NAME¶

std::ranges::find_end - std::ranges::find_end

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::forward_iterator I1, std::sentinel_for<I1> S1,

std::forward_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, (since
class Proj2 = std::identity > (1) C++20)
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr ranges::subrange<I1>
find_end( I1 first1, S1 last1, I2 first2, S2 last2,

Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
template< ranges::forward_range R1, ranges::forward_range R2,

class Pred = ranges::equal_to,
class Proj1 = std::identity,
class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>, (2) (since
ranges::iterator_t<R2>, C++20)
Pred, Proj1, Proj2>
constexpr ranges::borrowed_subrange_t<R1>
find_end( R1&& r1, R2&& r2, Pred pred = {},

Proj1 proj1 = {}, Proj2 proj2 = {} );

1) Searches for the last occurrence of the sequence [first2, last2) in the range
[first1, last1), after projection with proj1 and proj2 respectively. The projected
elements are compared using the binary predicate pred.
2) Same as (1), but uses r1 as the first source range and r2 as the second source
range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1,
ranges::begin(r2) as first2, and ranges::end(r2) as last2.

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¶

first1, last1 - the range of elements to examine (aka haystack)
first2, last2 - the range of elements to search for (aka needle)
r1 - the range of elements to examine (aka haystack)
r2 - the range of elements to search for (aka needle)
pred - binary predicate to compare the elements
proj1 - projection to apply to the elements in the first range
proj2 - projection to apply to the elements in the second range

Return value¶

1) ranges::subrange<I1>{} value initialized with expression {i, i + (i == last1 ? 0
: ranges::distance(first2, last2))} that denotes the last occurrence of the sequence
[first2, last2) in range [first1, last1) (after projections with proj1 and proj2).
If [first2, last2) is empty or if no such sequence is found, the return value is
effectively initialized with {last1, last1}.
2) Same as (1), except that the return type is ranges::borrowed_subrange_t<R1>.

Complexity¶

At most \(\scriptsize S\cdot(N-S+1)\)S·(N-S+1) applications of the corresponding
predicate and each projection, where \(\scriptsize S\)S is ranges::distance(first2,
last2) and \(\scriptsize N\)N is ranges::distance(first1, last1) for (1), or
\(\scriptsize S\)S is ranges::distance(r2) and \(\scriptsize N\)N is
ranges::distance(r1) for (2).

Notes¶

An implementation can improve efficiency of the search if the input iterators model
std::bidirectional_iterator by searching from the end towards the begin. Modelling
the std::random_access_iterator may improve the comparison speed. All this however
does not change the theoretical complexity of the worst case.

Possible implementation¶

struct find_end_fn
{
template<std::forward_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr ranges::subrange<I1>
operator()(I1 first1, S1 last1,
I2 first2, S2 last2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
if (first2 == last2)
{
auto last_it = ranges::next(first1, last1);
return {last_it, last_it};
}
auto result = ranges::search(
std::move(first1), last1, first2, last2, pred, proj1, proj2);

if (result.empty())
return result;

for (;;)
{
auto new_result = ranges::search(
std::next(result.begin()), last1, first2, last2, pred, proj1, proj2);
if (new_result.empty())
return result;
else
result = std::move(new_result);
}
}

template<ranges::forward_range R1, ranges::forward_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity,
class Proj2 = std::identity>
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr ranges::borrowed_subrange_t<R1>
operator()(R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), ranges::end(r1),
ranges::begin(r2), ranges::end(r2),
std::move(pred),
std::move(proj1), std::move(proj2));
}
};

inline constexpr find_end_fn find_end {};

Example¶

// Run this code

#include <algorithm>
#include <array>
#include <cctype>
#include <iostream>
#include <ranges>
#include <string_view>

void print(const auto haystack, const auto needle)
{
const auto pos = std::distance(haystack.begin(), needle.begin());
std::cout << "In \"";
for (const auto c : haystack)
std::cout << c;
std::cout << "\" found \"";
for (const auto c : needle)
std::cout << c;
std::cout << "\" at position [" << pos << ".." << pos + needle.size() << ")\n"
<< std::string(4 + pos, ' ') << std::string(needle.size(), '^') << '\n';
}

int main()
{
using namespace std::literals;
constexpr auto secret{"password password word..."sv};
constexpr auto wanted{"password"sv};

constexpr auto found1 = std::ranges::find_end(
secret.cbegin(), secret.cend(), wanted.cbegin(), wanted.cend());
print(secret, found1);

constexpr auto found2 = std::ranges::find_end(secret, "word"sv);
print(secret, found2);

const auto found3 = std::ranges::find_end(secret, "ORD"sv,
[](const char x, const char y) { // uses a binary predicate
return std::tolower(x) == std::tolower(y);
});
print(secret, found3);

const auto found4 = std::ranges::find_end(secret, "SWORD"sv, {}, {},
[](char c) { return std::tolower(c); }); // projects the 2nd range
print(secret, found4);

static_assert(std::ranges::find_end(secret, "PASS"sv).empty()); // => not found
}

Output:¶

In "password password word..." found "password" at position [9..17)
^^^^^^^^
In "password password word..." found "word" at position [18..22)
^^^^
In "password password word..." found "ord" at position [19..22)
^^^
In "password password word..." found "sword" at position [12..17)
^^^^^

See also¶

ranges::find_last
ranges::find_last_if
ranges::find_last_if_not finds the last element satisfying specific criteria
(C++23) (niebloid)
(C++23)
(C++23)
ranges::find
ranges::find_if
ranges::find_if_not finds the first element satisfying specific criteria
(C++20) (niebloid)
(C++20)
(C++20)
ranges::find_first_of searches for any one of a set of elements
(C++20) (niebloid)
ranges::adjacent_find finds the first two adjacent items that are equal (or
(C++20) satisfy a given predicate)
(niebloid)
ranges::search searches for a range of elements
(C++20) (niebloid)
ranges::search_n searches for a number consecutive copies of an element in a
(C++20) range
(niebloid)
find_end finds the last sequence of elements in a certain range
(function template)