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

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:

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.

Below is the program with CPU execution time which will illustrate the running time of both functions.



Program to illustrate of std::lower_bound():

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// 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 measured
void 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 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;
}

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Output:

Time taken by program is : 0.000046 sec

Program to illustrate of std::set::lower_bound():

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// C++ program to illustrate
// std::lower_bound
#include <bits/stdc++.h>
#include <sys/time.h>
using namespace std;
  
// Function whose time is to
// be measured
void 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 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;
}

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Output:

Time taken by program is : 0.000039 sec

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