Gray codes

Functions

sage.combinat.gray_codes.combinations(n, t)[source]

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 item i has to be removed and j 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 or Infinity; size of the ground set

  • t – 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]

>>> from sage.all import *
>>> from sage.combinat.gray_codes import combinations
>>> b = [Integer(1), Integer(1), Integer(1), Integer(0), Integer(0)]
>>> for i,j in combinations(Integer(5),Integer(3)):
...     b[i] = Integer(0); b[j] = Integer(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]

>>> s = set([Integer(0),Integer(1)])
>>> for i,j in combinations(Integer(4),Integer(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]

>>> from sage.all import *
>>> c = combinations(Infinity,Integer(4))
>>> s = set([Integer(0),Integer(1),Integer(2),Integer(3)])
>>> for _ in range(Integer(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]
>>> for _ in range(Integer(1000)):
...     i,j = next(c)
...     s.remove(i); s.add(j)
>>> sorted(s)
[0, 4, 13, 14]
sage.combinat.gray_codes.product(m)[source]

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 increment i at the position p. 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 – 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]

>>> from sage.all import *
>>> from sage.combinat.gray_codes import product
>>> l = [Integer(0),Integer(0),Integer(0)]
>>> for p,i in product([Integer(3),Integer(3),Integer(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]
>>> l = [Integer(0),Integer(0)]
>>> for i,j in product([Integer(2),Integer(1)]):
...     l[i] += j
...     print(l)
[1, 0]