Python Lists

Chapter 8

Python for Informatics: Exploring Information

www.pythonlearn.com

Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.

http://creativecommons.org/licenses/by/3.0/.

​

Copyright 2010- Charles Severance

A List is a kind of Collection

• A collection allows us to put many values in a single “variable”
• A collection is nice because we can carry all many values around in one convenient package.

friends = [ 'Joseph', 'Glenn', 'Sally' ]

​

carryon = [ 'socks', 'shirt', 'perfume' ]

What is not a “Collection”

• Most of our variables have one value in them - when we put a new value in the variable - the old value is over written

$ python

Python 2.5.2 (r252:60911, Feb 22 2008, 07:57:53)

[GCC 4.0.1 (Apple Computer, Inc. build 5363)] on darwin

>>> x = 2

>>> x = 4

>>> print x

4

List Constants

• List constants are surrounded by square brakets and the elements in the list are separated by commas.
• A list element can be any Python object - even another list
• A list can be empty

>>> print [1, 24, 76]

[1, 24, 76]

>>> print ['red', 'yellow', 'blue']

['red', 'yellow', 'blue']

>>> print ['red', 24, 98.6]

['red', 24, 98.599999999999994]

>>> print [ 1, [5, 6], 7]

[1, [5, 6], 7]

>>> print []

[]

We already use lists!

for i in [5, 4, 3, 2, 1] :

print i

print 'Blastoff!'

5

4

3

2

1

Blastoff!

Lists and definite loops - best pals

friends = ['Joseph', 'Glenn', 'Sally']

for friend in friends :

print 'Happy New Year:', friend

print 'Done!'

Happy New Year: JosephHappy New Year: GlennHappy New Year: SallyDone!

Looking Inside Lists

• Just like strings, we can get at any single element in a list using an index specified in square brackets

0

Joseph

>>> friends = [ 'Joseph', 'Glenn', 'Sally' ]

>>> print friends[1]

Glenn

>>>

1

Glenn

2

Sally

Lists are Mutable

• Strings are "immutable" - we cannot change the contents of a string - we must make a new string to make any change
• Lists are "mutable" - we can change an element of a list using the index operator

>>> fruit = 'Banana’

>>> fruit[0] = 'b’

Traceback

TypeError: 'str' object does not

support item assignment

>>> x = fruit.lower()

>>> print x

banana

>>> lotto = [2, 14, 26, 41, 63]

>>> print lotto[2, 14, 26, 41, 63]

>>> lotto[2] = 28

>>> print lotto

[2, 14, 28, 41, 63]

How Long is a List?

• The len() function takes a list as a parameter and returns the number of elements in the list
• Actually len() tells us the number of elements of any set or sequence (i.e. such as a string...)

>>> greet = 'Hello Bob’

>>> print len(greet)

9

>>> x = [ 1, 2, 'joe', 99]

>>> print len(x)

4

>>>

​

Using the range function

• The range function returns a list of numbers that range from zero to one less than the parameter
• We can construct an index loop using for and an integer iterator

>>> print range(4)

[0, 1, 2, 3]

>>> friends = ['Joseph', 'Glenn', 'Sally']

>>> print len(friends)

3

>>> print range(len(friends))

[0, 1, 2]

>>>

A tale of two loops...

friends = ['Joseph', 'Glenn', 'Sally']

​

for friend in friends :

print 'Happy New Year:', friend

​

for i in range(len(friends)) :

friend = friends[i]

print 'Happy New Year:', friend

Happy New Year: Joseph

Happy New Year: Glenn

Happy New Year: Sally

>>> friends = ['Joseph', 'Glenn', 'Sally']

>>> print len(friends)

3

>>> print range(len(friends))

[0, 1, 2]

>>>

Concatenating lists using +

• We can create a new list by adding two exsiting lists together

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

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

>>> c = a + b

>>> print c

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

>>> print a

[1, 2, 3]

Lists can be sliced using :

>>> t = [9, 41, 12, 3, 74, 15]

>>> t[1:3]

[41,12]

>>> t[:4]

[9, 41, 12, 3]

>>> t[3:]

[3, 74, 15]

>>> t[:]

[9, 41, 12, 3, 74, 15]

Remember: Just like in strings, the second number is "up to but not including"

List Methods

>>> x = list()

>>> type(x)<type 'list'>

>>> dir(x)['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

>>>

http://docs.python.org/tutorial/datastructures.html

Building a list from scratch

• We can create an empty list and then add elements using the append method
• The list stays in order and new elements are added at the end of the list

>>> stuff = list()

>>> stuff.append('book')

>>> stuff.append(99)

>>> print stuff

['book', 99]

>>> stuff.append('cookie')

>>> print stuff

['book', 99, 'cookie']

Is Something in a List?

