How to find the length of a "filter" object in python

Question

>>> n = [1,2,3,4]  >>> filter(lambda x:x>3,n) <filter object at 0x0000000002FDBBA8>  >>> len(filter(lambda x:x>3,n)) Traceback (most recent call last):   File "<pyshell#3>", line 1, in <module>     len(filter(lambda x:x>3,n)) TypeError: object of type 'filter' has no len() 

I could not get the length of the list I got. So I tried saving it to a variable, like this...

>>> l = filter(lambda x:x>3,n) >>> len(l) Traceback (most recent call last):   File "<pyshell#5>", line 1, in <module>     len(l) TypeError: object of type 'filter' has no len() 

Instead of using a loop, is there any way to get the length of this?

Answer 1

This is an old question, but I think this question needs an answer using the map-reduce ideology. So here:

from functools import reduce  def ilen(iterable):     return reduce(lambda sum, element: sum + 1, iterable, 0)  ilen(filter(lambda x: x > 3, n)) 

This is especially good if n doesn't fit in the computer memory.

Answer 2

You have to iterate through the filter object somehow. One way is to convert it to a list:

l = list(filter(lambda x: x > 3, n))  len(l)  # <-- 

But that might defeat the point of using filter() in the first place, since you could do this more easily with a list comprehension:

l = [x for x in n if x > 3] 

Again, len(l) will return the length.