std::equal

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	sequentialparpar_vec	sequential_execution_policyparallel_execution_policyparallel_vector_execution_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal lexicographical_compare	findfind_iffind_if_not (C++11) find_end find_first_of	adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy	uninitialized_copy_n	uninitialized_fill	uninitialized_fill_n
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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

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>> is true

The two ranges are considered equal if, 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.
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); The signature does not need to have const &, but the function must not modify the objects passed to it. 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 InputIterator.

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

1-4) At most last1 - first1 applications of the predicate

5-8) At most min(last1 - first1, last2 - first2) applications of the predicate.
However, if InputIt1 and InputIt2 meet the requirements of RandomAccessIterator and last1 - first1 != last2 - first2 then no applications of the predicate are made.

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,

• if policy is std::parallel_vector_execution_policy, std::terminate is called
• if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
• if policy is some other type, the behavior is implementation-defined

• If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), 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 equal() to test if a string is a palindrome

Run this code

#include <algorithm>
#include <iostream>
#include <string>
 
bool is_palindrome(const std::string& s)
{
    return std::equal(s.begin(), s.begin() + s.size()/2, s.rbegin());
}
 
void test(const std::string& 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

findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template)
lexicographical_compare	returns true if one range is lexicographically less than another (function template)
mismatch	finds the first position where two ranges differ (function template)
search	searches for a range of elements (function template)
std::experimental::parallel::equal (parallelism TS)	parallelized version of std::equal (function template)