Itertools Combinations/Permutations size

Question

Is there anyway to see the len() of an itertools.Combination or other object, really, without materializing it to a list?
I can get the cardinality of combs or permutations with the factorials,... but I want something that generalizes.

Thanks

score 10 · Accepted Answer · answered Jun 02 '18 at 00:23

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For any iterable it, you can do:

length = sum(1 for ignore in it)

That doesn't create a list, so the memory footprint is small. But for many kinds of iterables, it also consumes it (for example, if it is a generator, it's consumed and can't be restarted; if it is a list, it's not consumed). There is no generally "non-destructive" way to determine the length of an arbitrary iterable.

Also note that the code above will run "forever" if it delivers an unbounded sequence of objects.

answered Jun 02 '18 at 00:23

Tim Peters

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This modifies an iterator so that attempting to iterate over it again does nothing. To mitigate that where I'm using itertools, I did: `import itertools`, then `itCopy = itertools.tee(it)`, then `length = sum(1 for ignore in itCopy)`, then `for i in list(it): ..` – Alex Hall Mar 16 '20 at 23:39

score 2 · Answer 2 · answered Jun 02 '18 at 00:22

2

No need to create a list. You can count the number of items in an iterable without storing the entire set:

sum(1 for _ in myIterable)

answered Jun 02 '18 at 00:22

jspcal

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score 2 · Answer 3 · answered Jun 02 '18 at 00:23

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Yes,

def count_iterable(i):
return sum(1 for e in i)

Taken from: Is there any built-in way to get the length of an iterable in python?

answered Jun 02 '18 at 00:23

DecaK

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Itertools Combinations/Permutations size

3 Answers3