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      std::equal

      From cppreference.com
      <cpp‎ |algorithm
       
       
      Algorithm library
      Constrained algorithms and algorithms on ranges(C++20)
      Constrained algorithms, e.g.ranges::copy,ranges::sort, ...
      Execution policies(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
      Permutation operations
      C library
      Numeric operations
      Operations on uninitialized memory
       
      Defined in header<algorithm>
      template<class InputIt1,class InputIt2>

      bool equal( InputIt1 first1, InputIt1 last1,

                  InputIt2 first2);
      		 (1)(constexpr since C++20)
      template<class ExecutionPolicy,class ForwardIt1,class ForwardIt2>

      bool equal( ExecutionPolicy&& policy,
                  ForwardIt1 first1, ForwardIt1 last1,

                  ForwardIt2 first2);
      		 (2)(since C++17)
      template<class InputIt1,class InputIt2,class BinaryPred>

      bool equal( InputIt1 first1, InputIt1 last1,

                  InputIt2 first2, BinaryPred p);
      		 (3)(constexpr since C++20)
      template<class ExecutionPolicy,

               class ForwardIt1,class ForwardIt2,class BinaryPred>
      bool equal( ExecutionPolicy&& policy,
                  ForwardIt1 first1, ForwardIt1 last1,

                  ForwardIt2 first2, BinaryPred p);
      		 (4)(since C++17)
      template<class InputIt1,class InputIt2>

      bool equal( InputIt1 first1, InputIt1 last1,

                  InputIt2 first2, InputIt2 last2);
      		 (5)(since C++14)
      (constexpr since C++20)
      template<class ExecutionPolicy,class ForwardIt1,class ForwardIt2>

      bool equal( ExecutionPolicy&& policy,
                  ForwardIt1 first1, ForwardIt1 last1,

                  ForwardIt2 first2, ForwardIt2 last2);
      		 (6)(since C++17)
      template<class InputIt1,class InputIt2,class BinaryPred>

      bool equal( InputIt1 first1, InputIt1 last1,

                  InputIt2 first2, InputIt2 last2, BinaryPred p);
      		 (7)(since C++14)
      (constexpr since C++20)
      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[first1, last1) and a range starting fromfirst2 are equal:

      • For overloads(1-4), the second range hasstd::distance(first1, last1) elements.
      • For overloads(5-8), the second range is[first2, last2).
      1,5) Elements are compared usingoperator==.
      3,7) Elements are compared using the given binary predicatep.
      2,4,6,8) Same as(1,3,5,7), but executed according topolicy.
      These overloads participate in overload resolution only if all following conditions are satisfied:

      std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> istrue.

      		(until C++20)

      std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> istrue.

      		(since C++20)

      Contents

      [edit]Parameters

      first1, last1 - the pair of iterators defining the firstrange of elements to compare
      first2, last2 - the pair of iterators defining the secondrange of elements to compare
      policy - theexecution policy to use
      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 haveconst&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const)Type1 andType2 regardless ofvalue category (thus,Type1& is not allowed, nor isType1 unless forType1 a move is equivalent to a copy(since C++11)).
      The typesType1 andType2 must be such that objects of typesInputIt1 andInputIt2 can be dereferenced and then implicitly converted toType1 andType2 respectively.​

      Type requirements
      -
      InputIt1, InputIt2 must meet the requirements ofLegacyInputIterator.
      -
      ForwardIt1, ForwardIt2 must meet the requirements ofLegacyForwardIterator.
      -
      BinaryPred must meet the requirements ofBinaryPredicate.

      [edit]Return value

      1-4) If each corresponding elements in the two ranges are equal, returnstrue. Otherwise returnsfalse.
      5-8) Ifstd::distance(first1, last1) andstd::distance(first2, last2) are equal, and each corresponding elements in the two ranges are equal, returnstrue. Otherwise returnsfalse.

