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std::ranges::lower_bound(3) C++ Standard Libary std::ranges::lower_bound(3) NAME std::ranges::lower_bound - std::ranges::lower_bound Synopsis 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*, (1) (since C++20) std::projected<I, Proj>> Comp = ranges::less > constexpr I lower_bound( I first, S last, const T& value, Comp comp = {}, Proj proj = {} ); template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order< const T*, (2) (since C++20) 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 = {} ); 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 call- ing 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 per- formed are logarithmic in the distance between first and last (At most log 2(last - first) + O(1) comparisons and applications of the projec- tion). 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::iden- tity, 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::dis- tance(data.cbegin(), it); } Output: 4 4 4 4 8 found at index 3 See also ranges::equal_range returns range of elements matching a spe- cific key (C++20) (niebloid) ranges::partition divides a range of elements into two groups (C++20) (niebloid) ranges::partition_point locates the partition point of a partitioned range (C++20) (niebloid) ranges::upper_bound returns an iterator to the first element greater than a (C++20) certain value (niebloid) returns an iterator to the first element not less than the lower_bound given value (function template) http://cppreference.com 2022.07.31 std::ranges::lower_bound(3)
NAME | Synopsis | Parameters | Return value | Complexity | Possible implementation | Example | Output: | See also
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