Indexing and Slicing

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Indexing and Slicing#

Computers and people count differently:

indices for humans and computers

Computers treat an address in memory as the starting point of a body of data. In the same sense, an index in Python always refers to such a starting point, something that is in between two objects in memory. We humans in contrast always count the objects themselves.

This is why the indices used to slice lists are a bit unintuitive at first, e.g. in:

s = "my fat cat"

s[3:6]

'fat'

'fat'

'fat'

The diagram provides a practical model by which you can deduce indices yourself.

Indexing#

Many data types (lists, strings) allow to index items by their position:

    s = 'my fat cat'

    s[0]  # first

'm'

    s[2]  # third

' '

    s[-1] # last

't'

Using and index that does not exist causes an IndexError.

Slicing#

We can define intervals. This is called slicing:

    
    s = 'my fat cat'
    s

'my fat cat'

    s[3:6]  # -> 'fat'

'fat'

Slices may be open on either side:

    s[3:]  # -> 'fat cat'
    

'fat cat'

s[:6]  # -> 'my fat'

'my fat'

If you leave both number out, you copy the variable. This is sometimes a neat trick, if you want to manipulate a list, but preserve the original.

    d = [1, 2, 3]
    d

[1, 2, 3]

    e = d[:]
    e
    

[1, 2, 3]

    d.append(4)
    d
    

[1, 2, 3, 4]

    len(d)  # is now 4

4

    len(e)  # still 3

3

You can define slices with a step size:

    s = 'my fat cat'
    s
    

'my fat cat'

    s[1:8:2]  # -> 'yftc'    

'yftc'

    s[:8:2]   # -> 'm a '

'm a '

    s[1::2]   # -> 'yftct'

'yftct'

The step size may even be negative:

    s = 'my fat cat'
    s
    

'my fat cat'

    s[::-1]  # -> 'tac taf ym'

'tac taf ym'
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