      [edit]Complexity

      Given\(\scriptsize N_1\)N1 asstd::distance(first1, last1) and\(\scriptsize N_2\)N2 asstd::distance(first2, last2):

      1) At most\(\scriptsize N_1\)N1 comparisons usingoperator==.
      2)\(\scriptsize O(N_1)\)O(N1) comparisons usingoperator==.
      3) At most\(\scriptsize N_1\)N1 applications of the predicatep.
      4)\(\scriptsize O(N_1)\)O(N1) applications of the predicatep.
      5-8) IfInputIt1 andInputIt2 are bothLegacyRandomAccessIterator, andlast1- first1!= last2- first2 istrue, no comparison will be made.
      Otherwise, given\(\scriptsize N\)N as\(\scriptsize \min(N_1,N_2)\)min(N1,N2):
      5) At most\(\scriptsize N\)N comparisons usingoperator==.
      6)\(\scriptsize O(N)\)O(N) comparisons usingoperator==.
      7) At most\(\scriptsize N\)N applications of the predicatep.
      8)\(\scriptsize O(N)\)O(N) applications of the predicatep.

      [edit]Exceptions

      The overloads with a template parameter namedExecutionPolicy report errors as follows:

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

      [edit]Possible implementation

      equal (1)
      template<class InputIt1,class InputIt2>constexpr//< since C++20bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2){for(; first1!= last1;++first1,++first2)if(!(*first1==*first2))returnfalse; returntrue;}
      equal (3)
      template<class InputIt1,class InputIt2,class BinaryPred>constexpr//< since C++20bool equal(InputIt1 first1, InputIt1 last1,           InputIt2 first2, BinaryPred p){for(; first1!= last1;++first1,++first2)if(!p(*first1,*first2))returnfalse; returntrue;}
      equal (5)
      namespace detail{// random-access iterator implementation (allows quick range size detection)template<class RandomIt1,class RandomIt2>constexpr//< since C++20bool equal(RandomIt1 first1, RandomIt1 last1, RandomIt2 first2, RandomIt2 last2,std::random_access_iterator_tag,std::random_access_iterator_tag){if(last1- first1!= last2- first2)returnfalse; for(; first1!= last1;++first1,++first2)if(!(*first1==*first2))returnfalse; returntrue;} // input iterator implementation (needs to manually compare with “last2”)template<class InputIt1,class InputIt2>constexpr//< since C++20bool 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))returnfalse; return first1== last1&& first2== last2;}} template<class InputIt1,class InputIt2>constexpr//< since C++20bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2){    details::equal(first1, last1, first2, last2,typenamestd::iterator_traits<InputIt1>::iterator_category(),typenamestd::iterator_traits<InputIt2>::iterator_category());}
      equal (7)
      namespace detail{// random-access iterator implementation (allows quick range size detection)template<class RandomIt1,class RandomIt2,class BinaryPred>constexpr//< since C++20bool 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)returnfalse; for(; first1!= last1;++first1,++first2)if(!p(*first1,*first2))returnfalse; returntrue;} // input iterator implementation (needs to manually compare with “last2”)template<class InputIt1,class InputIt2,class BinaryPred>constexpr//< since C++20bool 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))returnfalse; return first1== last1&& first2== last2;}} template<class InputIt1,class InputIt2,class BinaryPred>constexpr//< since C++20bool equal(InputIt1 first1, InputIt1 last1,           InputIt2 first2, InputIt2 last2, BinaryPred p){    details::equal(first1, last1, first2, last2, p,typenamestd::iterator_traits<InputIt1>::iterator_category(),typenamestd::iterator_traits<InputIt2>::iterator_category());}

      [edit]Notes

      std::equal should not be used to compare the ranges formed by the iterators fromstd::unordered_set,std::unordered_multiset,std::unordered_map, orstd::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 containersor string views(since C++17) for equality,operator== for the corresponding type are usually preferred.

      Sequentialstd::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 withstd::memcmp or implementation-specific vectorized algorithms.

      [edit]Example

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

      Run this code
      #include <algorithm>#include <iomanip>#include <iostream>#include <string_view> constexprbool is_palindrome(conststd::string_view& s){return std::equal(s.cbegin(), s.cbegin()+ s.size()/2, s.crbegin());} void test(conststd::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

      [edit]See also

      finds the first element satisfying specific criteria
      (function template)[edit]
      returnstrue if one range is lexicographically less than another
      (function template)[edit]
      finds the first position where two ranges differ
      (function template)[edit]
      searches for the first occurrence of a range of elements
      (function template)[edit]
      determines if two sets of elements are the same
      (algorithm function object)[edit]
      function object implementingx== y
      (class template)[edit]
      returns range of elements matching a specific key
      (function template)[edit]
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