What does l[-1] mean? I found somewhere , l2.append(l[-1])

Answer 1

l[-n] means the n-th element counting backwards from the last. Specifically, l[-1] means the last element, l[-2] means the second to last and so on.

You can use negative indexes also for slicing:
"abcdefghij"[-6:-3] == "efg"

and for slicing with a step for backwards slicing:
"abcdefghij"[-3:-6:-1] == "hgf"

(I gave string examples but it works analogically for lists and all iterables)

Answer 2

the idea is like (bit different and bit confusing)
int in c range
-32768 to 32767
char in c range
-128 to 127

its like.....(-a) to (a-1) ### a is +ve

WHY---->>
the negetives are a substituted method to deal with math or computing problems..
they are not real...
real things ( dont confuse with real numbers) starts with 0 then to infinity

if in the positive numberline...
we take upto 5 it means
0-1
1-2
2-3
3-4
4-5

means 5 steps...
and the address of each step is the beggining number
0 is address of 0-1
1 is address of 1-2
.........
4 is address of 4-5

so length 5 means...0, 1, 2, 3, 4

now in the negetive u too have to take such 5 address to nutralise
u need to start the address with -5...not -4
heres why..
-5 is address of (-5)-(-4)
-4 is address of (-4)-(-3)
..........
-1 is address of (-1)-0

so...length 5 means also..-5, -4, -3, -2, -1

now compare.....

string = "sololearn"
#### 01234567
#### string[0] to string[7]
#### -8-7-6-5-4-3-2-1
#### string[-8] to [-1]

thats why string[-1] is the last element

so the conclusion...

length=len(string)
string[n] = string[n-length]
where n is positive..
put n=7 ; n-length=7-8=-1
put n=8 ; n-length=8-8=0

##################thats how the concept came
all abut steps...and beggining address