I have written a generator that puzzles me:
The generator is supposed to create ordered selections of a set of
objects. repetition of objects is allowed and the selections should be
of a size determined by a pramter to the generator.
Now, if I try to accummulate the generated selections into a list I get
some peculiar behaviour that I hope maybe some of you can help me
understand:
Help much appreciated
-Edvin
#straightforward acumulation. Does not give the expected result
>>> d=[]
>>> for f in orderedCombinations([1,2],3):
... d.append(f)
...
>>> d
[[1], [2], [1], [2], [1], [2], [1], [2]]
#accumulating shallow copies of the genereated combinations works:
>>> d=[]
>>> for f in orderedCombinations([1,2],3):
... d.append(f[:])
...
>>> d
[[1, 1, 1], [1, 1, 2], [1, 2, 1], [1, 2, 2], [2, 1, 1], [2, 1, 2], [2,
2, 1], [2, 2, 2]]
#The generator:
def orderedCombinations(pool, k):
"""
Generator yielding ordered selections of size k with repetition from
pool.
"""
if k == 1:
for m in pool:
yield [m]
if k > 1:
for m in pool:
for combo in orderedCombinations(pool, k-1):
#insert and pop to avoid copying entire list
combo.insert(0,m)
yield combo
combo.pop(0)
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