Learning to use sets and dictionaries in Python
Photo by Hitesh Choudhary on Unsplash
In the previous installments of the Zero to Hero Python series, we covered:
- Dealing with primitive data types like integers, strings, and boolean values.
- Interacting with the console and learning about lists and tuples.
In this piece, we will be looking at Python?s in-built collections for supporting mathematical concepts like sets and key-value pairs.
Let?s get started!
Working With Sets
A set is used to store a sequence of unique values.
Initialization
A set can be declared by making use of curly braces {}.
>>> numSet = {1, 2, 3, 4, 5}>>> print(numSet){1, 2, 3, 4, 5}
If you put duplicate elements in the initialization of a set, only one instance of the elements will be retained.
>>> numSet = {1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5}>>> print(numSet){1, 2, 3, 4, 5}
You can also initialize an empty set using the in-built set function.
>>> emptySet = set()>>> print(emptySet)set()
Note: Elements of a set must be immutable. Adding mutable objects to a set will raise an error.
>>> tuple1 = (1, 2, 3)>>> tuple2 = (4, 5, 6)>>> tupleSet = {tuple1, tuple2} # no error as tuples are immutable>>> print(tupleSet){(4, 5, 6), (1, 2, 3)}>>> list1 = [1, 2, 3]>>> list2 = [4, 5, 6]>>> listSet = {list1, list2} #will raise error as lists are mutableTraceback (most recent call last): File “<stdin>”, line 1, in <module>TypeError: unhashable type: ‘list’
Adding elements
Add elements to a set by using the in-built add function.
>>> numSet = {1, 2, 3, 4, 5}>>> numSet.add(6)>>> print(numSet){1, 2, 3, 4, 5, 6}
Note: If you attempt to add a duplicate element to a set, there will be no change to the set. No error will be raised by the add function in this case.
>>> numSet = {1, 2, 3, 4, 5}>>> numSet.add(5)>>> print(numSet){1, 2, 3, 4, 5}
Deleting elements
Remove elements from a set by using the in-built remove function.
>>> numSet = {1, 2, 3, 4, 5}>>> numSet.remove(5)>>> print(numSet){1, 2, 3, 4}
Note: Trying to remove an element that does not exist will raise an error.
>>> numSet = {1, 2, 3, 4, 5}>>> numSet.remove(99)Traceback (most recent call last): File “<stdin>”, line 1, in <module>KeyError: 99
Length
Find the length of a set using the in-built len function.
>>> numSet = {1, 2, 3, 4, 5}>>> len(numSet)5
Existence
Check for the existence of an element in a set using the in operator.
>>> numSet = {1, 2, 3, 4, 5}>>> 2 in numSetTrue>>> 99 in numSetFalse
Now, let us look at performing set operations.
Set intersection
Find the intersection of two sets using the & operator.
>>> setA = {1, 2, 3, 4, 5}>>> setB = {3, 4, 5, 6, 7}>>> intersection = setA & setB>>> print(intersection){3, 4, 5}
Set union
Find the intersection of two sets using the | operator.
>>> setA = {1, 2, 3, 4, 5}>>> setB = {3, 4, 5, 6, 7}>>> union = setA | setB>>> print(union){1, 2, 3, 4, 5, 6, 7}
Set difference
Set difference returns a set of elements present in the first set and not present in the second set.
Find difference of a set from some other set using the – operator.
>>> setA = {1, 2, 3, 4, 5}>>> setB = {3, 4, 5, 6, 7}>>> difference = setA – setB>>> print(difference){1, 2}>>> reverseDifference = setB – setA>>> print(reverseDifference){6, 7}
Set symmetric difference
Symmetric difference returns a set of elements present in exactly one of the two sets, but not both sets.
Find the symmetric difference of two sets using the ^ operator.
>>> setA = {1, 2, 3, 4, 5}>>> setB = {3, 4, 5, 6, 7}>>> symmDiff = setA ^ setB>>> print(symmDiff){1, 2, 6, 7}
Check superset
A set A is a superset of a set B if all elements present in set B are also present in set A.
Check if the left-hand side set is a superset of the right-hand side set using the >= operator.
