Difference between revisions of "cpp/algorithm/equal"
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{{cpp/algorithm/navbar}} | {{cpp/algorithm/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
− | {{dcl header | algorithm}} | + | {{dcl header|algorithm}} |
− | {{ | + | {{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 | since=c++17 | | + | {{dcl|num=2|since=c++17| |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > | template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > | ||
− | bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, | + | bool equal( ExecutionPolicy&& policy, |
+ | ForwardIt1 first1, ForwardIt1 last1, | ||
ForwardIt2 first2 ); | ForwardIt2 first2 ); | ||
}} | }} | ||
− | {{ | + | {{dcla|num=3|notes={{mark constexpr since c++20}}| |
− | template< class InputIt1, class InputIt2, class | + | template< class InputIt1, class InputIt2, class BinaryPred > |
− | bool equal( InputIt1 first1, InputIt1 last1, | + | bool equal( InputIt1 first1, InputIt1 last1, |
− | InputIt2 first2, | + | InputIt2 first2, BinaryPred p ); |
− | + | ||
− | + | ||
− | + | ||
− | + | ||
}} | }} | ||
− | {{dcl | since=c++17 | | + | {{dcl|num=4|since=c++17| |
− | template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, | + | template< class ExecutionPolicy, |
− | + | class ForwardIt1, class ForwardIt2, class BinaryPred > | |
− | bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, | + | bool equal( ExecutionPolicy&& policy, |
− | ForwardIt2 first2, | + | 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 | since=c++17 | | + | {{dcl|num=6|since=c++17| |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > | template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > | ||
− | bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, | + | bool equal( ExecutionPolicy&& policy, |
+ | ForwardIt1 first1, ForwardIt1 last1, | ||
ForwardIt2 first2, ForwardIt2 last2 ); | ForwardIt2 first2, ForwardIt2 last2 ); | ||
}} | }} | ||
− | {{ | + | {{dcla|num=7|since=c++14|notes={{mark constexpr since c++20}}| |
− | template< class InputIt1, class InputIt2, class | + | template< class InputIt1, class InputIt2, class BinaryPred > |
− | bool equal( InputIt1 first1, InputIt1 last1, | + | bool equal( InputIt1 first1, InputIt1 last1, |
− | InputIt2 first2, InputIt2 last2, | + | InputIt2 first2, InputIt2 last2, BinaryPred p ); |
− | + | ||
− | + | ||
− | + | ||
− | + | ||
− | + | ||
− | + | ||
}} | }} | ||
− | {{dcl | since=c++17 | | + | {{dcl|num=8|since=c++17| |
− | template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, | + | template< class ExecutionPolicy, |
− | + | class ForwardIt1, class ForwardIt2, class BinaryPred > | |
− | bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, | + | bool equal( ExecutionPolicy&& policy, |
− | ForwardIt2 first2, ForwardIt2 last2, | + | ForwardIt1 first1, ForwardIt1 last1, |
− | + | ForwardIt2 first2, ForwardIt2 last2, BinaryPred p ); | |
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
− | + | 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 exec pol}} | {{par exec pol}} | ||
− | {{par pred2 eq | p | p1=InputIt1 | p2=InputIt2}} | + | {{par pred2 eq|p|p1=InputIt1|p2=InputIt2}} |
{{par hreq}} | {{par hreq}} | ||
− | {{par req named | InputIt1, InputIt2 | InputIterator}} | + | {{par req named|InputIt1, InputIt2|InputIterator}} |
− | {{par req named | ForwardIt1, ForwardIt2 | ForwardIterator}} | + | {{par req named|ForwardIt1, ForwardIt2|ForwardIterator}} |
+ | {{par req named|BinaryPred|BinaryPredicate}} | ||
{{par end}} | {{par end}} | ||
===Return value=== | ===Return value=== | ||
− | @ | + | @1-4@ If each corresponding elements in the two ranges are equal, returns {{c|true}}. Otherwise returns {{c|false}}. |
− | If | + | @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}}. |
− | + | ===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)}}: | ||
− | + | @1@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} comparisons using {{c/core|1=operator==}}. | |
− | {{ | + | |
− | + | @2@ {{mathjax-or|\(\scriptsize O(N_1)\)|O(N{{su|b=1}})}} comparisons using {{c/core|1=operator==}}. | |
