Sets & Dictionaries

Python

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

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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}

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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}

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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

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• Contains all elements that are in set A or in set B

A

B

Boolean operations on sets (II)

• Intersection of two sets

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• Contains all elements that are in both sets A and B

A

B

Boolean operations on sets (III)

• Difference of two sets

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• Contains all elements that are in A but not in B

A

B

Boolean operations on sets (IV)

• Symmetric difference of two sets

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• 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}

https://blog.hubspot.com/website/python-set

https://intellipaat.com/blog/tutorial/python-tutorial/data-structures-with-python-cheat-sheet/

Towards Dictionaries

• Lists and tuples hold elements with only integer indices

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• So in essence, each element has an index (or a key) which can only be an integer, and a value which can be of any type (e.g., in the above list/tuple, the first element has key 0 and value 45)
• What if we want to store elements with non-integer indices (or keys)?

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Integer �Indices

Dictionaries

• In Python, you can use a dictionary to store elements with keys of any types (not necessarily only integers like lists and tuples) and values of any types as well

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• The above dictionary can be defined in Python as follows:

dic = {"NUM":45, 1000:"coding", 2000:4.5, 3.4:7, "XXX":89}

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keys of different types

Each element is a key:value pair, and elements are separated by commas

key

value

Values of different types

Dictionaries

• In summary, dictionaries:
• Can contain any and different types of elements (i.e., keys and values)
• Can contain only unique keys but duplicate values

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• Can be indexed but only through keys (i.e., dic2[“a”] will return 1 but dic2[0] will return an error since there is no element with key 0 in dic2 above)

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Output: {'a': 2, 'b': 2}

dic2 = {"a":1, "a":2, "b":2}

print(dic2)

The element “a”:2 will override the element “a”:1

because only ONE element can have key “a”

Dictionaries

• In summary, dictionaries:
• CANNOT be concatenated
• CANNOT be repeated
• Can be nested (e.g., d = {"first":{1:1}, "second":{2:"a"}}
• Can be passed to a function and will result in a pass-by-reference and not pass-by-value behavior since it is immutable (like lists)

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Output:

{'first': {1: 1}, 'second': {2: 'a'}}

{'first': [1, 2, 3], 'second': {2: 'a'}}

def func1(d):

d["first"] = [1, 2, 3]

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dic = {"first":{1:1}, "second":{2:"a"}}

print(dic)

func1(dic)

print(dic)

Dictionaries

• In summary, dictionaries:
• Can be iterated over

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Output:

dic = {"first": 1, "second": 2, "third": 3}

for i in dic:

print(i)

How to get the values?

first

second

third

ONLY the keys will be returned.

Dictionaries

• In summary, dictionaries:
• Can be iterated over

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Output:

dic = {"first": 1, "second": 2, "third": 3}

for i in dic:

print(dic[i])

1

2

3

Values can be

accessed via indexing!

Adding Elements to a Dictionary

• How to add elements to a dictionary?
• By indexing the dictionary via a key and assigning a corresponding value

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Output:

dic = {"first": 1, "second": 2, "third": 3}

print(dic)

dic["fourth"] = 4

print(dic)

{'first': 1, 'second': 2, 'third': 3}

{'first': 1, 'second': 2, 'third': 3, 'fourth': 4}

Adding Elements to a Dictionary

• How to add elements to a dictionary?
• By indexing the dictionary via a key and assigning a corresponding value

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Output:

dic = {"first": 1, "second": 2, "third": 3}

print(dic)

dic[”second"] = 4

print(dic)

{'first': 1, 'second': 2, 'third': 3}

{'first': 1, 'second’: 4, 'third': 3}

If the key already exists, �the value will be overridden

Deleting Elements to a Dictionary

• How to delete elements in a dictionary?
• By using del

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Output:

dic = {"first": 1, "second": 2, "third": 3}

print(dic)

dic["fourth"] = 4

print(dic)

del dic["first"]

print(dic)

{'first': 1, 'second': 2, 'third': 3}

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{'first': 1, 'second': 2, 'third': 3, 'fourth': 4}

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{'second': 2, 'third': 3, 'fourth': 4}

Deleting Elements to a Dictionary

• How to delete elements in a dictionary?
• Or by using the function pop(key)

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Output:

dic = {"first": 1, "second": 2, "third": 3}

print(dic)

dic["fourth"] = 4

print(dic)

dic.pop(“first”)

print(dic)

{'first': 1, 'second': 2, 'third': 3}

{'first': 1, 'second': 2, 'third': 3, 'fourth': 4}

{'second': 2, 'third': 3, 'fourth': 4}

Dictionary Functions

• Many other functions can also be used with dictionaries

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Dictionary Functions

• Many other functions can also be used with dictionaries

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