Lists

What is a list?

A list is a data structure that stores a number of items in a sequential form. Lists are useful for collecting variables together for processing as a whole.

When should I use a list?

Lists should be used when you need to collect variable values together in one place. Some examples:

• For storing a number of values entered by the user
• To store a set of co-ordinates that make up a shape
• To collect items of interest from a dataset (parse through some text, store long words in a list)

What can lists do?

Quite a few things.

List construction:

Interactive session:

>>> mylist = ["a", "b", "c", "d"]
>>> print mylist
['a', 'b', 'c', 'd']

>>> mylist = ["a"] * 4
>>> print mylist
['a', 'a', 'a', 'a']
>>>

>>> mylist = ["a"] * 3 + ["b"]
>>> print mylist
['a', 'a', 'a', 'b']

List access

Interactive session:

>>> mylist = ["a", "b", "c", "d"]
>>> mylist[0]
'a'
>>> mylist[3]
'd'
>>> mylist[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IndexError: list index out of range
>>> mylist[-1]
'd'

Other list operations

List length:

>>> mylist = ["a", "b", "c", "d"]
>>> print len(mylist)
4

Sorting:

>>> mylist = ["d", "b", "c", "a"]
>>> mylist.sort()
>>> print mylist
['a', 'b', 'c', 'd']

List slicing

List slicing is a way to get access to a few items of a list at once:

>>> mylist = ["a", "b", "c", "d"]
>>> mylist[0:2]
['a', 'b']
>>> mylist[1:2]
['b']
>>> mylist[:-1]
['a', 'b', 'c']
>>> mylist[::-1]
['d', 'c', 'b', 'a']
>>> mylist[2:]
['c', 'd']
>>>

Lists in other languages

Not all languages have lists as a data type like Python does. Other languages use a thing called an "array" which is like a list but much less flexible and (as a result) much more efficient for the machine to process. These languages often implement lists separately, so you have the choice of a flexible slow data structure (list) or an awkward fast one (array).

More help

The web is your friend.