# Subsets¶

The set of subsets of a finite set. The set can be given as a list or a Set or else as an integer $$n$$ which encodes the set $$\{1,2,...,n\}$$. See Subsets for more information and examples.

AUTHORS:

• Mike Hansen: initial version
• Florent Hivert (2009/02/06): doc improvements + new methods
class sage.combinat.subset.SubMultiset_s(s)

The combinatorial class of the sub multisets of s.

EXAMPLES:

sage: S = Subsets([1,2,2,3], submultiset=True)
sage: S.cardinality()
12
sage: S.list()
[[],
[1],
[2],
[3],
[1, 2],
[1, 3],
[2, 2],
[2, 3],
[1, 2, 2],
[1, 2, 3],
[2, 2, 3],
[1, 2, 2, 3]]
sage: S.first()
[]
sage: S.last()
[1, 2, 2, 3]

cardinality()

Return the cardinality of self

EXAMPLES:

sage: S = Subsets([1,1,2,3],submultiset=True)
sage: S.cardinality()
12
sage: len(S.list())
12

sage: S = Subsets([1,1,2,2,3],submultiset=True)
sage: S.cardinality()
18
sage: len(S.list())
18

sage: S = Subsets([1,1,1,2,2,3],submultiset=True)
sage: S.cardinality()
24
sage: len(S.list())
24

generating_serie(variable='x')

Return the serie (here a polynom) associated to the counting of the element of self weighted by the number of element they contain.

EXAMPLES:

sage: Subsets([1,1],submultiset=True).generating_serie()
x^2 + x + 1
sage: Subsets([1,1,2,3],submultiset=True).generating_serie()
x^4 + 3*x^3 + 4*x^2 + 3*x + 1
sage: Subsets([1,1,1,2,2,3,3,4],submultiset=True).generating_serie()
x^8 + 4*x^7 + 9*x^6 + 14*x^5 + 16*x^4 + 14*x^3 + 9*x^2 + 4*x + 1

sage: S = Subsets([1,1,1,2,2,3,3,4],submultiset=True)
sage: S.cardinality()
72
sage: sum(S.generating_serie())
72

random_element()

Return a random element of self with uniform law

EXAMPLES:

sage: S = Subsets([1,1,2,3], submultiset=True)
sage: S.random_element()
[2]

class sage.combinat.subset.SubMultiset_sk(s, k)

The combinatorial class of the subsets of size k of a multiset s. Note that each subset is represented by a list of the elements rather than a set since we can have multiplicities (no multiset data structure yet in sage).

EXAMPLES:

sage: S = Subsets([1,2,3,3],2,submultiset=True)
sage: S._k
2
sage: S.cardinality()
4
sage: S.first()
[1, 2]
sage: S.last()
[3, 3]
sage: [sub for sub in S]
[[1, 2], [1, 3], [2, 3], [3, 3]]

cardinality()

Return the cardinality of self

EXAMPLES:

sage: S = Subsets([1,2,2,3,3,3],4,submultiset=True)
sage: S.cardinality()
5
sage: len(list(S))
5

sage: S = Subsets([1,2,2,3,3,3],3,submultiset=True)
sage: S.cardinality()
6
sage: len(list(S))
6

generating_serie(variable='x')

Return the serie (this case a polynom) associated to the counting of the element of self weighted by the number of element they contains

EXAMPLES:

sage: x = ZZ['x'].gen()
sage: l = [1,1,1,1,2,2,3]
sage: for k in xrange(len(l)):
....:    S = Subsets(l,k,submultiset=True)
....:    print S.generating_serie(x) == S.cardinality()*x**k
True
True
True
True
True
True
True

random_element()

Return a random submultiset of given length

EXAMPLES:

sage: Subsets(7,3).random_element()
{1, 4, 7}
sage: Subsets(7,5).random_element()
{1, 3, 4, 5, 7}

sage.combinat.subset.Subsets(s, k=None, submultiset=False)

Return the combinatorial class of the subsets of the finite set s. The set can be given as a list, Set or any iterable convertible to a set. Alternatively, a non-negative integer $$n$$ can be provided in place of s; in this case, the result is the combinatorial class of the subsets of the set $$\{1,2,\dots,n\}$$ (i.e. of the Sage range(1,n+1)).

