If you don't want to reinvent the wheel all the time you can use this
one:
def inplacefilter(pred, alist):
"""inplacefilter(pred, alist): filters the given list like
filter(),
but works inplace, minimizing the used memory. It returns None.
>>> pr = lambda x: x > 2
>>> l = []
>>> inplacefilter(pr, l)
>>> l
[]
>>> l = [1,2,2]
>>> inplacefilter(pr, l)
>>> l
[]
>>> l = [3]
>>> inplacefilter(pr, l)
>>> l
[3]
>>> l = [1,2,3,1,5,1,6,0]
>>> r = filter(pr, l) # normal filter
>>> r
[3, 5, 6]
>>> inplacefilter(pr, l)
>>> r == l
True
"""
slow = 0
for fast, item in enumerate(alist):
if pred(item):
if slow != fast:
alist[slow] = alist[fast]
slow += 1
del alist[slow:]
If you use Psyco you can replace the enumerate() with a manually
incremented counter to speed up the code a bit, like this (untested):
def inplacefilter(pred, alist):
slow = 0
fast = 0
for item in alist:
if pred(item):
if slow != fast:
alist[slow] = alist[fast]
slow += 1
fast += 1
del alist[slow:]
Bye,
bearophile
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