Slicing a NumPy matrix in Python

Question

Why this statement is not giving any output ?
import numpy as np
letters = np.array([1, 3, 5, 7, 9, 7, 5])
print(letters[0:-5:-1])

OUTPUT:
[]

Answer 1

Try print(letters[-5:0:-1]).

letters[-5] is the fifth-last element in your array. letters[0] is your first. So you are currently telling it to print letters, starting from its first element going backwards and ending at its fifth from last element which doesn't make sense.

Answer 2

Russ
Then why this works
print(letters[:-3:-1])

OUTPUT:
[5 7]

It also starts from 0 and goes backward.

Answer 3

It doesn't "start from 0" actually. If you don't specify a starting index, it will start at the starting limit which, when going backwards, is the end if the list.

Answer 4

That's another way of saying that the element at the index at which you set the end of your slice (the number after the first colon ':') is not included in the slice.

letters[0:3] will include elements at indexes 0, 1 and 2, but not 3.

Answer 5

Russ thanks.
Also, I came across one statement " :stop value represents the first value that is not in the selected slice".
What does that mean ?

Answer 6

Thanks Russ