Can you explain the output?

Question

I found this code while doing a quiz and the output doesn't make any sense, can anyone explain it to me plz:

a = [0, 1, 2, 3]
for a[-1] in a:
print(a[-1])

Output :
0
1
2
2

Answer 1

a[-1] will become an iterator, and it will change in every loop, by the value in a:

1st loop: a[-1] = 0; a = [0, 1, 2, 0]
2nd loop: a[-1] = 1; a = [0, 1, 2, 1]
3rd loop: a[-1] = 2; a = [0, 1, 2, 2]

In the 4th loop it will take the last value of a, which is a[-1] itself, so same output again:

4th loop: a[-1] = 2; a = [0, 1, 2, 2]

Answer 2

med gazzeh yes, it assigns to itself the valies of a. Since it is already in a, there will be a point where it reaches itself, so it will take the same value twice in a row.

If for example you looped throught:

for a[-1] in range(100):
pass

a[-1] will take the last value it was, which is 99 in this case.

Answer 3

med gazzeh No problem

Answer 4

med gazzeh you can consider a as a variable.
When you use a for loop, it is like saying every time, put i = 1, i = 2, i = 3, i = 4... think of it as changing the variable name.

After every for loop, the iterator used does not actually disappear, but it is created as a variable. Say we have this program:

for i in range(10):
pass

print(i) # output: 9

Same goes to a[-1], it is like writing:

a[-1] = 0
a[-1] = 1
...

Answer 5

Aymane Boukrouh [INACTIVE]
Thanks of the quick answer :)

Answer 6

Aymane Boukrouh [INACTIVE]
ok, I got it, thanks :D

Answer 7

Aymane Boukrouh [INACTIVE]
I already knew that but the fact that the element a[-1] can be used to iterate through 'a' seemed very strange to me at first, but after your very first comment I came to realize that a[-1] is a variable in itself (feel free to correct me if I am wrong) all became clear. So thanks again :)
Ps : Sorry for late response

Answer 8

Aymane Boukrouh [INACTIVE]
So a[-1] iterates through a and reassigns it's current value to itself?

Answer 9

Aymane Boukrouh [INACTIVE]
It just struck me, I kept thinking how weard is this behaviour however if you think of a[-1] in terms of the allocated memory then it makes much more sense as the values in 'a' will just cycle through that part of memory (refered to by a[-1]) until it reachs it's own.