound() function
    // to find 5
    set::iterator it;
    it = s.lower_bound(5);
}
  
// Driver Code
int main()
{
    // Use function gettimeofday()
    // can get the time
    struct timeval start, end;
  
    // Start timer
    gettimeofday(&start, NULL);
  
    // unsync the I/O of C and C++.
    ios_base::sync_with_stdio(false);
  
    fun();
  
    // Stop timer
    gettimeofday(&end, NULL);
  
    // Calculating total time taken
    // by the program.
    double time_taken;
  
    time_taken = (end.tv_sec
                  - start.tv_sec)
                 * 1e6;
  
    time_taken = (time_taken
                  + (end.tv_usec
                     - start.tv_usec))
                 * 1e-6;
  
    cout << "Time taken by program is : "
         << fixed
         << time_taken << setprecision(6);
    cout << " sec" << endl;
    return 0;
}

输出：

Time taken by program is : 0.000039 sec

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