Writing the filter() function using for/while loops

Question

I am trying to understand how the filter() function works and would like to know how I could write:

test = filter(lambda x: x == y, lst)

using for or while loops.

Not sure why this was down voted a couple of times, seems like a good question, albeit basic. Beginner questions are welcome here! — Codie CodeMonkey
– Codie CodeMonkey, Commented Jul 23, 2013 at 11:08

TerryA · Accepted Answer · 2013-07-23 12:09:47Z

3

filter() is pretty much making a new list with a for loop and a conditional. In your example, it is identical to:

L = []
for i in lst:
    if i == y:
        L.append(i)

Or as a list comprehension:

[i for i in lst if i == y]

answered Jul 23, 2013 at 11:01

TerryA

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1 Comment

There was a time restriction on when I could accept it so I was just waiting.

poke · Accepted Answer · 2013-07-23 11:19:30Z

3

Adding to Haidro’s answer, in Python 3 filter is a generator, so you could reimplement it like this:

def filter (test, lst):
    for x in lst:
        if test(x):
            yield x

answered Jul 23, 2013 at 11:19

poke

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