output of lists slicing.....?? HOW

Question

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

OUTPUT [ 1, 4, 9, 16, 25, 36, 49, 64]

sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(sqs[1:7:-1])

OUTPUT []

sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(sqs[7:5:-1])

output [49, 36]

Answer 1

Some a bit tricky things here are:
1. List indexes are always not negative (positive or 0). list index [-1] is just an alias for [len(list)-1], so it is always equal or greater then zero.

2. Slice step could be both positive and negative. So you can walk throught the list forward or backward. That is why you have
>>> sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> print(sqs[7:5:-1])
[49, 36]

3. With negative step, you obviously can not go forward. That is why you got an empty list:
>>> sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> print(sqs[1:7:-1])
[]
This is true with an opposite case:
>>> sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> print(sqs[7:5:1])
[]
- you can not go back stepping forward.

Note ,that you got a list, not a None object, because you created list by the sqs[...] statement, but got an empty sequence of elements in it.

Answer 2

Answer=5

Answer 3

slicing is kind of self explanatory if you count the numbers