C++ Algorithm Library - lower_bound() Function


The C++ function std::algorithm::lower_bound() finds the first element not less than the given value. This function excepts element in sorted order. It uses operator< for comparison.


Following is the declaration for std::algorithm::lower_bound() function form std::algorithm header.


template <class ForwardIterator, class T>
ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last,
   const T& val);


  • first − Forward iterator to the initial position.

  • last − Forward iterator to the final position.

  • val − Value of the lower bound to search for in the range.

Return value

Returns an iterator to the first element not less than the given value. If all the element in the range compare less than val, then function returns last.


Throws exception if either the element comparison or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Time complexity



The following example shows the usage of std::algorithm::lower_bound() function.

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(void) {
   vector<int> v = {1, 2, 5, 13, 14};
   auto it = lower_bound(v.begin(), v.end(), 2);

   cout << "First element which greater than 2 is " << *it << endl;

   it = lower_bound(v.begin(), v.end(), 30);

   if (it == end(v))
      cout << "All elements are less than 30" << endl;
   return 0;

Let us compile and run the above program, this will produce the following result −

First element which greater than 2 is 2
All elements are less than 30