Nov-11-2022, 01:18 PM
(This post was last modified: Nov-11-2022, 01:18 PM by Gribouillis.)
Instances of this class
MIT License added so that you can freely use it and modify it.
Combinations
are slow replacements for itertools.combinations()
if they are iterated. On the other hand, they have the Sequence interface, meaning a length and random access. For example one can ask the 2**85
th item of Combinations(range(1_000_000), 9)
without iterating the instance.MIT License added so that you can freely use it and modify it.
""" MIT License Copyright (c) 2022 User Gribouillis at www.python-forum.io Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. """ __doc__ = "" import collections.abc from scipy.special import comb __version__ = '2022.11.11' class Combinations(collections.abc.Sequence): """Slow version of itertools.combinations with indexed access Instances implement the Sequence interface of collections.abc """ def __init__(self, seq, k): """Initialises the Combinations instance. Arguments: seq : a sequence implementing indexed access k a number in range(len(seq)) """ self.seq = seq self.n = len(seq) self.k = k self.co = comb(self.n, k, exact=True) def __len__(self): return self.co def _gen_item(self, idx): n, k, co = self.n, self.k, self.co if not (0 <= idx < co): raise IndexError(f'argument out of range({co}), got {idx}') i = -1 while 0 < k <= n: i += 1 c = k * co / n # k-1 among n-1 if idx >= c: idx -= c co = (n - k) * co / n # k among n-1 else: k -= 1 co = c yield i n -= 1 def __getitem__(self, idx): s = self.seq return tuple(s[i] for i in self._gen_item(idx)) def __iter__(self): for i in range(len(self)): yield self[i] def __reversed__(self): for i in range(len(self)-1, -1, -1): yield self[i] def main(): w = Combinations(range(6), 3) for i in range(len(w)): print(w[i]) print('-' * 30) for t in reversed(w): print(t) if __name__ == '__main__': main()
Output:(0, 1, 2)
(0, 1, 3)
(0, 1, 4)
(0, 1, 5)
(0, 2, 3)
(0, 2, 4)
(0, 2, 5)
(0, 3, 4)
(0, 3, 5)
(0, 4, 5)
(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)
------------------------------
(3, 4, 5)
(2, 4, 5)
(2, 3, 5)
(2, 3, 4)
(1, 4, 5)
(1, 3, 5)
(1, 3, 4)
(1, 2, 5)
(1, 2, 4)
(1, 2, 3)
(0, 4, 5)
(0, 3, 5)
(0, 3, 4)
(0, 2, 5)
(0, 2, 4)
(0, 2, 3)
(0, 1, 5)
(0, 1, 4)
(0, 1, 3)
(0, 1, 2)