std::ranges::search_n
Defined in header <algorithm> |
||
|---|---|---|
| Call signature | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T,
class Pred = ranges::equal_to, class Proj = std::identity >
requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
search_n( I first, S last, std::iter_difference_t<I> count,
const T& value, Pred pred = {}, Proj proj = {} ); |
(1) | (since C++20) |
template< ranges::forward_range R, class T, class Pred = ranges::equal_to,
class Proj = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
search_n( R&& r, ranges::range_difference_t<R> count,
const T& value, Pred pred = {}, Proj proj = {} ); |
(2) | (since C++20) |
1) Searches the range
[first, last) for the
first sequence of
count elements whose projected values are each equal to the given
value according to the binary predicate
pred.
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 niebloids, that is:
- Explicit template argument lists may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special compiler extensions.
Parameters
| first, last | - | the range of elements to examine (aka haystack) |
| r | - | the range of elements to examine (aka haystack) |
| count | - | the length of the sequence to search for |
| value | - | the value to search for (aka needle) |
| pred | - | the binary predicate that compares the projected elements with value |
| proj | - | the projection to apply to the elements of the range to examine |
Return value
1) Returns
std::ranges::subrange object that contains a pair of iterators in the range
[first, last) that designate the found subsequence.
If no such subsequence is found, returns std::ranges::subrange{last, last}. If count <= 0, returns std::ranges::subrange{first, first}.
Complexity
Linear: at most ranges::distance(first, last) applications of the predicate and the projection.
Notes
An implementation can improve efficiency of the search in average if the iterators model std::random_access_iterator.
Possible implementation
struct search_n_fn
{
template<std::forward_iterator I, std::sentinel_for<I> S, class T,
class Pred = ranges::equal_to, class Proj = std::identity>
requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
operator()(I first, S last, std::iter_difference_t<I> count,
const T& value, Pred pred = {}, Proj proj = {}) const {
if (count <= 0)
return {first, first};
for (; first != last; ++first) {
if (std::invoke(pred, std::invoke(proj, *first), value)) {
I start = first;
std::iter_difference_t<I> n{1};
for (;;) {
if (n++ == count)
return {start, std::next(first)}; // found
if (++first == last)
return {first, first}; // not found
if (!std::invoke(pred, std::invoke(proj, *first), value))
break; // not equ to value
}
}
}
return {first, first};
}
template<ranges::forward_range R, class T, class Pred = ranges::equal_to,
class Proj = std::identity>
requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
operator()(R&& r, ranges::range_difference_t<R> count,
const T& value, Pred pred = {}, Proj proj = {}) const {
return (*this)(ranges::begin(r), ranges::end(r),
std::move(count), value,
std::move(pred), std::move(proj));
}
};
inline constexpr search_n_fn search_n{};
|
Example
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <string>
int main()
{
static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1};
constexpr int count{ 3 };
constexpr int value{ 2 };
constexpr auto result1 = std::ranges::search_n(
nums.begin(), nums.end(), count, value
);
static_assert( // found
result1.size() == count &&
std::distance(nums.begin(), result1.begin()) == 6 &&
std::distance(nums.begin(), result1.end()) == 9
);
constexpr auto result2 = std::ranges::search_n(nums, count, value);
static_assert( // found
result2.size() == count &&
std::distance(nums.begin(), result2.begin()) == 6 &&
std::distance(nums.begin(), result2.end()) == 9
);
constexpr auto result3 = std::ranges::search_n(nums, count, /* value */ 5);
static_assert( // not found
result3.size() == 0 &&
result3.begin() == result3.end() &&
result3.end() == nums.end()
);
constexpr auto result4 = std::ranges::search_n(nums, /* count */ 0, /* value */ 1);
static_assert( // not found
result4.size() == 0 &&
result4.begin() == result4.end() &&
result4.end() == nums.begin()
);
constexpr char symbol{'B'};
std::cout << std::boolalpha << "Find a sub-sequence: "
<< std::quoted(std::string(count, symbol)) << '\n';
auto result5 = std::ranges::search_n(nums, count, symbol,
[](const char x, const char y) { // binary predicate
const bool o{ x == y };
std::cout << "bin_op(" << x << ", " << y << ") == " << o << "\n";
return o;
},
[](const int z) { return 'A' + z - 1; } // projects nums -> ASCII
);
std::cout << "Found: " << !result5.empty() << '\n';
}
Output:
Find a sub-sequence: "BBB" bin_op(A, B) == false bin_op(B, B) == true bin_op(B, B) == true bin_op(C, B) == false bin_op(D, B) == false bin_op(A, B) == false bin_op(B, B) == true bin_op(B, B) == true bin_op(B, B) == true Found: true
See also
|
(C++20)
|
finds the first two adjacent items that are equal (or satisfy a given predicate) (niebloid) |
|
(C++20)(C++20)(C++20)
|
finds the first element satisfying specific criteria (niebloid) |
|
(C++20)
|
finds the last sequence of elements in a certain range (niebloid) |
|
(C++20)
|
searches for any one of a set of elements (niebloid) |
|
(C++20)
|
returns true if one sequence is a subsequence of another (niebloid) |
|
(C++20)
|
finds the first position where two ranges differ (niebloid) |
|
(C++20)
|
searches for a range of elements (niebloid) |
| searches a range for a number of consecutive copies of an element (function template) |
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