− | + | @3@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} applications of the predicate {{c|p}}. | |
− | @ | + | |
− | @5 | + | @4@ {{mathjax-or|\(\scriptsize O(N_1)\)|O(N{{su|b=1}})}} applications of the predicate {{c|p}}. |
− | + | ||
− | @ | + | @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=== | ===Exceptions=== | ||
− | {{cpp/algorithm/ | + | {{cpp/algorithm/parallel exceptions reporting behavior|singular=no}} |
===Possible implementation=== | ===Possible implementation=== | ||
− | {{eq | + | {{eq impl |
− | + | |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 |
− | + | 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; | ||
} | } | ||
− | + | |title2=equal (3)|ver2=3|2= | |
− | template<class InputIt1, class InputIt2, class | + | template<class InputIt1, class InputIt2, class BinaryPred> |
− | bool equal(InputIt1 first1, InputIt1 last1, | + | constexpr //< since C++20 |
− | InputIt2 first2, | + | 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 {{ttt|std::equal}} to test if a string is a palindrome. | |
− | + | |code= | |
#include <algorithm> | #include <algorithm> | ||
+ | #include <iomanip> | ||
#include <iostream> | #include <iostream> | ||
− | #include < | + | #include <string_view> |
− | bool is_palindrome(const std:: | + | constexpr bool is_palindrome(const std::string_view& s) |
{ | { | ||
− | return std::equal(s. | + | return std::equal(s.cbegin(), s.cbegin() + s.size() / 2, s.crbegin()); |
} | } | ||
− | void test(const std:: | + | void test(const std::string_view& s) |
{ | { | ||
− | std::cout << | + | std::cout << std::quoted(s) |
− | + | << (is_palindrome(s) ? " is" : " is not") | |
− | + | << " a palindrome\n"; | |
} | } | ||
Line 158: | Line 249: | ||
test("hello"); | test("hello"); | ||
} | } | ||
− | + | |output= | |
"radar" is a palindrome | "radar" is a palindrome | ||
"hello" is not a palindrome | "hello" is not a palindrome | ||
Line 165: | Line 256: | ||
===See also=== | ===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/algorithm/ranges/dsc equal}} |
− | {{dsc inc | cpp/utility/functional/dsc equal_to}} | + | {{dsc inc|cpp/utility/functional/dsc equal_to}} |
− | {{dsc inc | cpp/algorithm/dsc equal_range}} | + | {{dsc inc|cpp/algorithm/dsc equal_range}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} |
Latest revision as of 18:45, 11 November 2024
Defined in header <algorithm>
|
||
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, |
(1) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool equal( ExecutionPolicy&& policy, |
(2) | (since C++17) |
template< class InputIt1, class InputIt2, class BinaryPred > bool equal( InputIt1 first1, InputIt1 last1, |
(3) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > |
(4) | (since C++17) |
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, |
(5) | (since C++14) (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool equal( ExecutionPolicy&& policy, |
(6) | (since C++17) |
template< class InputIt1, class InputIt2, class BinaryPred > bool equal( InputIt1 first1, InputIt1 last1, |
(7) | (since C++14) (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > |
(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)
.
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) |
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
[edit] Complexity
Given N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):
InputIt1
and InputIt2
are both LegacyRandomAccessIterator, and last1 - first1 != last2 - first2 is true, no comparison will be made.[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 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++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.
#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) |
returns true if one range is lexicographically less than another (function template) | |
finds the first position where two ranges differ (function template) | |
searches for the first occurrence of a range of elements (function template) | |
(C++20) |
determines if two sets of elements are the same (niebloid) |
function object implementing x == y (class template) | |
returns range of elements matching a specific key (function template) |