Question about List Slices

Question

Hello dear community,

Why
squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[:1:-1])

Output:
[81, 64, 49, 36, 25, 16, 9, 4]

But
squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[0:1:-1])

Output:
[]

Aren’t these print statements equal?

Answer 1

Nope
Actually the first one is [-1:1:-1]
With a negative step(3rd value) u should go right to left, otherwise the result is empty.
The default first value is -1 in this case.

Answer 2

Ruslan Guliyev defaults:
going forward the first value is most left,
Going backwards the first value is most right
It is straightforward.

Answer 3

No!
print(squares[:1:-1]) identical to this:
print(squares[len(squares):1:-1])

Answer 4

First list:
As we know squares[start:end:step] , first list [:1:-1] your list will start in the end because you let the step equal -1 to the index 1 . Is simply equal to squares [len(squares): 1 : -1] the end is not included.

Answer 5

Oma Falk
But why is the first value converted to -1? Is it just a rule that I need to memorize or there is some logic behind it?

Answer 6

#index: 0, 1, 2, 3...
values = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
#index:-10,-9,-8,-7...

Answer 7

Oma Falk Solo Thanks a lot for your answers!

Answer 8

squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[:1:-1])
print(squares[-1:1:-1])
print(squares[len(squares):1:-1])