Namespaces
Variants

    std::equal

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

    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


     
    Defined in header <algorithm> 
    template< class InputIt1, class InputIt2 >
bool equal( InputIt1 first1, InputIt1 last1,
            InputIt2 first2 );
    		(1) (constexpr since C++20) 
    template< class InputIt1, class InputIt2, class BinaryPred >
bool equal( InputIt1 first1, InputIt1 last1,
            InputIt2 first2, BinaryPred p );
    		(2) (constexpr since C++20) 
    template< class InputIt1, class InputIt2 >
bool equal( InputIt1 first1, InputIt1 last1,
            InputIt2 first2, InputIt2 last2 );
    		(3) (since C++14)
    (constexpr since C++20)    		 
    template< class InputIt1, class InputIt2, class BinaryPred >
bool equal( InputIt1 first1, InputIt1 last1,
            InputIt2 first2, InputIt2 last2, BinaryPred p );
    		(4) (since C++14)
    (constexpr since C++20)    		 
    template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
bool equal( ExecutionPolicy&& policy,
            ForwardIt1 first1, ForwardIt1 last1,
            ForwardIt2 first2 );
    		(5) (since C++17) 
    template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class BinaryPred >
bool equal( ExecutionPolicy&& policy,
            ForwardIt1 first1, ForwardIt1 last1,
            ForwardIt2 first2, BinaryPred p );
    		(6) (since C++17) 
    template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
bool equal( ExecutionPolicy&& policy,
            ForwardIt1 first1, ForwardIt1 last1,
            ForwardIt2 first2, ForwardIt2 last2 );
    		(7) (since C++17) 
    template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class BinaryPred >
bool equal( ExecutionPolicy&& policy,
            ForwardIt1 first1, ForwardIt1 last1,
            ForwardIt2 first2, ForwardIt2 last2, BinaryPred p );
    		(8) (since C++17)

    Checks whether the two target ranges [first1, last1) and [first2, last2) are equal. For overloads without the last2 parameter, last2 is std::next(first2, std::distance(first1, last1).

    1,3) Elements are compared using operator==.
    2,4) Elements are compared using the given binary predicate p.
    5-8) Same as (1-4), but executed according to policy.
    These overloads participate in overload resolution only if the value of the following expression is true:

    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)

    Parameters

    first1, last1 - the pair of iterators defining the first target range
    first2, last2 - the pair of iterators defining the second target range
    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 InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

    policy - the execution policy to use
    Type requirements
    -
    InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
    -
    ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
    -
    BinaryPred must meet the requirements of BinaryPredicate.

    Return value

    If the two target ranges have the same size, and each corresponding elements in the two ranges are equal, returns true. Otherwise returns false.

    Complexity

    Given N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):

    1) At most N1 comparisons using operator==.
    2) At most N1 applications of p.
    3) At most min(N1,N2) comparisons using operator==.
    4) At most min(N1,N2) applications of p.
    5) 𝓞(N1) comparisons using operator==.
    6) 𝓞(N1) applications of p.
    7) 𝓞(min(N1,N2)) comparisons using operator==.
    8) 𝓞(min(N1,N2)) applications of p.

    Exceptions

    5-8) 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

    equal (1) 
    template<class InputIt1, class InputIt2>
constexpr //< since C++20
bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2)
{
    for (; first1 != last1; ++first1, ++first2)
        if (!(*first1 == *first2))
            return false;
    
    return true;
}
    equal (2) 
    template<class InputIt1, class InputIt2, class BinaryPred>
constexpr //< since C++20
bool equal(InputIt1 first1, InputIt1 last1,
           InputIt2 first2, BinaryPred p)
{
    for (; first1 != last1; ++first1, ++first2)
        if (!p(*first1, *first2))
            return false;
    
    return true;
}
    equal (3) 
    namespace detail
{
    // random-access iterator implementation (allows quick range size detection)
    template<class RandomIt1, class RandomIt2>
    constexpr //< since C++20
    bool equal(RandomIt1 first1, RandomIt1 last1, RandomIt2 first2, RandomIt2 last2,
               std::random_access_iterator_tag, std::random_access_iterator_tag)
    {
        if (last1 - first1 != last2 - first2)
            return false;
        
