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Difference between revisions of "cpp/algorithm/equal"

From cppreference.com
< cpp‎ | algorithm
m (Extract intention-revealing predicate)
m (→‎Notes)
 
(40 intermediate revisions by 18 users not shown)
Line 2: Line 2:
 
{{cpp/algorithm/navbar}}
 
{{cpp/algorithm/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | algorithm}}
+
{{dcl header|algorithm}}
−
{{dcl | num=1 |
+
{{dcla|num=1|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2 >
 
template< class InputIt1, class InputIt2 >
−
bool equal( InputIt1 first1, InputIt1 last1,  
+
bool equal( InputIt1 first1, InputIt1 last1,
 
             InputIt2 first2 );
 
             InputIt2 first2 );
 
}}
 
}}
−
{{dcl | num=2 |  
+
{{dcl|num=2|since=c++17|
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
−
bool equal( InputIt1 first1, InputIt1 last1,  
+
bool equal( ExecutionPolicy&& policy,
−
             InputIt2 first2, BinaryPredicate p );
+
            ForwardIt1 first1, ForwardIt1 last1,
 +
             ForwardIt2 first2 );
 
}}
 
}}
−
{{dcl | num=3 | since=c++14 |
+
{{dcla|num=3|notes={{mark constexpr since c++20}}|
 +
template< class InputIt1, class InputIt2, class BinaryPred >
 +
bool equal( InputIt1 first1, InputIt1 last1,
 +
            InputIt2 first2, BinaryPred p );
 +
}}
 +
{{dcl|num=4|since=c++17|
 +
template< class ExecutionPolicy,
 +
          class ForwardIt1, class ForwardIt2, class BinaryPred >
 +
bool equal( ExecutionPolicy&& policy,
 +
            ForwardIt1 first1, ForwardIt1 last1,
 +
            ForwardIt2 first2, BinaryPred p );
 +
}}
 +
{{dcla|num=5|since=c++14|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2 >
 
template< class InputIt1, class InputIt2 >
−
bool equal( InputIt1 first1, InputIt1 last1,  
+
bool equal( InputIt1 first1, InputIt1 last1,
 
             InputIt2 first2, InputIt2 last2 );
 
             InputIt2 first2, InputIt2 last2 );
 
}}
 
}}
−
{{dcl | num=4 | since=c++14 |
+
{{dcl|num=6|since=c++17|
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
−
bool equal( InputIt1 first1, InputIt1 last1,  
+
bool equal( ExecutionPolicy&& policy,
−
             InputIt2 first2, InputIt2 last2,
+
            ForwardIt1 first1, ForwardIt1 last1,
−
             BinaryPredicate p );
+
            ForwardIt2 first2, ForwardIt2 last2 );
 +
}}
 +
{{dcla|num=7|since=c++14|notes={{mark constexpr since c++20}}|
 +
template< class InputIt1, class InputIt2, class BinaryPred >
 +
bool equal( InputIt1 first1, InputIt1 last1,
 +
             InputIt2 first2, InputIt2 last2, BinaryPred p );
 +
}}
 +
{{dcl|num=8|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 );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Returns {{c|true}} if the ranges {{tt|[first1, last1)}} and {{tt|[first2, last2)}} are considered equal, where {{tt|last2}} denotes {{c|first2 + (last1 - first1)}} if it was not given. The overloads (1) and (3) use {{c|operator{{==}}}} to compare the elements, whereas overloads (2) and (4) use the given binary predicate {{tt|p}}.
+
Checks whether {{range|first1|last1}} and a range starting from {{c|first2}} are equal:
 +
* For overloads {{v|1-4}}, the second range has {{c|std::distance(first1, last1)}} elements.
 +
* For overloads {{v|5-8}}, the second range is {{range|first2|last2}}.
 +
 
 +
@1,5@ Elements are compared using {{c/core|1=operator==}}.
 +
 
 +
@3,7@ Elements are compared using the given binary predicate {{c|p}}.
 +
 
 +
@2,4,6,8@ Same as {{v|1,3,5,7}}, but executed according to {{c|policy}}.
 +
@@{{cpp/algorithm/parallel overload precondition|plural=yes}}
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | first1, last1 | the first range of the elements to compare}}
+
{{par|first1, last1|the first range of the elements to compare}}
−
{{par | first2, last2 | the second range of the elements to compare}}
+
{{par|first2, last2|the second range of the elements to compare}}
−
{{par pred2 eq | p | p1=InputIt1 | p2=InputIt2}}
+
{{par exec pol}}
 +
{{par pred2 eq|p|p1=InputIt1|p2=InputIt2}}
 
{{par hreq}}
 
{{par hreq}}
−
{{par req concept | InputIt1, InputIt2 | InputIterator}}
+
{{par req named|InputIt1, InputIt2|InputIterator}}
 +
{{par req named|ForwardIt1, ForwardIt2|ForwardIterator}}
 +
{{par req named|BinaryPred|BinaryPredicate}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
−
If {{c|last1 - first1 !{{=}} last2 - first2}}, returns {{c|false}}
+
@1-4@ If each corresponding elements in the two ranges are equal, returns {{c|true}}. Otherwise returns {{c|false}}.
  
