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std::equal(3) C++ Standard Libary std::equal(3) NAME std::equal - std::equal Synopsis Defined in header <algorithm> template< class InputIt1, class InputIt2 > (until bool equal( InputIt1 first1, InputIt1 last1, C++20) InputIt2 first2 ); template< class InputIt1, class InputIt2 > constexpr bool equal( InputIt1 first1, (since InputIt1 last1, C++20) InputIt2 first2 ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > (since bool equal( ExecutionPolicy&& policy, (2) C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2 ); template< class InputIt1, class InputIt2, class BinaryPredicate > (until bool equal( InputIt1 first1, C++20) InputIt1 last1, InputIt2 first2, BinaryPredicate p ); template< class InputIt1, class InputIt2, class BinaryPredicate > (since constexpr bool equal( InputIt1 first1, C++20) InputIt1 last1, InputIt2 first2, BinaryPredicate p ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > (since bool equal( ExecutionPolicy&& policy, (4) C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, (1) BinaryPredicate p ); template< class InputIt1, class InputIt2 > (since C++14) bool equal( InputIt1 first1, InputIt1 last1, (until C++20) InputIt2 first2, InputIt2 last2 ); template< class InputIt1, class InputIt2 > constexpr bool equal( InputIt1 first1, (since InputIt1 last1, (3) C++20) InputIt2 first2, InputIt2 last2 ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > (6) (since bool equal( ExecutionPolicy&& policy, C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 ); template< class InputIt1, class InputIt2, (since class BinaryPredicate > (5) C++14) bool equal( InputIt1 first1, InputIt1 last1, (until InputIt2 first2, InputIt2 last2, C++20) BinaryPredicate p ); template< class InputIt1, class InputIt2, class BinaryPredicate > (since constexpr bool equal( InputIt1 first1, C++20) InputIt1 last1, (7) InputIt2 first2, InputIt2 last2, BinaryPredicate p ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > (8) (since bool equal( ExecutionPolicy&& policy, C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, BinaryPredicate p ); 1,3) Returns true if the range [first1, last1) is equal to the range [first2, first2 + (last1 - first1)), and false otherwise. 5,7) Returns true if the range [first1, last1) is equal to the range [first2, last2), and false otherwise. 2,4,6,8) Same as (1,3,5,7), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true. Two ranges are considered equal if they have the same number of ele- ments and, for every iterator i in the range [first1,last1), *i equals *(first2 + (i - first1)). The overloads (1,2,5,6) use operator== to determine if two elements are equal, whereas overloads (3,4,7,8) use the given binary predicate p. Parameters first1, last1 - the first range of the elements to compare first2, last2 - the second range of the elements to compare policy - the execution policy to use. See execution policy for details. 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); p - 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 re- gardless 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 InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. Type requirements - InputIt1, InputIt2 must meet the requirements of LegacyInputItera- tor. - ForwardIt1, ForwardIt2 must meet the requirements of LegacyFor- wardIterator. Return value 5-8) If the length of the range [first1, last1) does not equal the length of the range [first2, last2), returns false If the elements in the two ranges are equal, returns true. Otherwise returns false. Notes std::equal should not be used to compare the ranges formed by the iterators from std::unordered_set, std::unordered_multiset, std::unordered_map, or std::unordered_multimap because the order in which the elements are stored in those containers may be different even if the two containers store the same elements. When comparing entire containers for equality, operator== for the corresponding container are usually preferred. Complexity 1,3) At most last1 - first1 applications of the predicate 5,7) At most min(last1 - first1, last2 - first2) applications of the predicate. However, if InputIt1 and InputIt2 meet the requirements of LegacyRandomAccessIterator and last1 - first1 != last2 - first2 then no applications of the predicate are made (size mismatch is detected without looking at any elements). 2,4,6,8) same, but the complexity is specified as O(x), rather than "at most x" 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::termi- nate is called. For any other ExecutionPolicy, the behavior is implementation- defined. * If the algorithm fails to allocate memory, std::bad_alloc is thrown. Possible implementation First version template<class InputIt1, class InputIt2> bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2) { for (; first1 != last1; ++first1, ++first2) { if (!(*first1 == *first2)) { return false; } } return true; } Second version template<class InputIt1, class InputIt2, class BinaryPredicate> bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p) { for (; first1 != last1; ++first1, ++first2) { if (!p(*first1, *first2)) { return false; } } return true; } Example The following code uses std::equal to test if a string is a palin- drome. // Run this code #include <algorithm> #include <iostream> #include <string_view> constexpr bool is_palindrome(const std::string_view& s) { return std::equal(s.begin(), s.begin() + s.size()/2, s.rbegin()); } void test(const std::string_view& s) { std::cout << "\"" << s << "\" " << (is_palindrome(s) ? "is" : "is not") << " a palindrome\n"; } int main() { test("radar"); test("hello"); } Output: "radar" is a palindrome "hello" is not a palindrome See also find find_if finds the first element satisfying specific criteria find_if_not (function template) (C++11) returns true if one range is lexicographi- cally less than lexicographical_compare another (function template) mismatch finds the first position where two ranges differ (function template) search searches for a range of elements (function template) ranges::equal determines if two sets of elements are the same (C++20) (niebloid) equal_to function object implementing x == y (class template) equal_range returns range of elements matching a spe- cific key (function template) http://cppreference.com 2022.07.31 std::equal(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Notes | Complexity | Exceptions | Possible implementation | First version | Second version | Example | Output: | See also
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