Difference between std::set::lower_bound and std::lower_bound in C++ - GeeksforGeeks

07 May, 2020
Prerequisite: Random-access Iterators in C++, Bidirectional Iterators in C++.
std::lower_bound in C++:The lower_bound() method in C++ is used to return an iterator pointing to the first element in the range [first, last) which has a value not less than the given value. This means that the function returns the index of the next smallest number just greater than that number.
std::set::lower_bound in C++:The set::lower_bound() is a built-in function in C++ STL which returns an iterator pointing to the element in the container which is equivalent to K passed in the parameter. In case K is not present in the set container, the function returns an iterator pointing to the immediate next element which is just greater than K. If the key passed in the parameter exceeds the maximum value in the container, then the iterator returned points to the last element in the set container.
Below is the difference between std::lower_bound and std::set::lower_bound:
Below is the program with CPU execution time which will illustrate the running time of both functions.
Program to illustrate of std::lower_bound():
// C++ program to illustrate// std::set::lower_bound#include <bits/stdc++.h>#include <sys/time.h>using namespace std;  // Function whose time is to// be measuredvoid fun(){    // Initialise the set    set<int> s;      // Insert element in the set    for (int i = 0; i < 10; i++) {        s.insert(i);    }      // Use lower_bound() function    // to find 5    set<int>::iterator it;    it = lower_bound(s.begin(), s.end(), 5);}  // Driver Codeint 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;}
Time taken by program is : 0.000046 sec

Program to illustrate of std::set::lower_bound():
// C++ program to illustrate// std::lower_bound#include <bits/stdc++.h>#include <sys/time.h>using namespace std;  // Function whose time is to// be measuredvoid fun(){    // Initialise the set    set<int> 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<int>::iterator it;    it = s.lower_bound(5);}  // Driver Codeint 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;}
Time taken by program is : 0.000039 sec

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