• Python provides two operators that let you check if an item is in a list
• These are logical operators that return True or False
• They do not modify the list

>>> some = [1, 9, 21, 10, 16]

>>> 9 in some

True

>>> 15 in some

False

>>> 20 not in some

True

>>>

A List is an Ordered Sequence

• A list can hold many items and keeps those items in the order until we do something to change the order
• A list can be sorted (i.e. change its order)
• The sort method (unlike in strings) means "sort yourself"

>>> friends = [ 'Joseph', 'Glenn', 'Sally' ]

>>> friends.sort()

>>> print friends

['Glenn', 'Joseph', 'Sally']

>>> print friends[1]

Joseph>>>

Built in Functions and Lists

• There are a number of functions built into Python that take lists as parameters
• Remember the loops we built? These are much simpler

>>> nums = [3, 41, 12, 9, 74, 15]

>>> print len(nums)

6

>>> print max(nums)

74>>> print min(nums)

3

>>> print sum(nums)

154

>>> print sum(nums)/len(nums)

25

http://docs.python.org/lib/built-in-funcs.html

numlist = list()

while True :

inp = raw_input('Enter a number: ')

if inp == 'done' : break

value = float(inp)

numlist.append(value)

​

average = sum(numlist) / len(numlist)

print 'Average:', average

total = 0

count = 0

while True :

inp = raw_input('Enter a number: ')

if inp == 'done' : break

value = float(inp)

total = total + value

count = count + 1

​

average = total / count

print 'Average:', average

Enter a number: 3

Enter a number: 9

Enter a number: 5

Enter a number: done

Average: 5.66666666667

Best Friends: Strings and Lists

>>> abc = 'With three words’

>>> stuff = abc.split()

>>> print stuff

['With', 'three', 'words']

>>> print len(stuff)

3

>>> print stuff[0]

With

>>> print stuff

['With', 'three', 'words']

>>> for w in stuff :

... print w

...

With

Three

Words

>>>

Split breaks a string into parts produces a list of strings. We think of these as words. We can access a particular word or loop through all the words.

>>> line = 'A lot of spaces’

>>> etc = line.split()

>>> print etc['A', 'lot', 'of', 'spaces']

>>>

>>> line = 'first;second;third’

>>> thing = line.split()

>>> print thing['first;second;third']

>>> print len(thing)

1

>>> thing = line.split(';')

>>> print thing['first', 'second', 'third']

>>> print len(thing)

3

>>>

When you do not specify a delimiter, multiple spaces are treated like “one” delimiter.

​

You can specify what delimiter character to use in the splitting.

fhand = open('mbox-short.txt')

for line in fhand:

line = line.rstrip()

if not line.startswith('From ') : continue

words = line.split()

print words[2]

Sat

Fri

Fri

Fri

...

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

>>> line = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008’

>>> words = line.split()

>>> print words

['From', 'stephen.marquard@uct.ac.za', 'Sat', 'Jan', '5', '09:14:16', '2008']

>>>

The Double Split Pattern

• Sometimes we split a line one way and then grab one of the pieces of the line and split that piece again

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

words = line.split()

email = words[1]

pieces = email.split('@')

print pieces[1]

stephen.marquard@uct.ac.za

['stephen.marquard', 'uct.ac.za']

'uct.ac.za'

The Double Split Pattern

• Sometimes we split a line one way and then grab one of the pieces of the line and split that piece again

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

words = line.split()

email = words[1]

pieces = email.split('@')

print pieces[1]

stephen.marquard@uct.ac.za

['stephen.marquard', 'uct.ac.za']

'uct.ac.za'

stephen.marquard

uct.ac.za

The Double Split Pattern

• Sometimes we split a line one way and then grab one of the pieces of the line and split that piece again

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

words = line.split()

email = words[1]

pieces = email.split('@')

print pieces[1]

stephen.marquard@uct.ac.za

['stephen.marquard', 'uct.ac.za']

'uct.ac.za'

The Double Split Pattern

• Sometimes we split a line one way and then grab one of the pieces of the line and split that piece again

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

words = line.split()

email = words[1]

pieces = email.split('@')

print pieces[1]

stephen.marquard@uct.ac.za

['stephen.marquard', 'uct.ac.za']

'uct.ac.za'

List Summary

• Concept of a collection
• Lists and definite loops
• Indexing and lookup
• List mutability
• Functions: len, min, max, sum
• Slicing lists
• List methods: append, remove
• Sorting lists
• Splitting strings into lists of words
• Using split to parse strings