Slicing lists in python

Question

There is a hell lot of confusion on how exactly does slice operation work on lists.

The main problem occurs when using negative indices.

It will be good if someone could show me the exact definition of slice in the built-in module.

Edit: Why do [1,2,3][::-1] and [1,2,3][0:3:-1] have different return values

Check this out diveintopython3.net/native-datatypes.html#lists — Vipin Yadav
– Vipin Yadav, Commented Mar 22, 2018 at 7:54
@imvpn22 all that is pretty basic. My question is more confusing. — vaanchit kaul
– vaanchit kaul, Commented Mar 22, 2018 at 7:58

Setop · Accepted Answer · 2020-07-29 15:31:03Z

1

List ['A', 'B', 'C', 'D']

Index from 0 to size-1.

Negative index means going through the list backward :

       negative index   |   positive index
-5   -4   -3   -2   -1  |  0    1    2    3    4
     'A'  'B', 'C', 'D',|['A', 'B', 'C', 'D']

Index > 4 or < -5 throw IndexError.

Slices follow this pattern : List[from:to:step]

step, defaults to one, indicates which indexes to keep in slice its sign gives the direction of slicing
- positive, for left to right,
- negative for right to left
from index where to start the slicing, inclusive
- defaults to 0 when step is positive,
- defaults to -1 when step is negative
to index where to end the slicing, exclusive
- default to size-1 when step is positive,
- (size-1) when step is negative

examples :

['A','B','C','D'][::-1] means from right to left, from -1 to -(size-1) => ['D', 'C', 'B', 'A']

['A','B','C','D'][1:-4] means from second element to first element excluded with step of one => nothing

['A','B','C','D'][1:-4:-1] means from second element to first element excluded with step of minus one, only second element left => [2]

Of course, the best is always to try a slice on samples before using it.

answered Mar 22, 2018 at 8:24

Setop

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4 Comments

Why is [1,2,3][::-1] and [1,2,3][0:3:-1] not same?

[::-1] means from end to start, step -1 which gives revered list. [0:3:-1] means from start to end step -1 which leads to empty.

This is an important answer. Other languages use [start:run length] and concentration can lapse.

@vaanchitkaul, [1,2,3][::-1] and [1,2,3][-1:-4:-1] are the same.