python: get number of items from list(sequence) with certain condition

Question

Assuming that I have a list with huge number of items.

l = [ 1, 4, 6, 30, 2, ... ]

I want to get the number of items from that list, where an item should satisfy certain condition. My first thought was:

count = len([i for i in l if my_condition(l)])

But if the my_condition() filtered list has also great number of items, I think that creating new list for filtered result is just waste of memory. For efficiency, IMHO, above call can't be better than:

count = 0
for i in l:
    if my_condition(l):
        count += 1

Is there any functional-style way to achieve to get the # of items that satisfy certain condition without generating temporary list?

Thanks in advance.

Answer 1

You can use a generator expression:

>>> l = [1, 3, 7, 2, 6, 8, 10]
>>> sum(1 for i in l if i % 4 == 3)
2

or even

>>> sum(i % 4 == 3 for i in l)
2

which uses the fact that int(True) == 1.

Alternatively, you could use itertools.imap (python 2) or simply map (python 3):

>>> def my_condition(x):
...     return x % 4 == 3
... 
>>> sum(map(my_condition, l))
2