std::lexicographical_compare
Defined in header <algorithm>
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template< class InputIt1, class InputIt2 > bool lexicographical_compare( InputIt1 first1, InputIt1 last1, |
(1) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > |
(2) | (since C++17) |
template< class InputIt1, class InputIt2, class Compare > bool lexicographical_compare( InputIt1 first1, InputIt1 last1, |
(3) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class Compare > |
(4) | (since C++17) |
Checks if the first range [
first1,
last1)
is lexicographically less than the second range [
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) |
Lexicographical comparison is an operation with the following properties:
- Two ranges are compared element by element.
- The first mismatching element defines which range is lexicographically less or greater than the other.
- If one range is a prefix of another, the shorter range is lexicographically less than the other.
- If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
- An empty range is lexicographically less than any non-empty range.
- Two empty ranges are lexicographically equal.
Contents |
[edit] Parameters
first1, last1 | - | the first range of elements to examine |
first2, last2 | - | the second range of elements to examine |
policy | - | the execution policy to use. See execution policy for details. |
comp | - | comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(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.
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-ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
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-Compare must meet the requirements of Compare.
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[edit] Return value
true if the first range is lexicographically less than the second, otherwise false.
[edit] Complexity
Given N
1 as std::distance(first1, last1) and N
2 as std::distance(first2, last2):
1,N
2) comparisons using operator<.
1,N
2) applications of the comparison function comp.
[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
lexicographical_compare (1) |
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template<class InputIt1, class InputIt2> bool lexicographical_compare(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2) { for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2) { if (*first1 < *first2) return true; if (*first2 < *first1) return false; } return (first1 == last1) && (first2 != last2); } |
lexicographical_compare (3) |
template<class InputIt1, class InputIt2, class Compare> bool lexicographical_compare(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp) { for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2) { if (comp(*first1, *first2)) return true; if (comp(*first2, *first1)) return false; } return (first1 == last1) && (first2 != last2); } |
[edit] Example
#include <algorithm> #include <iostream> #include <random> #include <vector> void print(const std::vector<char>& v, auto suffix) { for (char c : v) std::cout << c << ' '; std::cout << suffix; } int main() { std::vector<char> v1{'a', 'b', 'c', 'd'}; std::vector<char> v2{'a', 'b', 'c', 'd'}; for (std::mt19937 g{std::random_device{}()}; !std::lexicographical_compare(v1.begin(), v1.end(), v2.begin(), v2.end());) { print(v1, ">= "); print(v2, '\n'); std::shuffle(v1.begin(), v1.end(), g); std::shuffle(v2.begin(), v2.end(), g); } print(v1, "< "); print(v2, '\n'); }
Possible output:
a b c d >= a b c d d a b c >= c b d a b d a c >= a d c b a c d b < c d a b
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 142 | C++98 | at most min(N 1,N 2) comparisons were allowed, but that is not possible (equivalence is determined by 2 comparisons) |
doubled the limit |
LWG 1205 | C++98 | the results of lexicographical comparisons involving empty ranges were unclear | made clear |
[edit] See also
determines if two sets of elements are the same (function template) | |
compares two ranges using three-way comparison (function template) | |
returns true if one range is lexicographically less than another (niebloid) |