Python Notes – 4 : Lists

Welcome to our third note in our Python learning process. In this note we will talk mainly about lists, its functions and how to use it.

Lists - Creation

A list is an ordered set of values, where each value is identified by an index. The values that make up a list are called its elements. You could create it like:

>>> X = [12, 56, 87]

>>> print X

[12, 56, 87]

>>> Y = ["one", "three", "five"]

>>> print Y

["one", "three", "five"]

The list elements don't have to be the same type.

["hello", 2.0, 5, [10, 20]]

A list within another list is said to be nested.

Another way of creating special list of consecutive integers:

>>> range(1,5)

[1, 2, 3, 4]

If there is no start of the range, it will create starting from the 0

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If there is a third argument, it specifies the space between successive values,

which is called the step size

>>> range(1, 10, 2)

[1, 3, 5, 7, 9]

Lists - Accessing elements

You access list elements by index. Indices starts with zero. Indices could be any integer expression.

>>> numbers = range(1,10)

>>> print numbers[0]

1

>>> print numbers[5-3]

3

Lists - Length

You can get the list length using len() method. Syntax like that:

>>> len(numbers)

9

Although a list can contain another list, the nested list still counts as a single

element. The length of this list is four:

['spam!', 1, ['Brie', 'Roquefort', 'Pol le Veq'], [1, 2, 3]]

Lists - operators

in is a boolean operator that tests membership in a sequence.

>>> horsemen = ['war', 'famine', 'pestilence', 'death']

>>> 'pestilence' in horsemen

1

>>> 'debauchery' in horsemen

0

Lists - for loops

The syntax is like:

for VARIABLE in LIST:

BODY

for horseman in horsemen:

print horseman

This is equivalent to:

i = 0

while i < len(LIST):

VARIABLE = LIST[i]

BODY

i = i + 1

Any list expression can be used in a for loop:

for number in range(20):

if number % 2 == 0:

print number

List - operations

The + operator concatenates lists:

>>> a = [1, 2, 3]

>>> b = [4, 5, 6]

>>> c = a + b

>>> print c

[1, 2, 3, 4, 5, 6]

Similarly, the * operator repeats a list a given number of times:

>>> [0] * 4

[0, 0, 0, 0]

>>> [1, 2, 3] * 3

[1, 2, 3, 1, 2, 3, 1, 2, 3]

The head function takes a list as a parameter and returns the first element.

>>> a = [1, 2, 3]

>>> head(a)

1

List - slices

>>> list = ['a', 'b', 'c', 'd', 'e', 'f']

>>> list[1:3]

['b', 'c']

>>> list[:4]

['a', 'b', 'c', 'd']

>>> list[3:]

['d', 'e', 'f']

>>> list[:]

['a', 'b', 'c', 'd', 'e', 'f']

List - deletion

del removes an element from a list:

>>> a = ['one', 'two', 'three']

>>> del a[1]

>>> a

['one', 'three']

You can use a slice as an index for del:

>>> list = ['a', 'b', 'c', 'd', 'e', 'f']

>>> del list[1:5]

>>> print list

['a', 'f']

• Strings are lists and can be treated the same way you treat lists.

Nested lists

A nested list is a list that appears as an element in another list. In this list, the

three-eth element is a nested list:

>>> list = ["hello", 2.0, 5, [10, 20]]

To extract an element from the nested list, we can proceed in two steps:

>>> elt = list[3]

>>> elt[0]

10

Or we can combine them:

>>> list[3][1]

20

In this note we talked mainly lists which are very powerful feature in Python. We will continue our learning in the upcoming notes.