A second optional parameter k can be given. In this case, Subsets returns the combinatorial class of subsets of s of size k.

Warning

The subsets are returned as Sets. Do not assume that these Sets are ordered; they often are not! (E.g., Subsets(10).list()[619] returns {10, 4, 5, 6, 7} on my system.) See SubsetsSorted for a similar class which returns the subsets as sorted tuples.

Finally the option submultiset allows one to deal with sets with repeated elements, usually called multisets. The method then returns the class of all multisets in which every element is contained at most as often as it is contained in s. These multisets are encoded as lists.

EXAMPLES:

sage: S = Subsets([1, 2, 3]); S
Subsets of {1, 2, 3}
sage: S.cardinality()
8
sage: S.first()
{}
sage: S.last()
{1, 2, 3}
sage: S.random_element()  # random
{2}
sage: S.list()
[{}, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}]


Here is the same example where the set is given as an integer:

sage: S = Subsets(3)
sage: S.list()
[{}, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}]


We demonstrate various the effect of the various options:

sage: S = Subsets(3, 2); S
Subsets of {1, 2, 3} of size 2
sage: S.list()
[{1, 2}, {1, 3}, {2, 3}]

sage: S = Subsets([1, 2, 2], submultiset=True); S
SubMultiset of [1, 2, 2]
sage: S.list()
[[], [1], [2], [1, 2], [2, 2], [1, 2, 2]]

sage: S = Subsets([1, 2, 2, 3], 3, submultiset=True); S
SubMultiset of [1, 2, 2, 3] of size 3
sage: S.list()
[[1, 2, 2], [1, 2, 3], [2, 2, 3]]

sage: S = Subsets(['a','b','a','b'], 2, submultiset=True); S.list()
[['a', 'a'], ['a', 'b'], ['b', 'b']]


And it is possible to play with subsets of subsets:

sage: S = Subsets(3)
sage: S2 = Subsets(S); S2
Subsets of Subsets of {1, 2, 3}
sage: S2.cardinality()
256
sage: it = iter(S2)
sage: [it.next() for _ in xrange(8)]
[{}, {{}}, {{1}}, {{2}}, {{3}}, {{1, 2}},  {{1, 3}}, {{2, 3}}]
sage: S2.random_element()     # random
{{2}, {1, 2, 3}, {}}
sage: [S2.unrank(k) for k in xrange(256)] == S2.list()
True

sage: S3 = Subsets(S2)
sage: S3.cardinality()
115792089237316195423570985008687907853269984665640564039457584007913129639936
sage: S3.unrank(14123091480)
{{{1, 3}, {1, 2, 3}, {2}, {1}},
{{2}, {1, 2, 3}, {}, {1, 2}},
{},
{{2}, {1, 2, 3}, {}, {3}, {1, 2}},
{{1, 2, 3}, {}, {1}}, {{2}, {2, 3}, {}, {1, 2}}}

sage: T = Subsets(S2, 10)
sage: T.cardinality()
278826214642518400
sage: T.unrank(1441231049)
{{{3}, {1, 2}, {}, {2, 3}, {1}, {1, 3}, ..., {{2, 3}, {}}, {{}}}

class sage.combinat.subset.SubsetsSorted(s)

Lightweight class of all subsets of some set $$S$$, with each subset being encoded as a sorted tuple.

Used to model indices of algebras given by subsets (so we don’t have to explicitly build all $$2^n$$ subsets in memory). For example, CliffordAlgebra.

first()

Return the first element of self.

EXAMPLES:

sage: from sage.combinat.subset import SubsetsSorted
sage: S = SubsetsSorted(range(3))
sage: S.first()
()

last()

Return the last element of self.

EXAMPLES:

sage: from sage.combinat.subset import SubsetsSorted
sage: S = SubsetsSorted(range(3))
sage: S.last()
(0, 1, 2)

random_element()

Return a random element of self.

