Slicing in Python

Question

Can someone tell me slicing is supposed to work in Python? It would be great if you could explain all the different ways slicing is used.

Answer 1

This is what you're looking for:

a[start:end] # items from (start) to (end - 1)
a[start:] # items from start through the rest of the array
a[:end] # all items until and excluding (end)
a[:] # a copy of the whole array
a[start:end:step] #extract array elements in steps of the specified interval.

The "step" parameter can be a negative number too, in which the list will be scanned backwards in steps of the given length.

Answer 2

One way to remember how slices work is to think of the indices as pointing betweencharacters, with the left edge of the first character numbered 0. 

Then the right edge of the last character of a string of n characters has index n

 +---+---+---+---+---+
 | H | e | l | p | A |
 +---+---+---+---+---+
 0   1   2   3   4   5
-5  -4  -3  -2  -1

The first row of numbers gives the position of the indices 0...5 in the string, 
the second row gives the corresponding negative indices.

Answer 3

To understand slice assignment, it's helpful to add another concept to the ascii art:

                +---+---+---+---+---+---+
                | P | y | t | h | o | n |
                +---+---+---+---+---+---+
Slice position: 0   1   2   3   4   5   6
Index position:   0   1   2   3   4   5

>>> p = ['P','y','t','h','o','n']
# Why the two sets of numbers: 
# indexing gives items, not lists
>>> p[0]
 'P'
>>> p[5]
 'n'
# slicing gives lists
>>> p[0:1]
 ['P']
>>> p[0:2]
 ['P','y']

Answer 4

Slice notation works like this:

sequence[start:stop:step]

Answer 5

Couple of things that weren't immediately obvious to me when I first saw the slicing syntax:

>>> x = [1,2,3,4,5,6]
>>> x[::-1]
[6,5,4,3,2,1]

Easy way to reverse sequences!

And if you wanted, for some reason, every second item in the reversed sequence:

>>> x = [1,2,3,4,5,6]
>>> x[::-2]
[6,4,2]

Answer 6

Index:
      ------------>
  0   1   2   3   4
+---+---+---+---+---+
| a | b | c | d | e |
+---+---+---+---+---+
  0  -4  -3  -2  -1
      <------------

Slice:
    <---------------|
|--------------->
:   1   2   3   4   :
+---+---+---+---+---+
| a | b | c | d | e |
+---+---+---+---+---+
:  -4  -3  -2  -1   :
|--------------->
    <---------------|

I hope this will help you to model the list in Python.