Python : How to fill an array line by line?

Question

I have an issue with numpy that I can't solve. I have 3D arrays (x,y,z) filled with 0 and 1. For instance, one slice in the z axis :

array([[1, 0, 1, 0, 1, 1, 0, 0],
       [0, 0, 1, 1, 0, 1, 1, 0],
       [1, 0, 1, 1, 0, 0, 0, 1],
       [0, 0, 0, 0, 0, 0, 0, 0],
       [1, 1, 1, 0, 1, 0, 0, 1],
       [1, 0, 0, 0, 0, 1, 0, 1],
       [0, 0, 0, 0, 1, 0, 0, 0],
       [0, 0, 1, 0, 1, 1, 0, 1]])

And I want this result :

array([[1, 1, 1, 1, 1, 1, 0, 0],
       [0, 0, 1, 1, 1, 1, 1, 0],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [0, 0, 0, 0, 0, 0, 0, 0],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [0, 0, 0, 0, 1, 0, 0, 0],
       [0, 0, 1, 1, 1, 1, 1, 1]])

That is to say, what I want to do for each slice z is to scan line by line right to left and left to right (x axis) and the first time I have a 1 I want to fill the rest of the line with ones.

Is there an efficient way to compute that ?

Thanks a lot.

Nico !

Answer 1

Accessing NumPy array elements one by one is not very efficient. You may do better with just plain Python lists. They also have an index method which can search for the first entry of the value in the list.

from numpy import *

a = array([[1, 0, 1, 0, 1, 1, 0, 0],
       [0, 0, 1, 1, 0, 1, 1, 0],
       [1, 0, 1, 1, 0, 0, 0, 1],
       [0, 1, 0, 0, 1, 0, 1, 0],
       [1, 1, 1, 0, 1, 0, 0, 1],
       [1, 0, 0, 0, 0, 1, 0, 1],
       [0, 0, 0, 0, 1, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 1, 0, 1, 1, 0, 1]])

def idx_front(ln):
    try:
        return list(ln).index(1)
    except ValueError:
        return len(ln) # an index beyond line end

def idx_back(ln):
    try:
        return len(ln) - list(reversed(ln)).index(1) - 1
    except ValueError:
        return len(ln) # an index beyond line end

ranges = [ (idx_front(ln), idx_back(ln)) for ln in a ]
for ln, (lo,hi) in zip(a, ranges):
    ln[lo:hi] = 1  # attention: destructive update in-place

print "ranges =", ranges
print a

Output:

ranges = [(0, 5), (2, 6), (0, 7), (1, 6), (0, 7), (0, 7), (4, 4), (8, 8), (2, 7)]
[[1 1 1 1 1 1 0 0]
 [0 0 1 1 1 1 1 0]
 [1 1 1 1 1 1 1 1]
 [0 1 1 1 1 1 1 0]
 [1 1 1 1 1 1 1 1]
 [1 1 1 1 1 1 1 1]
 [0 0 0 0 1 0 0 0]
 [0 0 0 0 0 0 0 0]
 [0 0 1 1 1 1 1 1]]