Question or problem about Python programming:
I’m new to Python and numpy. I’ve figured out how to slice 1 dimensional sequence: arr[start:end], and access an element in the array: el = arr[row][col].
Trying something like slice = arr[0:2][0:2] (where arr is a numpy array) doesn’t give me the first 2 rows and columns, but repeats the first 2 rows. What did I just do, and how do I slice along another dimension?
How to solve the problem:
If you use
numpy, this is easy:
slice = arr[:2,:2]
or if you want the 0’s,
slice = arr[0:2,0:2]
You’ll get the same result.
slice is actually the name of a builtin-type. Generally, I would advise giving your object a different “name”.
Another way, if you’re working with lists of lists*:
slice = [arr[i][0:2] for i in range(0,2)]
(Note that the 0’s here are unnecessary:
[arr[i][:2] for i in range(2)] would also work.).
What I did here is that I take each desired row 1 at a time (
arr[i]). I then slice the columns I want out of that row and add it to the list that I’m building.
If you naively try:
arr[0:2] You get the first 2 rows which if you then slice again
arr[0:2][0:2], you’re just slicing the first two rows over again.
*This actually works for numpy arrays too, but it will be slow compared to the “native” solution I posted above.