Tuples, sets and dictionaries-Programming in Python

COSC 1306—COMPUTER
SCIENCE AND PROGRAMMING
PYTHON TUPLES, SETS
AND DICTIONARIES
Jehan-François Pâris
jfparis@uh.edu

Tuples
• Same as lists but
– Immutable
– Enclosed in parentheses
– A tuple with a single element must have a
comma inside the parentheses:
• a = (11,)

Examples
• >>> mytuple = (11, 22, 33)
• >>> mytuple[0]
11
• >>> mytuple[-1]
33
• >>> mytuple[0:1]
(11,)
• The comma is required!

Why?
• No confusion possible between [11] and 11
• (11) is a perfectly acceptable expression
– (11) without the comma is the integer 11
– (11, ) with the comma is a list containing the
integer 11
• Sole dirty trick played on us by tuples!

Tuples are immutable
• >>> mytuple = (11, 22, 33)
• >>> saved = mytuple
• >>> mytuple += (44,)
• >>> mytuple
(11, 22, 33, 44)
• >>> saved
(11, 22, 33)

Things that do not work
• mytuple += 55
Traceback (most recent call last):Z
…
TypeError:
can only concatenate tuple (not "int") to tuple
– Can understand that!

Sorting tuples
• >>> atuple = (33, 22, 11)
• >>> atuple.sort()
Traceback (most recent call last):
…
AttributeError:
'tuple' object has no attribute 'sort'
• >>> atuple = sorted(atuple)
• >>> atuple
[11, 22, 33]
Tuples are immutable!
sorted( ) returns a list!

Most other things work!
• >>> atuple = (11, 22, 33)
• >>> len(atuple)
3
• >>> 44 in atuple
False
• >>> [ i for [i for i in atuple]
[11, 22, 33]

The reverse does not work
• >>> alist = [11, 22, 33]
• >>> (i for i in alist)
<generator object <genexpr> at 0x02855DA0>
– Does not work!

Converting sequences into tuples
• >>> alist = [11, 22, 33]
• >>> atuple = tuple(alist)
• >>> atuple
(11, 22, 33)
• >>> newtuple = tuple('Hello World!')
• >>> newtuple
('H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!')

Sets
• Indentified by curly braces
– {'Alice', 'Bob', 'Carol'}
– {'Dean'} is a singleton
• Can only contain unique elements
– Duplicates are eliminated
• Immutable like tuples and strings

Sets do not contain duplicates
• >>> cset = {11, 11, 22}
• >>> cset
{11, 22}

Sets are immutable
• >>> aset = {11, 22, 33}
• >>> bset = aset
• >>> aset = aset | {55}
• >>> aset
{33, 11, 22, 55}
• >>> bset
{33, 11, 22}
Union of
two sets

Sets have no order
• >>> {1, 2, 3, 4, 5, 6, 7}
{1, 2, 3, 4, 5, 6, 7}
• >>> {11, 22, 33}
{33, 11, 22}

Sets do not support indexing
• >>> myset = {'Apples', 'Bananas', 'Oranges'}
• >>> myset
{'Bananas', 'Oranges', 'Apples'}
• >>> myset[0]
Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
myset[0]
TypeError: 'set' object does not support indexing

Examples
• >>> alist = [11, 22, 33, 22, 44]
• >>> aset = set(alist)
• >>> aset
{33, 11, 44, 22}
• >>> aset = aset + {55}
SyntaxError: invalid syntax

Boolean operations on sets (I)
• Union of two sets
• Contains all elements that are in set A or in set B
A B

Boolean operations on sets (II)
• Intersection of two sets
• Contains all elements that are in both sets A and B
A B

Boolean operations on sets (III)
• Difference of two sets
• Contains all elements that are in A but not in B
A B

Boolean operations on sets (IV)
• Symmetric difference of two sets
• Contains all elements that are either
– in set A but not in set B or
– in set B but not in set A
A B

Boolean operations on sets (V)
• >>> aset = {11, 22, 33}
• >>> bset = {12, 23, 33}
• Union of two sets
– >>> aset | bset
{33, 22, 23, 11, 12}
• Intersection of two sets:
– >>> aset & bset
{33}

