std::equal

 

C++

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
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)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1,             InputIt2 first2 );	(1)
template< class InputIt1, class InputIt2, class BinaryPredicate > bool equal( InputIt1 first1, InputIt1 last1,             InputIt2 first2, BinaryPredicate p );	(2)
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1,             InputIt2 first2, InputIt2 last2 );	(3)	(since C++14)
template< class InputIt1, class InputIt2, class BinaryPredicate > bool equal( InputIt1 first1, InputIt1 last1,             InputIt2 first2, InputIt2 last2,             BinaryPredicate p );	(4)	(since C++14)

Returns true if the ranges [first1, last1) and [first2, last2) are considered equal, where last2 denotes first2 + (last1 - first1) if it was not given. The overloads (1) and (3) use operator== to compare the elements, whereas overloads (2) and (4) 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
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

Return value

If std::distance(first1,last1) != std::distance(first2,last2) returns false. Otherwise if the elements in the two ranges are equal returns true. Otherwise returns false.

Notes

std::equal may 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== is usually preferred.

Complexity

At most min(last1 - first1, last2 - first2) applications of the predicate p.

However, if InputIt1 and InputIt2 meet the requirements of Template:concept and last1 - first1 != last2 - first2 then no applications of the predicate p are made.

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 <iostream>
#include <algorithm>
#include <string>
 
void test(const std::string& s)
{
    if(std::equal(s.begin(), s.begin() + s.size()/2, s.rbegin())) {
        std::cout << "\"" << s << "\" is a palindrome\n";
    } else {
        std::cout << "\"" << s << "\" is not palindrome\n";
    }
}
int main()
{
    test("radar");
    test("hello");
}

Output:

"radar" is a palindrome
"hello" is not 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 the first occurrence of a range of elements (function template)