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) |
true
if the range [first1, last1)
is equal to the range [first2, first2 + (last1 - first1))
, and false
otherwisetrue
if the range [first1, last1)
is equal to the range [first2, last2)
, and false
otherwise.The two ranges are considered equal if, for every iterator i
in the range [first1,last1)
, *i
equals *(first2 + (i - first1))
. The overloads (1,3) use operator==
to determine if two elements are equal, whereas overloads (2,4) 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:
The signature does not need to have |
Type requirements | ||
- InputIt1, InputIt2 must meet the requirements of InputIterator . |
Return value
[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
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==
for the corresponding container are usually preferred.
Complexity
last1
- first1
applications of the predicate p
last1
- first1
, last2
- first2
) applications of the predicate p
.However, if
InputIt1
and InputIt2
meet the requirements of RandomAccessIterator
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.
#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
(C++11) | finds the first element satisfying specific criteria (function template) |
returns true if one range is lexicographically less than another (function template) | |
finds the first position where two ranges differ (function template) | |
searches for a range of elements (function template) | |
std::experimental::parallel::equal
(parallelism TS) | parallelized version of std::equal (function template) |
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