std::ranges::lower_bound
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, |
(1) | (since C++20) |
| template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order< |
(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.
Contents |
[edit] 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 |
[edit] Return value
Iterator pointing to the first element that is not less than value, or last if no such element is found.
[edit] 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.
[edit] 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; |
[edit] 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
[edit] See also
| (C++20) |
returns range of elements matching a specific key (niebloid) |
| (C++20) |
divides a range of elements into two groups (niebloid) |
| (C++20) |
locates the partition point of a partitioned range (niebloid) |
| (C++20) |
returns an iterator to the first element greater than a certain value (niebloid) |
| returns an iterator to the first element not less than the given value (function template) |
