+ 1
List anomaly in Python
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)
10 ответов
+ 8
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
+ 3
"-" just negates the number at a[-1]
+ 2
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 :)
+ 2
Basic thing to remember is to always resolve the innermost groupings (parentheses/brackets) first, working your way outward, when evaluating a nested expression.
+ 1
Slick, thanks for the code example!
So, for instance, a[-a[2]] it's just a more complicated way to say a[-3], right?
+ 1
No worries, and that's correct if you're referring to my example
+ 1
UNKNOWN, thank you!
That's a very clear way to explain it!
+ 1
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.
0
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.
0
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 😅