std::ranges::lexicographical_compare
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I1, std::sentinel_for<I1> S1,
std::input_iterator I2, std::sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<I1, Proj1>,
std::projected<I2, Proj2>> Comp = ranges::less >
constexpr bool lexicographical_compare
( I1 first1, S1 last1, I2 first2, S2 last2,
Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
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(1) | (since C++20) |
template< ranges::input_range R1, ranges::input_range R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>> Comp =
ranges::less >
constexpr bool lexicographical_compare
( R1&& r1, R2&& r2,
Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
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(2) | (since C++20) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I1, std::sized_sentinel_for<I1> S1,
std::random_access_iterator I2, std::sized_sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<I1, Proj1>,
std::projected<I2, Proj2>> Comp = ranges::less >
bool lexicographical_compare
( Ep&& policy, I1 first1, S1 last1, I2 first2, S2 last2,
Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep,
/*sized-random-access-range*/ R1, /*sized-random-access-range*/ R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>> Comp =
ranges::less >
bool lexicographical_compare
( Ep&& policy, R1&& r1, R2&& r2,
Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
1,2) Checks if the first target range
[first1, last1) or r1 is lexicographically less than the second target range [first2, last2) or r2. Elements of the two ranges are projected by proj1 and proj2 respectively, and compared using comp.3,4) Same as (1,2), but executed according to
policy.Lexicographical comparison is an operation with the following properties:
- Two empty ranges are lexicographically equal.
- An empty range is lexicographically less than any non-empty range.
- Non-empty ranges are compared element by element:
- The first pair of mismatching elements defines which range is lexicographically less or greater than the other.
- If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
- If one range is a prefix of another, the shorter range is lexicographically less than the other.
The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Parameters
| first1, last1 | - | the iterator-sentinel pair defining the first target range |
| r1 | - | the first target range |
| first2, last2 | - | the iterator-sentinel pair defining the second target range |
| r2 | - | the second target range |
| comp | - | the comparator to be applied to the (projected) elements |
| proj1 | - | the projection to be applied to the elements in the first target range |
| proj2 | - | the projection to be applied to the elements in the second target range |
| policy | - | the execution policy to use |
Return value
true if the first target range is lexicographically less than the second target range, otherwise false.
Complexity
Given
- N1 as
ranges::distance(first1, last1)orranges::distance(r1), - N2 as
ranges::distance(first2, last2)orranges::distance(r2):
1,2) At most 2min(1,N2) applications of
comp, proj1 and proj2.3,4) 𝓞(min(1,N2)) applications of
comp, proj1 and proj2.Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Possible implementation
struct lexicographical_compare_fn
{
template<std::input_iterator I1, std::sentinel_for<I1> S1,
std::input_iterator I2, std::sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<I1, Proj1>,
std::projected<I2, Proj2>> Comp = ranges::less>
constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2)
{
if (std::invoke(comp, std::invoke(proj1, *first1),
std::invoke(proj2, *first2)))
return true;
if (std::invoke(comp, std::invoke(proj2, *first2),
std::invoke(proj1, *first1)))
return false;
}
return (first1 == last1) && (first2 != last2);
}
template<ranges::input_range R>
constexpr get_end(R&& r)
{
return ranges::end(r);
}
template<ranges::forward_range R>
constexpr get_end(R&& r)
{
return ranges::next(ranges::begin(r), ranges::end(r));
}
template<ranges::input_range R1, ranges::input_range R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_strict_weak_order
<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less>
constexpr bool operator()(R1&& r1, R2&& r2, Comp comp = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), get_end(r1),
ranges::begin(r2), get_end(r2),
std::ref(comp), std::ref(proj1), std::ref(proj2));
}
};
inline constexpr lexicographical_compare_fn lexicographical_compare;
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Example
Run this code
#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>
int main()
{
std::vector<char> v1{'a', 'b', 'c', 'd'};
std::vector<char> v2{'a', 'b', 'c', 'd'};
namespace ranges = std::ranges;
auto os = std::ostream_iterator<char>(std::cout, " ");
std::mt19937 g{std::random_device{}()};
while (not ranges::lexicographical_compare(v1, v2))
{
ranges::copy(v1, os);
std::cout << ">= ";
ranges::copy(v2, os);
std::cout << '\n';
ranges::shuffle(v1, g);
ranges::shuffle(v2, g);
}
ranges::copy(v1, os);
std::cout << "< ";
ranges::copy(v2, os);
std::cout << '\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
See also
| compares two ranges lexicographically (function template) | |
(C++20) |
determines if two sets of elements are the same (algorithm function object) |