Efficient Algorithm For List Edits

I have a 3D-point cloud, saved in two lists. Example with 5 points (x,y,z): (3,3,3), (1,1,1), (4,4,4), (2,2,2), (5,5,5) -> My lists looks like this: z = [3, 1, 4, 2, 5] # the z

Solution 1:

z, pts = zip(*[(z, pt) for z, pt in zip(z, pts) if z <= 3])
print z, pts

Output

(3, 1, 2) ((3, 3), (1, 1), (2, 2))

Solution 2:

With itertools:

from itertools import izip, ifilter

zip together

zipped_coords = izip(z, pts)

filter (in post you mention higher, but expeted results are in fact lower, choosed first one)

filtered_coords = ifilter(lambda x: x[0]>=3, zipped_coords )

unzip

znew, ptsnew = map(list, izip(*filtered_coords))

or all-in-one oneliner

>>> znew, ptsnew = map(list, izip(*ifilter(lambda x: x[0]>=3, izip(z, pts))))
>>> print znew, ptsnew
[3, 4, 5] [(3, 3), (4, 4), (5, 5)]

Solution 3:

Because you are removing elements from the list as you iterate it is a O(N^2) algorithm. You can use a simple list comprehension along with zip to do this in linear time.

z = [3, 1, 4, 2, 5] # the z values
pts = [(3,3), (1,1), (4,4), (2,2), (5,5)] # the x and y values

merged = zip(z, pts)    
filtered = [x for x in merged if x[0] <= 3]
z, pts = zip(*filtered)