EXAMPLES:

sage: from sage.combinat.subset import SubsetsSorted
sage: S = SubsetsSorted(range(3))
sage: isinstance(S.random_element(), tuple)
True

unrank(r)

Return the subset which has rank r.

EXAMPLES:

sage: from sage.combinat.subset import SubsetsSorted
sage: S = SubsetsSorted(range(3))
sage: S.unrank(4)
(0, 1)

class sage.combinat.subset.Subsets_s(s)

Subsets of a given set.

EXAMPLES:

sage: S = Subsets(4); S
Subsets of {1, 2, 3, 4}
sage: S.cardinality()
16
sage: Subsets(4).list()
[{}, {1}, {2}, {3}, {4},
{1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4}, {3, 4},
{1, 2, 3}, {1, 2, 4}, {1, 3, 4}, {2, 3, 4},
{1, 2, 3, 4}]

sage: S = Subsets(Subsets(Subsets(GF(3)))); S
Subsets of Subsets of Subsets of Finite Field of size 3
sage: S.cardinality()
115792089237316195423570985008687907853269984665640564039457584007913129639936
sage: S.unrank(3149254230)
{{{1, 2}, {0, 1, 2}, {0, 2}, {0, 1}},
{{1, 2}, {}, {0, 2}, {1}, {0, 1, 2}, {2}},
{{1, 2}, {0}}, {{1, 2}, {0, 1}, {0, 1, 2}, {1}},
{{0, 2}, {1}}}

an_element()

Returns an example of subset.

EXAMPLES:

sage: Subsets(0).an_element()
{}
sage: Subsets(3).an_element()
{1, 2}
sage: Subsets([2,4,5]).an_element()
{2, 4}

cardinality()

Return the number of subsets of the set s.

This is given by $$2^{|s|}$$.

EXAMPLES:

sage: Subsets(Set([1,2,3])).cardinality()
8
sage: Subsets([1,2,3,3]).cardinality()
8
sage: Subsets(3).cardinality()
8

element_class

alias of Set_object_enumerated

first()

Returns the first subset of s. Since we aren’t restricted to subsets of a certain size, this is always the empty set.

EXAMPLES:

sage: Subsets([1,2,3]).first()
{}
sage: Subsets(3).first()
{}

last()

Return the last subset of s. Since we aren’t restricted to subsets of a certain size, this is always the set s itself.

EXAMPLES:

sage: Subsets([1,2,3]).last()
{1, 2, 3}
sage: Subsets(3).last()
{1, 2, 3}

random_element()

Return a random element of the class of subsets of s (in other words, a random subset of s).

EXAMPLES:

sage: Subsets(3).random_element()           # random
{2}
sage: Subsets([4,5,6]).random_element()     # random
{5}

sage: S = Subsets(Subsets(Subsets([0,1,2])))
sage: S.cardinality()
115792089237316195423570985008687907853269984665640564039457584007913129639936
sage: s = S.random_element()
sage: s     # random
{{{1, 2}, {2}, {0}, {1}}, {{1, 2}, {0, 1, 2}, {0, 2}, {0}, {0, 1}}, ..., {{1, 2}, {2}, {1}}, {{2}, {0, 2}, {}, {1}}}
sage: s in S
True

rank(sub)

Return the rank of sub as a subset of s.

EXAMPLES:

sage: Subsets(3).rank([])
0
sage: Subsets(3).rank([1,2])
4
sage: Subsets(3).rank([1,2,3])
7
sage: Subsets(3).rank([2,3,4])
Traceback (most recent call last):
...
ValueError: {2, 3, 4} is not a subset of {1, 2, 3}

underlying_set()

Return the set of elements.

EXAMPLES:

sage: Subsets(GF(13)).underlying_set()
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}

unrank(r)

Return the subset of s that has rank k.

EXAMPLES:

sage: Subsets(3).unrank(0)
{}
sage: Subsets([2,4,5]).unrank(1)
{2}
sage: Subsets([1,2,3]).unrank(257)
Traceback (most recent call last):
...
IndexError: index out of range

class sage.combinat.subset.Subsets_sk(s, k)

Subsets of fixed size of a set.