Boolean operations on sets (VI)
• >>> aset = {11, 22, 33}
• >>> bset = {12, 23, 33}
• Difference:
– >>> aset – bset
{11, 22}
• Symmetric difference:
– >>> aset ^ bset
{11, 12, 22, 23}

Dictionaries (I)
• Store pairs of entries called items
{ 'CS' : '743-713-3350', 'UHPD' : '713-743-3333'}
• Each pair of entries contains
– A key
– A value
• Key and values are separated by a colon
• Paris of entries are separated by commas
• Dictionary is enclosed within curly braces

Usage
• Keys must be unique within a dictionary
– No duplicates
• If we have
age = {'Alice' : 25, 'Bob' :28}
then
age['Alice'] is 25
and
age[Bob'] is 28

Dictionaries are mutable
• >>> age = {'Alice' : 25, 'Bob' : 28}
• >>> saved = age
• >>> age['Bob'] = 29
• >>> age
{'Bob': 29, 'Alice': 25}
• >>> saved
{'Bob': 29, 'Alice': 25}

Keys must be unique
• >>> age = {'Alice' : 25, 'Bob' : 28, 'Alice' : 26}
• >>> age
{'Bob': 28, 'Alice': 26}

Displaying contents
• >>> age = {'Alice' : 25, 'Carol': 'twenty-two'}
• >>> age.items()
dict_items([ ('Alice', 25), ('Carol', 'twenty-two')])
• >>> age.keys()
dict_keys([ 'Alice', 'Carol'])
• age.values()
dict_values([28, 25, 'twenty-two'])

Updating directories
• >>> age = {'Alice': 26 , 'Carol' : 22}
• >>> age.update({'Bob' : 29})
• >>> age
{'Bob': 29, 'Carol': 22, 'Alice': 26}
• >>> age.update({'Carol' : 23})
• >>> age
{'Bob': 29, 'Carol': 23, 'Alice': 26}

Returning a value
• >>> age = {'Bob': 29, 'Carol': 23, 'Alice': 26}
• >>> age.get('Bob')
29
• >>> age['Bob']
29

Removing a specific item (I)
• >>> a = {'Alice' : 26, 'Carol' : 'twenty-two'}
• >>> a
{'Carol': 'twenty-two', 'Alice': 26}
• >>> a.pop('Carol’)
'twenty-two'
• >>> a
{'Alice': 26}

Removing a specific item (II)
• >>> a.pop('Alice')
26
• >>> a
{}
• >>>

Remove a random item
• >>> age = {'Bob': 29, 'Carol': 23, 'Alice': 26}
• >>> age.popitem()
('Bob', 29)
• >>> age
• {'Carol': 23, 'Alice': 26}
• >>> age.popitem()
('Carol', 23)
• >>> age
{'Alice': 26}

Summary
• Strings, lists, tuples, sets and dictionaries all
deal with aggregates
• Two big differences
– Lists and dictionaries are mutable
• Unlike strings, tuples and sets
– Strings, lists and tuples are ordered
• Unlike sets and dictionaries

Mutable aggregates
• Can modify individual items
– x = [11, 22, 33]
x[0] = 44
will work
• Cannot save current value
– x = [11,22, 33]
y = x
will not work

Immutable aggregates
• Cannot modify individual items
– s = 'hello!'
s[0] = 'H'
is an ERROR
• Can save current value
– s= 'hello!'
t = s
will work

Ordered aggregates
• Entities in the collection can be accessed through a
numerical index
– s= 'Hello!'
s[0]
– x = ['Alice', 'Bob', 'Carol']
x[-1]
– t = (11, 22)
t[1]

Other aggregates
• Cannot index sets
– myset = {'Apples', 'Bananas', 'Oranges'}
myset[0] is WRONG
• Can only index dictionaries through their keys
– age = {'Bob': 29, 'Carol': 23, 'Alice': 26}
age['Alice'] works
age[0] is WRONG

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