List slicing with negative step size

Question

Why does the below code returns an empty list when it should return [4, 3, 2] (because of the negative step size):

i = [  0,  1,  2,  3,  4,  5,  6,  7,  8,  9]
   #[  0,  1,  2,  3,  4,  5,  6,  7,  8,  9]
   #[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1]
s = i[2:-5:-1]
print(s)

Possible duplicate of [Understanding Python's slice notation](https://stackoverflow.com/questions/509211/understanding-pythons-slice-notation) — internet_user, Feb 14 '18 at 16:48
I corrected my question, -1 should return the list in reverse order but its not happening. — Akshay J, Feb 14 '18 at 16:51
Slices are `start:end`, so if `start` comes before `end` with a negative step, the result will be empty. `[-4:1:-1]` should do what you want. — internet_user, Feb 14 '18 at 16:52

score 4 · Answer 1 · answered Feb 14 '18 at 16:53

4

It starts counting from "2" going backwards (step -1) until "-5" is reached - but reaching the element "-5" would require positive step in this case.

For example the output of:

i[2::-1]

is: [2, 1, 0]

answered Feb 14 '18 at 16:53

Lemurata

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2
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How can I get the output of [4, 3, 2] ? – Akshay J Feb 14 '18 at 16:56
Something like: i[4:1:-1] – Lemurata Feb 14 '18 at 16:58
Thanks, it works. Can you please add a small explanation of how it works. – Akshay J Feb 14 '18 at 17:01
i[1:4] gives elements 1,2,3 (right limit is always not inclusive). But negative step i[1:4:-1]: causes similar effect like getting i[4:1] – Lemurata Feb 14 '18 at 17:21

score 0 · Answer 2 · answered Feb 14 '18 at 17:30

As it turns out, when going from left to right (using negative or positive version of index), a positive step size is required.

Going from right to left requires a negative step size:

i = [  0,  1,  2,  3,  4,  5,  6,  7,  8,  9]
   #[  0,  1,  2,  3,  4,  5,  6,  7,  8,  9]
   #[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1]

# to get 6, 7, 8
print(i[6:9])
print(i[-4:-1:1])
print(i[6:-1:1])
print(i[-4:9:1])


# to get 8, 7, 6
print(i[8:5:-1])
print(i[-2:-5:-1])
print(i[-2:5:-1])
print(i[8:-5:-1])

List slicing with negative step size

2 Answers2