Gray codes¶
Functions¶
- sage.combinat.gray_codes.combinations(n, t)¶
Iterator through the switches of the revolving door algorithm.
The revolving door algorithm is a way to generate all combinations of a set (i.e. the subset of given cardinality) in such way that two consecutive subsets differ by one element. At each step, the iterator output a pair
(i,j)
where the itemi
has to be removed andj
has to be added.The ground set is always \(\{0, 1, ..., n-1\}\). Note that
n
can be infinity in that algorithm.See [Knu2011] Section 7.2.1.3, “Generating All Combinations”.
INPUT:
n
– (integer orInfinity
) – size of the ground sett
– (integer) – size of the subsets
EXAMPLES:
sage: from sage.combinat.gray_codes import combinations sage: b = [1, 1, 1, 0, 0] sage: for i,j in combinations(5,3): ....: b[i] = 0; b[j] = 1 ....: print(b) [1, 0, 1, 1, 0] [0, 1, 1, 1, 0] [1, 1, 0, 1, 0] [1, 0, 0, 1, 1] [0, 1, 0, 1, 1] [0, 0, 1, 1, 1] [1, 0, 1, 0, 1] [0, 1, 1, 0, 1] [1, 1, 0, 0, 1] sage: s = set([0,1]) sage: for i,j in combinations(4,2): ....: s.remove(i) ....: s.add(j) ....: print(sorted(s)) [1, 2] [0, 2] [2, 3] [1, 3] [0, 3]
Note that
n
can be infinity:sage: c = combinations(Infinity,4) sage: s = set([0,1,2,3]) sage: for _ in range(10): ....: i,j = next(c) ....: s.remove(i); s.add(j) ....: print(sorted(s)) [0, 1, 3, 4] [1, 2, 3, 4] [0, 2, 3, 4] [0, 1, 2, 4] [0, 1, 4, 5] [1, 2, 4, 5] [0, 2, 4, 5] [2, 3, 4, 5] [1, 3, 4, 5] [0, 3, 4, 5] sage: for _ in range(1000): ....: i,j = next(c) ....: s.remove(i); s.add(j) sage: sorted(s) [0, 4, 13, 14]
- sage.combinat.gray_codes.product(m)¶
Iterator over the switch for the iteration of the product \([m_0] \times [m_1] \ldots \times [m_k]\).
The iterator return at each step a pair
(p,i)
which corresponds to the modification to perform to get the next element. More precisely, one has to apply the incrementi
at the positionp
. By construction, the increment is either+1
or-1
.This is algorithm H in [Knu2011] Section 7.2.1.1, “Generating All \(n\)-Tuples”: loopless reflected mixed-radix Gray generation.
INPUT:
m
– a list or tuple of positive integers that correspond to the size of the sets in the product
EXAMPLES:
sage: from sage.combinat.gray_codes import product sage: l = [0,0,0] sage: for p,i in product([3,3,3]): ....: l[p] += i ....: print(l) [1, 0, 0] [2, 0, 0] [2, 1, 0] [1, 1, 0] [0, 1, 0] [0, 2, 0] [1, 2, 0] [2, 2, 0] [2, 2, 1] [1, 2, 1] [0, 2, 1] [0, 1, 1] [1, 1, 1] [2, 1, 1] [2, 0, 1] [1, 0, 1] [0, 0, 1] [0, 0, 2] [1, 0, 2] [2, 0, 2] [2, 1, 2] [1, 1, 2] [0, 1, 2] [0, 2, 2] [1, 2, 2] [2, 2, 2] sage: l = [0,0] sage: for i,j in product([2,1]): ....: l[i] += j ....: print(l) [1, 0]