How to slice an array of arrays in Python

Question

I'm trying to write my own machine learning scripts on python (I know there are libraries for that but this is purely for the fun of it - I'm in the process of learning Python). I have the following array;

[array([[[  5,   5,   5, 255],
    [  6,   6,   6, 255],
    [  6,   6,   6, 255],
    ..., 
    [ 12,  12,  12, 255],
    [ 10,  10,  10, 255],
    [ 10,  10,  10, 255]],

   [[  8,   8,   8, 255],
    [ 10,  10,  10, 255],
    [ 14,  14,  14, 255],
    ..., 
    [ 15,  15,  15, 255],
    [ 13,  13,  13, 255],
    [ 13,  13,  13, 255]],

It continues on like this for some time. I've got this array using the follow code:

imagesList = listdir("someaddress")
loadedImages = []
for image in imagesList:
    #img = PImage.open()
    loadedImages.append(misc.imread("someaddress" + image))

My logic here is I want to read in image files as arrays of pixel values for use in a image classification problem. As you can tell from the data above, the images are grey scale. I'd like to remove a dimension from this data and just have a single value for each pixel (eg, ([[[5],[6],[6],[12]...) The 255's are just the alpha values (which I don't care about). I know this is array splicing that I need to use, but boy do I have no idea how to apply it to this problem.

I've tried; loadedImages[:,1]

I get the following error;

TypeError: list indices must be integers, not tuple

The result I really want out of this would look as follows

[array([[  5,
  6,
  6,
..., 
 12,
 10,
 10],

 [  8,
   10,
   14,
    ..., 
   15,
   13,
   13,

Why did you put a comma in the index? In addition, [this question](https://stackoverflow.com/questions/509211/explain-slice-notation) may help. — Jerrybibo, Sep 27 '17 at 16:59
@Jerrybibo if I'm honest I've browsed a bunch of SO questions and that's what they seemed to do. I'm a bit at sea with this! — user6916458, Sep 27 '17 at 17:00

jabargas · Accepted Answer · 2017-09-27T17:41:26.967

2

Have you tried removing the comma on your splice, like loadedimages[:1]? For this array arr = [ 5, 5, 5, 255], to remove the 255, you could do arr = arr[:3]. For multi-dimensional arrays, you could nest the for loops as needed so you could iterate over the individual arrays. e.g.

main_array = [[  5,   5,   5, 255],
              [  6,   6,   6, 255],
              [  6,   6,   6, 255]]

for sub_array in main_array:
  spliced_array = sub_array[:3]
  print spliced_array

will yield

[5, 5, 5]
[6, 6, 6]
[6, 6, 6]

Also, in Python, this kind of data structure is called a list. It might be the reason why you're having a hard time finding information about it.

Edit: You could try using list comprehensions as well. It's something that's really useful in python. e.g.

arr = [[  5,   5,   5, 255],
       [  6,   6,   6, 255],
       [  6,   6,   6, 255]]

filtered_arr = [a[0] for a in arr]
filtered_arr
>>>[5,6,6]

edited Sep 27 '17 at 17:41

answered Sep 27 '17 at 17:10

jabargas

210
1
3

Hey, thanks for the input. All I really want is one number in each row (eg. 5... 6.... 6...) no need to have it all 3 times! – user6916458 Sep 27 '17 at 17:11
if that's the case, you could just do arr[:1] or simply arr[0]. It will get the first element of the list. Just use the same logic in the example I gave. – jabargas Sep 27 '17 at 17:14
Thanks. How do I then put this back into an array of the same format? I still want the arrays to be of the same form just with the inner most array replaced by a number. – user6916458 Sep 27 '17 at 17:17

score 2 · Answer 2 · answered Sep 27 '17 at 17:16

2

just use numpy and you can do the kind of extended slicing you want to do:

import numpy as np
a = np.array([[  5,   5,   5, 255],
 [  6,   6,   6, 255],
 [  6,   6,   6, 255]])
# first column:
print(a[:, 0])

in this example, the : represents all rows of the array and the 0 represents the first column

answered Sep 27 '17 at 17:16

acushner

9,595
1
34
34

I have one extra set of square brackets at the start, how would that change the syntax of the *slicing*? :) – user6916458 Sep 27 '17 at 17:23
fair question. basically, the ordering goes from outer to inner, so to get the first column, you would want to do `a[:, :, 0]` – acushner Sep 27 '17 at 17:35
@acuchner that gives me the error stated in the question (TypeError: list indices must be integers, not tuple) – user6916458 Sep 27 '17 at 18:57
you have a list of arrays, if you want to access all of them, make the outermost list an array as well. you'll probably need something like `arrays[:, :, :, 0]` in this case – acushner Sep 27 '17 at 19:02
I've made it an array of arrays now (it's initialised with - loadedImages = np.empty(0, dtype=object)) But still no luck. IndexError: too many indices for array – user6916458 Sep 27 '17 at 22:17

How to slice an array of arrays in Python

2 Answers2