std::ranges::lower_bound

Defined in header `<algorithm>`
Call signature
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, std::indirect_strict_weak_order< const T*, std::projected<I, Proj>> Comp = ranges::less > constexpr I lower_bound( I first, S last, const T& value, Comp comp = {}, Proj proj = {} );	(1)	(since C++20)
template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order< const T*, std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr ranges::borrowed_iterator_t<R> lower_bound( R&& r, const T& value, Comp comp = {}, Proj proj = {} );	(2)	(since C++20)

1) Returns an iterator pointing to the first element in the range [first, last) that is not less than (i.e. greater or equal to) value, or last if no such element is found. The range [first, last) must be partitioned with respect to the expression comp(element, value), i.e., all elements for which the expression is true must precede all elements for which the expression is false. A fully-sorted range meets this criterion.

2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	iterator-sentinel pair defining the partially-ordered range to examine
r	-	the partially-ordered range to examine
value	-	value to compare the elements to
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements

Return value

Iterator pointing to the first element that is not less than value, or last if no such element is found.

Complexity

The number of comparisons and applications of the projection performed are logarithmic in the distance between first and last (At most $log 2 (last - first) + O(1)$ comparisons and applications of the projection). However, for an iterator that does not model random_access_iterator, the number of iterator increments is linear.

Possible implementation

struct lower_bound_fn {
  template<std::forward_iterator I, std::sentinel_for<I> S,
           class T, class Proj = std::identity,
           std::indirect_strict_weak_order<
               const T*,
               std::projected<I, Proj>> Comp = ranges::less>
  constexpr I operator()( I first, S last, const T& value,
                          Comp comp = {}, Proj proj = {} ) const
  {
      I it;
      std::iter_difference_t<I> count, step;
      count = std::ranges::distance(first, last);
 
      while (count > 0) {
          it = first;
          step = count / 2;
          ranges::advance(it, step, last);
          if (comp(std::invoke(proj, *it), value)) {
              first = ++it;
              count -= step + 1;
          }
          else {
              count = step;
          }
      }
      return first;
  }
 
  template<ranges::forward_range R, class T, class Proj = std::identity,
           std::indirect_strict_weak_order<
               const T*,
               std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
  constexpr ranges::borrowed_iterator_t<R>
  operator()( R&& r, const T& value, Comp comp = {}, Proj proj = {} ) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), value,
                   std::ref(comp), std::ref(proj));
  }
};
 
inline constexpr lower_bound_fn lower_bound;

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
namespace ranges = std::ranges;
 
template<std::forward_iterator I, std::sentinel_for<I> S, class T,
         class Proj = std::identity,
         std::indirect_strict_weak_order<
               const T*,
               std::projected<I, Proj>> Comp = ranges::less>
constexpr
I binary_find(I first, S last, const T& value, Comp comp = {}, Proj proj = {})
{
    first = ranges::lower_bound(first, last, value, comp, proj);
    return first != last && !comp(value, proj(*first)) ? first : last;
}
 
int main()
{
    std::vector data = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5 };
 
    auto lower = ranges::lower_bound(data, 4);
    auto upper = ranges::upper_bound(data, 4);
 
    ranges::copy(lower, upper, std::ostream_iterator<int>(std::cout, " "));
 
    std::cout << '\n';
 
    // classic binary search, returning a value only if it is present
 
    data = { 1, 2, 4, 8, 16 }; 
 
    auto it = binary_find(data.cbegin(), data.cend(), 8); //< choosing '5' will return end()
 
    if(it != data.cend())
        std::cout << *it << " found at index "<< ranges::distance(data.cbegin(), it);
}

Output:

4 4 4 4
8 found at index 3

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::equal_range (C++20)	returns range of elements matching a specific key (niebloid)
ranges::partition (C++20)	divides a range of elements into two groups (niebloid)
ranges::partition_point (C++20)	locates the partition point of a partitioned range (niebloid)
ranges::upper_bound (C++20)	returns an iterator to the first element greater than a certain value (niebloid)
lower_bound	returns an iterator to the first element not less than the given value (function template)