List anomaly in Python

Question

Hello, I've recently had a pretty peculiar task in a Python challenge. It had the following code:
a = [1, 2, 0, 3, 4]
for x in range(len(a)):
if x >= 4:
print(a[-a[-1]])

What this code does is pretty clear to me. But what drew my attention was the last line and the "-a" thing there. Looks like it works this way:
- first we take our list "a"
- we "reverse" it with this a[-a] thing
- and then we specify the penultimate item from the reversed version of the list with a[-a[-1]]
So it's basically the same as a[1].

a[-a] doesn't work on its own, it seems to require a particular item indication.
What is this "-a" thing called? How can it be used? Where can I learn more about it? (sorry, I just don't know how to Google it)

Answer 1

It's not weird, it's making the list value negative.

a[-a[-1]]

a[-1] == 4
but because the negative in front of 'a' it's -4

a[-4] == 2

Look at this code:

https://code.sololearn.com/cy2xE91f5cRQ/?ref=app

Answer 2

"-" just negates the number at a[-1]

Answer 3

First
a=[1,2,0,3,4]
Here there is a[-a[-1]]
First let us keep -a aside
Here let's start from inside
i.e a[-1] =4
Now a[-a[-1]]= a[-4] =>2
Hope it helps :)

Answer 4

Basic thing to remember is to always resolve the innermost groupings (parentheses/brackets) first, working your way outward, when evaluating a nested expression.

Answer 5

Slick, thanks for the code example!
So, for instance, a[-a[2]] it's just a more complicated way to say a[-3], right?

Answer 6

No worries, and that's correct if you're referring to my example

Answer 7

UNKNOWN, thank you!
That's a very clear way to explain it!

Answer 8

Erik, that's a great advice, thanks!
I usually stick to it but this time I got really confused with the minus before the variable.

Answer 9

Lisa, thanks for the fast response!
But would that not return -4, if it was just a negation? It returns 2 for me.
Or perhaps I just got your response wrongly, sorry.

Answer 10

Lisa, nevermind, now I understand what you meant.
So it's basically like this:
a[-a[-1]] == a[-4]

Yeah, so that was just an awkward way to mean something as simple as that 😅