The inner loop logic is wrong but particularly, this line:
image[c:c+3], image[-c-3: -c] = image[-c-3: -c], image[c:c+3]
You are changing the image
variable in-place, but you seemed to forget the row variable r
. So right now, you are changing rows. And your negative slicing is a bit off. For c=0
, you'll get image[-3:0]
and this is not a valid slice and it will return []
.
But judging from your code you don't mean to change image
in-place, you rather want to create new_image
. What you should be doing is inserting slices at the end of new_row
:
def horizontal_flip(image):
rows = len(image)
cols = len(image[0])
new_image = []
for r in range(rows):
new_row = []
for c in range(0,cols,3):
new_row = image[r][c:c+3] + new_row
new_image.append(new_row)
return new_image
By the way, you can also change the image
in-place, but be careful. As you are passing a list
, you should copy it before changing so that original is unchanged. Here is that version:
def horizontal_flip(image):
cols = len(image[0])/3
#make a copy so that original image is not altered
image = [row[:] for row in image]
for row in image:
for c in range(int(cols/2)): # int() is not needed for Python 2.x, since integer division yields integer
# This also takes care of odd n cases, middle chunk is not changed.
row[3*c:3*c+3], row[3*(cols-c-1):3*(cols-c-1)+3] = row[3*(cols-c-1):3*(cols-c-1)+3], row[3*c:3*c+3]
return image
This can also be done with a list comprehension in single line, but it will be less readable. If you like, here is how you can do it:
from itertools import chain
flipped_image = [list(chain(*[row[3*i:3*i+3] for i in range(len(image[0])/3-1,-1,-1)])) for row in image]