EXAMPLES:

sage: S = Subsets([0,1,2,5,7], 3); S
Subsets of {0, 1, 2, 5, 7} of size 3
sage: S.cardinality()
10
sage: S.first(), S.last()
({0, 1, 2}, {2, 5, 7})
sage: S.random_element()  # random
{0, 5, 7}
sage: S([0,2,7])
{0, 2, 7}
sage: S([0,3,5])
Traceback (most recent call last):
...
ValueError: {0, 3, 5} not in Subsets of {0, 1, 2, 5, 7} of size 3
sage: S([0])
Traceback (most recent call last):
...
ValueError: {0} not in Subsets of {0, 1, 2, 5, 7} of size 3

an_element()

Returns an example of subset.

EXAMPLES:

sage: Subsets(0,0).an_element()
{}
sage: Subsets(3,2).an_element()
{1, 3}
sage: Subsets([2,4,5],2).an_element()
{2, 5}

cardinality()

EXAMPLES:

sage: Subsets(Set([1,2,3]), 2).cardinality()
3
sage: Subsets([1,2,3,3], 2).cardinality()
3
sage: Subsets([1,2,3], 1).cardinality()
3
sage: Subsets([1,2,3], 3).cardinality()
1
sage: Subsets([1,2,3], 0).cardinality()
1
sage: Subsets([1,2,3], 4).cardinality()
0
sage: Subsets(3,2).cardinality()
3
sage: Subsets(3,4).cardinality()
0

first()

Returns the first subset of s of size k.

EXAMPLES:

sage: Subsets(Set([1,2,3]), 2).first()
{1, 2}
sage: Subsets([1,2,3,3], 2).first()
{1, 2}
sage: Subsets(3,2).first()
{1, 2}
sage: Subsets(3,4).first()
Traceback (most recent call last):
...
EmptySetError

last()

Returns the last subset of s of size k.

EXAMPLES:

sage: Subsets(Set([1,2,3]), 2).last()
{2, 3}
sage: Subsets([1,2,3,3], 2).last()
{2, 3}
sage: Subsets(3,2).last()
{2, 3}
sage: Subsets(3,4).last()
Traceback (most recent call last):
...
EmptySetError

random_element()

Return a random element of the class of subsets of s of size k (in other words, a random subset of s of size k).

EXAMPLES:

sage: Subsets(3, 2).random_element()
{1, 2}
sage: Subsets(3,4).random_element()
Traceback (most recent call last):
...
EmptySetError

rank(sub)

Return the rank of sub as a subset of s of size k.

EXAMPLES:

sage: Subsets(3,2).rank([1,2])
0
sage: Subsets([2,3,4],2).rank([3,4])
2
sage: Subsets([2,3,4],2).rank([2])
Traceback (most recent call last):
...
ValueError: {2} is not a subset of length 2 of {2, 3, 4}
sage: Subsets([2,3,4],4).rank([2,3,4,5])
Traceback (most recent call last):
...
ValueError: {2, 3, 4, 5} is not a subset of length 4 of {2, 3, 4}

unrank(r)

Return the subset of s of size k that has rank r.

EXAMPLES:

sage: Subsets(3,2).unrank(0)
{1, 2}
sage: Subsets([2,4,5],2).unrank(0)
{2, 4}
sage: Subsets([1,2,8],3).unrank(42)
Traceback (most recent call last):
...
IndexError: index out of range

sage.combinat.subset.dict_to_list(d)

Return a list whose elements are the elements of i of d repeated with multiplicity d[i].

EXAMPLES:

sage: from sage.combinat.subset import dict_to_list
sage: dict_to_list({'a':1, 'b':3})
['a', 'b', 'b', 'b']

sage.combinat.subset.list_to_dict(l)

Return a dictionnary whose keys are the elements of l and values are the multiplicity they appear in l.

EXAMPLES:

sage: from sage.combinat.subset import list_to_dict
sage: list_to_dict(['a', 'b', 'b', 'b'])
{'a': 1, 'b': 3}


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