−
Otherwise, if the elements in the two ranges are equal, returns {{c|true}}.
+
@5-8@ If {{c|std::distance(first1, last1)}} and {{c|std::distance(first2, last2)}} are equal, and each corresponding elements in the two ranges are equal, returns {{c|true}}. Otherwise returns {{c|false}}.
  
−
Otherwise returns {{c|false}}.
+
===Complexity===
 +
Given {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} as {{c|std::distance(first1, last1)}} and {{mathjax-or|\(\scriptsize N_2\)|N{{su|b=2}}}} as {{c|std::distance(first2, last2)}}:
  
−
===Notes===
+
@1@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} comparisons using {{c/core|1=operator==}}.
  
−
{{tt|std::equal}} may not be used to compare the ranges formed by the iterators from {{lc|std::unordered_set}}, {{lc|std::unordered_multiset}}, {{lc|std::unordered_map}}, or {{lc|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.  
+
@2@ {{mathjax-or|\(\scriptsize O(N_1)\)|O(N{{su|b=1}})}} comparisons using {{c/core|1=operator==}}.
  
−
When comparing entire containers for equality, operator== is usually preferred.
+
@3@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} applications of the predicate {{c|p}}.
  
−
===Complexity===
+
@4@ {{mathjax-or|\(\scriptsize O(N_1)\)|O(N{{su|b=1}})}} applications of the predicate {{c|p}}.
−
At most min({{tt|last1}} - {{tt|first1}}, {{tt|last2}} - {{tt|first2}}) applications of the predicate {{tt|p}}.
+
  
−
However, if {{tt|InputIt1}} and {{tt|InputIt2}} meet the requirements of {{concept|RandomAccessIterator}} and {{c|last1 - first1 !{{=}} last2 - first2}} then no applications of the predicate {{tt|p}} are made.
+
@5-8@ If {{tt|InputIt1}} and {{tt|InputIt2}} are both {{named req|RandomAccessIterator}}, and {{c|1=last1 - first1 != last2 - first2}} is {{c|true}}, no comparison will be made.
 +
@@ Otherwise, given {{mathjax-or|\(\scriptsize N\)|N}} as {{mathjax-or|\(\scriptsize \min(N_1,N_2)\)|min(N{{su|b=1}},N{{su|b=2}})}}:
 +
 
 +
:@5@ At most {{mathjax-or|\(\scriptsize N\)|N}} comparisons using {{c/core|1=operator==}}.
 +
 
 +
:@6@ {{mathjax-or|\(\scriptsize O(N)\)|O(N)}} comparisons using {{c/core|1=operator==}}.
 +
 
 +
:@7@ At most {{mathjax-or|\(\scriptsize N\)|N}} applications of the predicate {{c|p}}.
 +
 
 +
:@8@ {{mathjax-or|\(\scriptsize O(N)\)|O(N)}} applications of the predicate {{c|p}}.
 +
 
 +
===Exceptions===
 +
{{cpp/algorithm/parallel exceptions reporting behavior|singular=no}}
  
 
===Possible implementation===
 
===Possible implementation===
−
{{eq fun
+
{{eq impl
−
| 1=
+
|title1=equal (1)|ver1=1|1=
 
template<class InputIt1, class InputIt2>
 
template<class InputIt1, class InputIt2>
−
bool equal(InputIt1 first1, InputIt1 last1,  
+
constexpr //< since C++20
−
          InputIt2 first2)
+
bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2)
 
{
 
{
−
     for (; first1 != last1; ++first1, ++first2) {
+
     for (; first1 != last1; ++first1, ++first2)
−
         if (!(*first1 == *first2)) {
+
         if (!(*first1 == *first2))
 
             return false;
 
             return false;
−
        }
+
      
−
     }
+
 
     return true;
 
     return true;
 
}
 
}
−
| 2=
+
|title2=equal (3)|ver2=3|2=
−
template<class InputIt1, class InputIt2, class BinaryPredicate>
+
template<class InputIt1, class InputIt2, class BinaryPred>
−
bool equal(InputIt1 first1, InputIt1 last1,  
+
constexpr //< since C++20
−
           InputIt2 first2, BinaryPredicate p)
+
bool equal(InputIt1 first1, InputIt1 last1,
 +
           InputIt2 first2, BinaryPred p)
 
