std::ranges::lower_bound
| Defined in header <algorithm>
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| Call signature |
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| (1) | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, |
(since C++20) (until C++26) |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(since C++26) | |
| (2) | ||
| template< ranges::forward_range R, class T, class Proj = std::identity, |
(since C++20) (until C++26) |
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| template< ranges::forward_range R, class Proj = std::identity, |
(since C++26) | |
[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 std::invoke(comp, std::invoke(proj, 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.The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Contents |
[edit] Parameters
| first, last | - | the iterator-sentinel pair defining the partially-ordered range of elements to examine |
| r | - | the partially-ordered range to examine |
| value | - | value to compare the projected elements to |
| comp | - | comparison 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 log2(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] Notes
On a range that's fully sorted (or more generally, partially ordered with respect to value) after projection, std::ranges::lower_bound implements the binary search algorithm. Therefore, std::ranges::binary_search can be implemented in terms of it.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | List-initialization for algorithms (1,2) |
[edit] Possible implementation
struct lower_bound_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>, 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 Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj>, 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
#include <algorithm> #include <cassert> #include <complex> #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); std::cout << "found a range [" << ranges::distance(data.cbegin(), lower) << ", " << ranges::distance(data.cbegin(), upper) << ") = { "; 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); // '5' would return end() if (it != data.cend()) std::cout << *it << " found at index " << ranges::distance(data.cbegin(), it); using CD = std::complex<double>; std::vector<CD> nums{{1, 0}, {2, 2}, {2, 1}, {3, 0}}; auto cmpz = [](CD x, CD y) { return x.real() < y.real(); }; #ifdef __cpp_lib_algorithm_default_value_type auto it2 = ranges::lower_bound(nums, {2, 0}, cmpz); #else auto it2 = ranges::lower_bound(nums, CD{2, 0}, cmpz); #endif assert((*it2 == CD{2, 2})); }
Output:
found a range [6, 10) = { 4 4 4 4 }
8 found at index 3[edit] See also
| (C++20) |
returns range of elements matching a specific key (algorithm function object) |
| (C++20) |
divides a range of elements into two groups (algorithm function object) |
| (C++20) |
locates the partition point of a partitioned range (algorithm function object) |
| (C++20) |
returns an iterator to the first element greater than a certain value (algorithm function object) |
| returns an iterator to the first element not less than the given value (function template) |
