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Difference between revisions of "cpp/algorithm/find end"

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< cpp‎ | algorithm
m (Minor tweak.)
m (Minor fix.)
 
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===Complexity===
 
===Complexity===
−
Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first1, last1)}} and {{mathjax-or|\(\scriptsize S\)|S}} as {{c|std::distance(first2, last2)}}:
+
Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}} and {{mathjax-or|\(\scriptsize S\)|S}} as {{c|std::distance(s_first, s_last)}}:
−
@1,2@ At most {{mathjax-or|\(\scriptsize S\cdot(N-S+1)\)|S&middot;(N-S+1)}} comparisons using {{c/core|operator{{==}}}}.
+
@1,2@ At most {{mathjax-or|\(\scriptsize S\cdot(N-S+1)\)|S&middot;(N-S+1)}} comparisons using {{c/core|1=operator==}}.
 
@3,4@ At most {{mathjax-or|\(\scriptsize S\cdot(N-S+1)\)|S&middot;(N-S+1)}} applications of the predicate {{c|p}}.
 
@3,4@ At most {{mathjax-or|\(\scriptsize S\cdot(N-S+1)\)|S&middot;(N-S+1)}} applications of the predicate {{c|p}}.
  

Latest revision as of 09:17, 28 July 2024

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
template< class ForwardIt1, class ForwardIt2 >

ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,

                     ForwardIt2 s_first, ForwardIt2 s_last );
 (1) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >

ForwardIt1 find_end( ExecutionPolicy&& policy,
                     ForwardIt1 first, ForwardIt1 last,

                     ForwardIt2 s_first, ForwardIt2 s_last );
 (2) (since C++17)
template< class ForwardIt1, class ForwardIt2, class BinaryPred >

ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,

                     BinaryPred p );
 (3) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_end( ExecutionPolicy&& policy,
                     ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,

                     BinaryPred p );
 (4) (since C++17)

Searches for the last occurrence of the sequence [s_first, s_last) in the range [first, last).

1) Elements are compared using operator==.
3) Elements are compared using the given binary predicate p.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - the range of elements to examine
s_first, s_last - the range of elements to search for
policy - the execution policy to use. See execution policy for details.
p - binary predicate which returns ​true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

Type requirements
-
ForwardIt1 must meet the requirements of LegacyForwardIterator.
-
ForwardIt2 must meet the requirements of LegacyForwardIterator.

[edit] Return value

Iterator to the beginning of last occurrence of the sequence [s_first, s_last) in range [first, last).

If [s_first, s_last) is empty or if no such sequence is found, last is returned.

[edit] Complexity

Given N as std::distance(first, last) and S as std::distance(s_first, s_last):

1,2) At most S·(N-S+1) comparisons using operator==.
3,4) At most S·(N-S+1) applications of the predicate p.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

find_end (1) 
template<class ForwardIt1, class ForwardIt2>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last)
{
    if (s_first == s_last)
        return last;
 
    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}
find_end (3) 
template<class ForwardIt1, class ForwardIt2, class BinaryPred>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last,
                    BinaryPred p)
{
    if (s_first == s_last)
        return last;
 
    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last, p);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}

[edit] Example

Run this code
#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
 
auto print_result = [](auto result, const auto& v)
{
    result == v.end()
        ? std::cout << "Sequence not found\n"
        : std::cout << "Last occurrence is at: " << std::distance(v.begin(), result)
                    << '\n';
};
 
int main()
{
    const auto v = {1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4};
 
    for (auto const& x : {std::array{1, 2, 3}, {4, 5, 6}})
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end()); // overload (1)
        print_result(iter, v);
    }
 
    for (auto const& x : {std::array{-1, -2, -3}, {-4, -5, -6}})
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end(), // overload (3)
                                  [](int x, int y)
                                  {
                                      return std::abs(x) == std::abs(y);
                                  });
        print_result(iter, v);
    }
}

Output: 

Last occurrence is at: 8
Sequence not found
Last occurrence is at: 8
Sequence not found

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 1205 C++98 the return value was unclear if [s_first, s_last) is empty returns last in this case
LWG 2150 C++98 the condition of “sequence occurence” was incorrect corrected

[edit] See also

 searches for the first occurrence of a range of elements
(function template) [edit]
returns true if one sequence is a subsequence of another
(function template) [edit]
finds the first two adjacent items that are equal (or satisfy a given predicate)
(function template) [edit]
(C++11)
 finds the first element satisfying specific criteria
(function template) [edit]
searches for any one of a set of elements
(function template) [edit]
searches for the first occurrence of a number consecutive copies of an element in a range
(function template) [edit]
(C++20)
 finds the last sequence of elements in a certain range
(niebloid)[edit]
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