{
 
{
−
     for (; first1 != last1; ++first1, ++first2) {
+
     for (; first1 != last1; ++first1, ++first2)
−
         if (!p(*first1, *first2)) {
+
         if (!p(*first1, *first2))
 
             return false;
 
             return false;
−
        }
+
      
−
     }
+
 
     return true;
 
     return true;
 +
}
 +
|title3=equal (5)|ver3=5|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());
 +
}
 +
|title4=equal (7)|ver4=7|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===
 +
{{tt|std::equal}} should not be used to compare the ranges formed by the iterators from {{lc|std::unordered_set}}, {{lc|std::unordered_multiset}}, {{lc|std::unordered_map}}, or {{lc|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 {{rev inl|since=c++17|or string views}} for equality, {{c/core|1=operator==}} for the corresponding type are usually preferred.
 +
 +
Sequential {{tt|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 {{lc|std::memcmp}} or implementation-specific vectorized algorithms.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| The following code uses {{c|equal()}} to test if a string is a palindrome
+
|The following code uses {{ttt|std::equal}} to test if a string is a palindrome.
−
| code=
+
|code=
 
#include <algorithm>
 
#include <algorithm>
 +
#include <iomanip>
 
#include <iostream>
 
#include <iostream>
−
#include <string>
+
#include <string_view>
  
−
bool is_palindrome(const std::string& s)
+
constexpr bool is_palindrome(const std::string_view& s)
 
{
 
{
−
     return std::equal(s.begin(), s.begin() + s.size()/2, s.rbegin());
+
     return std::equal(s.cbegin(), s.cbegin() + s.size() / 2, s.crbegin());
 
}
 
}
  
−
void test(const std::string& s)
+
void test(const std::string_view& s)
 
{
 
{
−
     std::cout << "\"" << s << "\" "
+
     std::cout << std::quoted(s)
−
        << (is_palindrome(s) ? "is" : "is not")
+
              << (is_palindrome(s) ? " is" : " is not")
−
        << " a palindrome\n";
+
              << " a palindrome\n";
 
}
 
}
  
Line 108: Line 249:
 
     test("hello");
 
     test("hello");
 
}
 
}
−
| output=
+
|output=
 
"radar" is a palindrome
 
"radar" is a palindrome
−
"hello" is not palindrome
+
"hello" is not a palindrome
 
}}
 
}}
  
 +
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/algorithm/dsc find}}
+
{{dsc inc|cpp/algorithm/dsc find}}
−
{{dsc inc | cpp/algorithm/dsc lexicographical_compare}}
+
{{dsc inc|cpp/algorithm/dsc lexicographical_compare}}
−
{{dsc inc | cpp/algorithm/dsc mismatch}}
+
{{dsc inc|cpp/algorithm/dsc mismatch}}
−
{{dsc inc | cpp/algorithm/dsc search}}
+
{{dsc inc|cpp/algorithm/dsc search}}
 +
{{dsc inc|cpp/algorithm/ranges/dsc equal}}
 +
{{dsc inc|cpp/utility/functional/dsc equal_to}}
 +
{{dsc inc|cpp/algorithm/dsc equal_range}}
 
{{dsc end}}
 
{{dsc end}}
  
−
[[de:cpp/algorithm/equal]]
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
−
[[es:cpp/algorithm/equal]]
+
−
[[fr:cpp/algorithm/equal]]
+
−
[[it:cpp/algorithm/equal]]
+
−
[[ja:cpp/algorithm/equal]]
+
−
[[pt:cpp/algorithm/equal]]
+
−
[[ru:cpp/algorithm/equal]]
+
−
[[zh:cpp/algorithm/equal]]
+

Latest revision as of 18:45, 11 November 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)
equal

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 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 from first2 are equal:

  • For overloads (1-4), the second range has std::distance(first1, last1) elements.
  • For overloads (5-8), the second range is [first2, last2).
1,5) Elements are compared using operator==.
3,7) Elements are compared using the given binary predicate p.
2,4,6,8) Same as (1,3,5,7), 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

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.
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. ​

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.

[edit] Return value

1-4) If each corresponding elements in the two ranges are equal, returns true. Otherwise returns false.
5-8) If std::distance(first1, last1) and std::distance(first2, last2) are equal, and each corresponding elements in the two ranges are equal, returns true. Otherwise returns false.

[edit] Complexity

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

1) At most N1 comparisons using operator==.
2) O(N1) comparisons using operator==.
3) At most N1 applications of the predicate p.
4) O(N1) applications of the predicate p.
5-8) If InputIt1 and InputIt2 are both LegacyRandomAccessIterator, and last1 - first1 != last2 - first2 is true, no comparison will be made.
Otherwise, given N as min(N1,N2):
5) At most N comparisons using operator==.
6) O(N) comparisons using operator==.
7) At most N applications of the predicate p.
8) O(N) 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

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 (3) 
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 (5) 
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 (7) 
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());
}

[edit] 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.

[edit] 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

[edit] See also

(C++11)
 finds the first element satisfying specific criteria
(function template) [edit]
returns true 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]
(C++20)
 determines if two sets of elements are the same
(niebloid)[edit]
function object implementing x == y
(class template) [edit]
returns range of elements matching a specific key
(function template) [edit]
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