std::lexicographical_compare
Defined in header
template< class InputIt1, class InputIt2 >
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2 );
template< class ExecutionPolicy, class InputIt1, class InputIt2 >
bool lexicographical_compare( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2 );
template< class InputIt1, class InputIt2, class Compare >
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2,
Compare comp );
template< class ExecutionPolicy, class InputIt1, class InputIt2, class Compare >
bool lexicographical_compare( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2,
Compare comp );
Checks if the first range [first1, last1) is lexicographically less than the second range [first2, last2).
1) Elements are compared using operator<.
3) Elements are compared using the given binary comparison function comp.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t> is true
Lexicographical comparison is a 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.
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);
The signature does not need to have const &, but the function object 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 both Type1 and Type2.
Type requirements
InputIt1, InputIt2 must meet the requirements of InputIterator.
Return value
true if the first range is lexicographically less than the second.
Complexity
At most 2·min(N1, N2) applications of the comparison operation, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).
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
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);
}
Second version
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);
}
#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
int main()
{
std::vector<char> v1 {'a', 'b', 'c', 'd'};
std::vector<char> v2 {'a', 'b', 'c', 'd'};
std::srand(std::time(0));
while (!std::lexicographical_compare(v1.begin(), v1.end(),
v2.begin(), v2.end())) {
for (auto c : v1) std::cout << c << ' ';
std::cout << ">= ";
for (auto c : v2) std::cout << c << ' ';
std::cout << '\n';
std::random_shuffle(v1.begin(), v1.end());
std::random_shuffle(v2.begin(), v2.end());
}
for (auto c : v1) std::cout << c << ' ';
std::cout << "< ";
for (auto c : v2) std::cout << c << ' ';
std::cout << '\n';
}
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