        for (; first1 != last1; ++first1, ++first2)
            if (!(*first1 == *first2))
                return false;
        
        return true;
    }
    
    // input iterator implementation (needs to manually compare with “last2”)
    template<class InputIt1, class InputIt2>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2,
               std::input_iterator_tag, std::input_iterator_tag)
    {
        for (; first1 != last1 && first2 != last2; ++first1, ++first2)
            if (!(*first1 == *first2))
                return false;
        
        return first1 == last1 && first2 == last2;
    }
}

template<class InputIt1, class InputIt2>
constexpr //< since C++20
bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2)
{
    details::equal(first1, last1, first2, last2,
                   typename std::iterator_traits<InputIt1>::iterator_category(),
                   typename std::iterator_traits<InputIt2>::iterator_category());
}
    equal (4) 
    namespace detail
{
    // random-access iterator implementation (allows quick range size detection)
    template<class RandomIt1, class RandomIt2, class BinaryPred>
    constexpr //< since C++20
    bool equal(RandomIt1 first1, RandomIt1 last1,
               RandomIt2 first2, RandomIt2 last2, BinaryPred p,
               std::random_access_iterator_tag, std::random_access_iterator_tag)
    {
        if (last1 - first1 != last2 - first2)
            return false;
        
        for (; first1 != last1; ++first1, ++first2)
            if (!p(*first1, *first2))
                return false;
        
        return true;
    }
    
    // input iterator implementation (needs to manually compare with “last2”)
    template<class InputIt1, class InputIt2, class BinaryPred>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2, BinaryPred p,
               std::input_iterator_tag, std::input_iterator_tag)
    {
        for (; first1 != last1 && first2 != last2; ++first1, ++first2)
            if (!p(*first1, *first2))
                return false;
        
        return first1 == last1 && first2 == last2;
    }
}

template<class InputIt1, class InputIt2, class BinaryPred>
constexpr //< since C++20
bool equal(InputIt1 first1, InputIt1 last1,
           InputIt2 first2, InputIt2 last2, BinaryPred p)
{
    details::equal(first1, last1, first2, last2, p,
                   typename std::iterator_traits<InputIt1>::iterator_category(),
                   typename std::iterator_traits<InputIt2>::iterator_category());
}

    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 or string views(since C++17) for equality, operator== for the corresponding type are usually preferred.

    Sequential std::equal is not guaranteed to be short-circuit. E.g. if the first pair elements of both ranges do not compare equal, the rest of elements may also be compared. Non-short-circuit comparison may happen when the ranges are compared with std::memcmp or implementation-specific vectorized algorithms.

    Example

    The following code uses std::equal to test if a string is a palindrome.

    Run this code
    #include <algorithm>
#include <iomanip>
#include <iostream>
#include <string_view>

constexpr bool is_palindrome(const std::string_view& s)
{
    return std::equal(s.cbegin(), s.cbegin() + s.size() / 2, s.crbegin());
}

void test(const std::string_view& s)
{
    std::cout << std::quoted(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

    (C++20)
    		 determines if two sets of elements are the same
    (algorithm function object)[edit]
    (C++11)
    		 finds the first element satisfying specific criteria
    (function template & algorithm function object)[edit]
    (C++20)(C++20)(C++20)
    		compares two ranges lexicographically
    (function template & algorithm function object)[edit]
    (C++20)
    		finds the first position where two ranges differ
    (function template & algorithm function object)[edit]
    (C++20)
    		searches for the first occurrence of a range of elements
    (function template & algorithm function object)[edit]
    (C++20)
    		finds the range of elements matching the given value using binary search
    (function template & algorithm function object)[edit]
    (C++20)
    		function object implementing x == y
    (class template) [edit]
    Morty Proxy This is a proxified and sanitized view of the page, visit original site.