std::ranges::lower_bound - cppreference.com (original) (raw)

| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | --------------------------- | | Call signature | | | | (1) | | | | 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 = {} ); | | (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 = {} ); | | (since C++26) | | (2) | | | | 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 = {} ); | | (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 = {} ); | | (since C++26) |

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 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.

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 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

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[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.

[edit] Possible implementation

struct lower_bound_fn { template<std::forward_iterator I, std::sentinel_for 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 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, Proj>, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t, Proj>> Comp = ranges::less> constexpr ranges::borrowed_iterator_t 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 #include #include #include #include #include   namespace ranges = std::ranges;   template<std::forward_iterator I, std::sentinel_for 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(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; std::vector 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