# Combinations¶

AUTHORS:

• Mike Hansen (2007): initial implementation
• Vincent Delecroix (2011): cleaning, bug corrections, doctests
class sage.combinat.combination.ChooseNK(mset, k)
sage.combinat.combination.Combinations(mset, k=None)

Return the combinatorial class of combinations of the multiset mset. If k is specified, then it returns the combinatorial class of combinations of mset of size k.

A combination of a multiset $$M$$ is an unordered selection of $$k$$ objects of $$M$$, where every object can appear at most as many times as it appears in $$M$$.

The combinatorial classes correctly handle the cases where mset has duplicate elements.

EXAMPLES:

sage: C = Combinations(range(4)); C
Combinations of [0, 1, 2, 3]
sage: C.list()
[[],
[0],
[1],
[2],
[3],
[0, 1],
[0, 2],
[0, 3],
[1, 2],
[1, 3],
[2, 3],
[0, 1, 2],
[0, 1, 3],
[0, 2, 3],
[1, 2, 3],
[0, 1, 2, 3]]
sage: C.cardinality()
16

sage: C2 = Combinations(range(4),2); C2
Combinations of [0, 1, 2, 3] of length 2
sage: C2.list()
[[0, 1], [0, 2], [0, 3], [1, 2], [1, 3], [2, 3]]
sage: C2.cardinality()
6

sage: Combinations([1,2,2,3]).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: Combinations([1,2,3], 2).list()
[[1, 2], [1, 3], [2, 3]]

sage: mset = [1,1,2,3,4,4,5]
sage: Combinations(mset,2).list()
[[1, 1],
[1, 2],
[1, 3],
[1, 4],
[1, 5],
[2, 3],
[2, 4],
[2, 5],
[3, 4],
[3, 5],
[4, 4],
[4, 5]]

sage: mset = ["d","a","v","i","d"]
sage: Combinations(mset,3).list()
[['d', 'd', 'a'],
['d', 'd', 'v'],
['d', 'd', 'i'],
['d', 'a', 'v'],
['d', 'a', 'i'],
['d', 'v', 'i'],
['a', 'v', 'i']]

sage: X = Combinations([1,2,3,4,5],3)
sage: [x for x in X]
[[1, 2, 3],
[1, 2, 4],
[1, 2, 5],
[1, 3, 4],
[1, 3, 5],
[1, 4, 5],
[2, 3, 4],
[2, 3, 5],
[2, 4, 5],
[3, 4, 5]]


It is possible to take combinations of Sage objects:

sage: Combinations([vector([1,1]), vector([2,2]), vector([3,3])], 2).list()
[[(1, 1), (2, 2)], [(1, 1), (3, 3)], [(2, 2), (3, 3)]]

class sage.combinat.combination.Combinations_mset(mset)
cardinality()
class sage.combinat.combination.Combinations_msetk(mset, k)
cardinality()

Return the size of combinations(mset,k).

IMPLEMENTATION: Wraps GAP’s NrCombinations.

EXAMPLES:

sage: mset = [1,1,2,3,4,4,5]
sage: Combinations(mset,2).cardinality()
12

class sage.combinat.combination.Combinations_set(mset)
rank(x)

EXAMPLES:

sage: c = Combinations([1,2,3])
sage: list(range(c.cardinality())) == list(map(c.rank, c))
True

unrank(r)

EXAMPLES:

sage: c = Combinations([1,2,3])
sage: c.list() == list(map(c.unrank, range(c.cardinality())))
True

class sage.combinat.combination.Combinations_setk(mset, k)
list()

EXAMPLES:

sage: Combinations([1,2,3,4,5],3).list()
[[1, 2, 3],
[1, 2, 4],
[1, 2, 5],
[1, 3, 4],
[1, 3, 5],
[1, 4, 5],
[2, 3, 4],
[2, 3, 5],
[2, 4, 5],
[3, 4, 5]]

rank(x)

EXAMPLES:

sage: c = Combinations([1,2,3], 2)
sage: list(range(c.cardinality())) == list(map(c.rank, c.list()))
True

unrank(r)

EXAMPLES:

sage: c = Combinations([1,2,3], 2)
sage: c.list() == list(map(c.unrank, range(c.cardinality())))
True

sage.combinat.combination.from_rank(r, n, k)

Returns the combination of rank r in the subsets of range(n) of size k when listed in lexicographic order.

The algorithm used is based on combinadics and James McCaffrey’s MSDN article. See: Wikipedia article Combinadic

EXAMPLES:

sage: import sage.combinat.combination as combination
sage: combination.from_rank(0,3,0)
()
sage: combination.from_rank(0,3,1)
(0,)
sage: combination.from_rank(1,3,1)
(1,)
sage: combination.from_rank(2,3,1)
(2,)
sage: combination.from_rank(0,3,2)
(0, 1)
sage: combination.from_rank(1,3,2)
(0, 2)
sage: combination.from_rank(2,3,2)
(1, 2)
sage: combination.from_rank(0,3,3)
(0, 1, 2)

sage.combinat.combination.rank(comb, n, check=True)

Return the rank of comb in the subsets of range(n) of size k where k is the length of comb.

The algorithm used is based on combinadics and James McCaffrey’s MSDN article. See: Wikipedia article Combinadic.

EXAMPLES:

sage: import sage.combinat.combination as combination
sage: combination.rank((), 3)
0
sage: combination.rank((0,), 3)
0
sage: combination.rank((1,), 3)
1
sage: combination.rank((2,), 3)
2
sage: combination.rank((0,1), 3)
0
sage: combination.rank((0,2), 3)
1
sage: combination.rank((1,2), 3)
2
sage: combination.rank((0,1,2), 3)
0

sage: combination.rank((0,1,2,3), 3)
Traceback (most recent call last):
...
ValueError: len(comb) must be <= n
sage: combination.rank((0,0), 2)
Traceback (most recent call last):
...
ValueError: comb must be a subword of (0,1,...,n)

sage: combination.rank([1,2], 3)
2
sage: combination.rank([0,1,2], 3)
0