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 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.
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 projected elements to |
| comp | - | comparison 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.
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) |
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; |
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 3See 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) |