lstrip(), rstrip() for lists

Question

I have a bunch of huge lists with integers. These lists may start or end with a couple of zeroes.

Is there an easy way for strip either the zeroes on the left or right side from the list? Something analogous to lstrip() or rstrip() for strings?

The data looks like

[0,0,0,1,2,3,4]

or

[1,2,3,4,0,0,0]

I must be able to individually lstrip() or rstrip(). I do not need a strip from both sides of the list.

Answer 1

You could use itertools.dropwhile():

>>> L = [0, 0, 1, 1, 2, 2, 0]
>>> list(itertools.dropwhile(lambda x: x == 0, L))
[1, 1, 2, 2, 0]

Answer 2

There is a more efficient solution than the built-in itertools.dropwhile(). You can use the almighty collections.deque, which would be the ideal data structure for this task, because its left or right pop is O(1). Here is the left-strip case, and the right-strip is going to be just the mirror image of it:

from collections import deque

def noLeadingZero(l):
    d = deque(l)
    for e in l:
        if e == 0:
            d.popleft()
        else:
            break
    return list(d)

l = [0, 0, 1, 1, 2, 2, 0]
print(noLeadingZero(l))
# Result:
# [1, 1, 2, 2, 0]

Let's test its performance against the following code that utilizes the built-in itertools.dropwhile():

from itertools import dropwhile
print(list(dropwhile(lambda x: x == 0, l)))

Here is the performance test:

import timeit

print timeit.timeit(
setup= """from itertools import dropwhile
l = [0, 0, 1, 1, 2, 2, 0]""",
stmt="""list(dropwhile(lambda x: x == 0, l))""") #2.308

print timeit.timeit(
setup= """from collections import deque
l = [0, 0, 1, 1, 2, 2, 0]
def noLeadingZero(l):
    d = deque(l)
    for e in l:
        if e == 0:
            d.popleft()
        else:
            break
    return list(d)""",
stmt="""noLeadingZero(l)""") #1.684 -> Win!