std::is_permutation() algorithm
- od C++20
- od C++14
- od C++11
// (1)
template< class ForwardIt1, class ForwardIt2 >
constexpr bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2 );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
constexpr bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, BinaryPredicate p );
// (3)
template< class ForwardIt1, class ForwardIt2 >
constexpr bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 );
// (4)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
constexpr bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2, BinaryPredicate p );
// (1)
template< class ForwardIt1, class ForwardIt2 >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2 );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, BinaryPredicate p );
// (3)
template< class ForwardIt1, class ForwardIt2 >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 );
// (4)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, BinaryPredicate p );
// (1)
template< class ForwardIt1, class ForwardIt2 >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2 );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, BinaryPredicate p );
Returns true
if there exists a permutation of the elements in the range [first1
; last1
)
that makes that range equal to the range [first2
; last2
), where last2
denotes first2 + (last1 - first1)
if it was not given.
- (1, 3) Elements are compared using
operator==
. - (2, 4) Elements are compared using the given binary predicate
p
.
The behavior is undefined
if it is not an equivalence relation.Parameters
first1 last1 | The range of elements to compare. |
first1 last1 | The second range to compare. |
p | Unary predicate which returns
|
Type requirements
ForwardIt1 ForwardIt2 | LegacyForwardIterator |
ForwardIt1 ForwardIt2 | Must have the same value type. |
Return value
true
if the range [first1
; last1
) is a permutation of the range [first2
; last2
).
Complexity
Given N
as std::distance(first1, last1)
:
At most O(N2) applications of the predicate, or exactly N
if the sequences are already equal, where N is .
However if ForwardIt1
and ForwardIt2
meet the requirements of LegacyRandomAccessIterator
and std::distance(first1, last1) != std::distance(first2, last2)
no applications of the predicate are made.
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.
Possible implementation
is_permutation (1)
template<class ForwardIt1, class ForwardIt2>
bool is_permutation(ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first)
{
// skip common prefix
std::tie(first, d_first) = std::mismatch(first, last, d_first);
// iterate over the rest, counting how many times each element
// from [first, last) appears in [d_first, d_last)
if (first != last)
{
ForwardIt2 d_last = std::next(d_first, std::distance(first, last));
for (ForwardIt1 i = first; i != last; ++i)
{
if (i != std::find(first, i, *i))
continue; // this *i has been checked
auto m = std::count(d_first, d_last, *i);
if (m == 0 || std::count(i, last, *i) != m)
return false;
}
}
return true;
}
Notes
The std::is_permutation
can be used in testing, namely to check the correctness of rearranging algorithms (e.g. sorting, shuffling, partitioning).
If x
is an original range and y
is a permuted range then std::is_permutation(x, y) == true
means that y
consist of "the same" elements, maybe staying at other positions.
Examples
#include <algorithm>
#include <iostream>
template<typename Os, typename V>
Os& operator<<(Os& os, V const& v)
{
os << "{ ";
for (auto const& e : v)
os << e << ' ';
return os << '}';
}
int main()
{
static constexpr auto v1 = {1, 2, 3, 4, 5};
static constexpr auto v2 = {3, 5, 4, 1, 2};
static constexpr auto v3 = {3, 5, 4, 1, 1};
std::cout << v2 << " is a permutation of " << v1 << ": " << std::boolalpha
<< std::is_permutation(v1.begin(), v1.end(), v2.begin()) << '\n'
<< v3 << " is a permutation of " << v1 << ": "
<< std::is_permutation(v1.begin(), v1.end(), v3.begin()) << '\n';
}
{ 3 5 4 1 2 } is a permutation of { 1 2 3 4 5 }: true
{ 3 5 4 1 1 } is a permutation of { 1 2 3 4 5 }: false
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