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,
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 = {} );
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(1) | (since C++20) (until C++26) |
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 lower_bound( I first, S last, const T& value,
Comp comp = {}, Proj proj = {} );
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(since C++26) | |
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 = {} );
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(2) | (since C++20) (until C++26) |
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>
lower_bound( R&& r, const T& value, Comp comp = {}, Proj proj = {} );
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(since C++26) | |
Searches for the first element (projected by proj) in the partitioned source range [first, last) or r which is not ordered before value with the comparator comp.
If the elements e of the source range are not partitioned with respect to the expression bool(std::invoke(comp, std::invoke(proj, e), value)), the behavior is undefined.
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.
Parameters
| first, last | - | the iterator-sentinel pair defining the source range |
| r | - | the source range |
| value | - | the value to be compared with the (projected) elements |
| comp | - | the comparator to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
Return value
Iterator to the first element which is not ordered before value, or the past-the-end iterator of the source range if no such element is found.
Complexity
Given N as ranges::distance(first, last) or ranges::distance(r):
1,2) At most log2(N)+𝓞(1) applications of
comp and proj.Notes
| 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::next(ranges::begin(r), ranges::end(r)),
value, std::ref(comp), std::ref(proj));
}
};
inline constexpr lower_bound_fn lower_bound;
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Example
Run this code
#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