std::ranges::adjacent_find
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 Proj = std::identity,
std::indirect_binary_predicate
<std::projected<I, Proj>,
std::projected<I, Proj>> Pred = ranges::equal_to >
constexpr I
adjacent_find( I first, S last, Pred pred = {}, Proj proj = {} );
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(1) | (since C++20) |
template< ranges::forward_range R, class Proj = std::identity,
std::indirect_binary_predicate
<std::projected<ranges::iterator_t<R>, Proj>,
std::projected<ranges::iterator_t<R>, Proj>> Pred
= ranges::equal_to >
constexpr ranges::borrowed_iterator_t<R>
adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );
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(2) | (since C++20) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Proj = std::identity,
std::indirect_binary_predicate
<std::projected<I, Proj>,
std::projected<I, Proj>> Pred = ranges::equal_to >
I adjacent_find( Ep&& policy, I first, S last,
Pred pred = {}, Proj proj = {} );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Proj = std::identity,
std::indirect_binary_predicate
<std::projected<ranges::iterator_t<R>, Proj>,
std::projected<ranges::iterator_t<R>, Proj>> Pred
= ranges::equal_to >
ranges::borrowed_iterator_t<R>
adjacent_find( Ep&& policy, R&& r, Pred pred = {}, Proj proj = {} );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
Searches the source range for the first pair of adjacent elements (projected by proj) satisfying the given binary predicate pred.
1) The source range is
[first, last).2) The source range is
r.3,4) Same as (1,2), but executed according to
policy.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
Parameters
| first, last | - | the iterator-sentinel pair defining the source range |
| r | - | the source range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
The first iterator iter in the source range such that
bool(std::invoke(pred, std::invoke(proj, *iter),
std::invoke(proj, *ranges::next(iter))))
evaluates to true.
If no such iterator is found, returns:
1,3)
ranges::next(first, last)2,4)
ranges::next(ranges::begin(r), ranges::end(r))Complexity
Given
resultas the return value ofadjacent_find,- M as
ranges::distance(first, result)orranges::distance(ranges::begin(r), result), and - N as
ranges::distance(first, last)orranges::distance(r):
1,2) Exactly min(M+1,N-1) applications of
pred.3,4) 𝓞(N) applications of
pred.proj is applied no more than double of the number of applications of pred.
Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Possible implementation
struct adjacent_find_fn
{
template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_binary_predicate
<std::projected<I, Proj>,
std::projected<I, Proj>> Pred = ranges::equal_to>
constexpr I operator()(I first, S last, Pred pred = {}, Proj proj = {}) const
{
if (first == last)
return first;
auto next = ranges::next(first);
for (; next != last; ++next, ++first)
if (std::invoke(pred, std::invoke(proj, *first), std::invoke(proj, *next)))
return first;
return next;
}
template<ranges::forward_range R, class Proj = std::identity,
std::indirect_binary_predicate
<std::projected<ranges::iterator_t<R>, Proj>,
std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, Pred pred = {}, Proj proj = {}) const
{
return (*this)(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)),
std::ref(pred), std::ref(proj));
}
};
inline constexpr adjacent_find_fn adjacent_find;
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Example
Run this code
#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>
constexpr bool some_of(auto&& r, auto&& pred) // some but not all
{
return std::ranges::cend(r) != std::ranges::adjacent_find(r,
[&pred](const auto& x, const auto& y)
{
return pred(x) != pred(y);
});
}
// test some_of
constexpr auto a = {0, 0, 0, 0}, b = {1, 1, 1, 0}, c = {1, 1, 1, 1};
auto is_one = [](auto x){ return x == 1; };
static_assert(!some_of(a, is_one) && some_of(b, is_one) && !some_of(c, is_one));
int main()
{
const auto v = {0, 1, 2, 3, 40, 40, 41, 41, 5}; /*
^^ ^^ */
namespace ranges = std::ranges;
if (auto it = ranges::adjacent_find(v.begin(), v.end()); it == v.end())
std::cout << "No matching adjacent elements\n";
else
std::cout << "The first adjacent pair of equal elements is at ["
<< ranges::distance(v.begin(), it) << "] == " << *it << '\n';
if (auto it = ranges::adjacent_find(v, ranges::greater()); it == v.end())
std::cout << "The entire vector is sorted in ascending order\n";
else
std::cout << "The last element in the non-decreasing subsequence is at ["
<< ranges::distance(v.begin(), it) << "] == " << *it << '\n';
}
Output:
The first adjacent pair of equal elements is at [4] == 40
The last element in the non-decreasing subsequence is at [7] == 41
See also
| finds the first two adjacent items that are equal (or satisfy a given predicate) (function template) | |
(C++20) |
removes consecutive duplicate elements in a range (algorithm function object) |