Question about negative value

Question

I understand about standard question like this:

sqs = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

if I print sqs[2:8:2] (meaning: start:end:step)
result : [4, 16, 36]

but the problem is negative value

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

I don't understand this result, so I try like this
print(sqs[7:5] and the result is [] (I guess end index is smaller than start)

Could you explain the negative value of step part?

Answer 1

The step is negative so you go backwards.
sqs[7:5:-1] starts at 49, steps back 1, until it reaches 25
So output is [49,36]
sqs[7:1:-2]
Output: [49,25,9] because you move back to steps each time.