>>> bigSet = {1, 2, 3, 4, 5}>>> smallSet = {3, 4}>>> isSuperSet = bigSet >= smallSet>>> print(isSuperSet)True
To check if the set on the right-hand side set is a superset of the left-hand set, use the <= operator.
>>> bigSet = {1, 2, 3, 4, 5}>>> smallSet = {3, 4}>>> isSuperSet = smallSet <= bigSet>>> print(isSuperSet)True
Working With Dictionaries
Dictionaries are used to store key-value pairs in Python.
Initialization
Dictionaries are also initialized using the curly braces {}, and the key-value pairs are declared using the key:value syntax.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3}
You can also initialize an empty dictionary by using the in-built dict function.
>>> emptyDict = dict()>>> print(emptyDict){}
Empty dictionaries can also be initialized by simply using empty curly braces.
>>> emptyDict = {}>>> print(emptyDict){}
Note: The keys in a dictionary must be immutable. Attempts to create a dict with mutable keys will raise an error.
>>> tupleA = (1, 2, 3) # tuples are immutable>>> stringA = “I love Python!” # strings are immutable>>> floatA = 3.14 # float values are immutable>>> dictA = {tupleA : True, stringA : False, floatA : True} # no error as all keys are immutable>>> print(dictA){(1, 2, 3): True, ‘I love Python!’: False, 3.14: True}>>> listB = [1, 2, 3] #list is mutable>>> dictB = {listB : True} # raises an error as lists are mutableTraceback (most recent call last): File “<stdin>”, line 1, in <module>TypeError: unhashable type: ‘list’
Fetching data
Get a value from a dictionary using its key using square brackets ().
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> JohnsNumber = nameToNumber[“John”]>>> print(JohnsNumber)1
Note: Trying to fetch a key that does not exist will raise an error.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> nameToNumber[“Sam”]Traceback (most recent call last): File “<stdin>”, line 1, in <module>KeyError: ‘Sam’
To avoid this error, use the in-built get function. Fetching a non existent key using the get function will return None, but will not throw any error.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> johnsNumber = nameToNumber.get(“John”)>>> print(johnsNumber)1>>> samsNumber = nameToNumber.get(“Sam”)>>> print(samsNumber)None
If a key is missing in a dictionary, we can use the get function to return a default value. Pass the default value you want as the second parameter in the get function.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> johnsNumber = nameToNumber.get(“John”, 99)>>> print(johnsNumber)1>>> samsNumber = nameToNumber.get(“Sam”, 99)>>> print(samsNumber)99
Modifying data
Insert data into a dictionary using the in-built setdefault function.
The setdefault function will create a new key-value pair into the dictionary only if the key does not exist in the dictionary. If the key already exists, it will not be overridden.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> nameToNumber.setdefault(“Sam”, 4)4>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 4}>>> nameToNumber.setdefault(“Sam”, 99) # no changes as the key already exists4>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 4}
To modify existing data in a dictionary, use the in-built update function.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> nameToNumber.update({“Sam” : 4}) # creates new entry>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 4}>>> nameToNumber.update({“Sam” : 99}) # updates existing entry>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 99}
Existing data can also be modified by using the square bracket syntax.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> nameToNumber[“Sam”] = 4 # creates new entry>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 4}>>> nameToNumber[“Sam”] = 99 # updates existing entry>>> print(nameToNumber){‘John’: 1, ‘Harry’: 2, ‘Jacob’: 3, ‘Sam’: 99}
Deleting data
Delete keys from a dictionary using the del command.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> del nameToNumber[“John”]>>> print(nameToNumber){‘Harry’: 2, ‘Jacob’: 3}
Note: Trying to delete a key that does not exist will result in an error.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> del nameToNumber[“Sam”]Traceback (most recent call last): File “<stdin>”, line 1, in <module>KeyError: ‘Sam’
Iterations
We can iterate over the keys in a dictionary using the in-built keys function.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> names = list(nameToNumber.keys()) # using list() to store in a list>>> print(names)[‘John’, ‘Harry’, ‘Jacob’]
We can iterate over values in a dictionary using the in-built values function.
>>> nameToNumber = {“John” : 1, “Harry” : 2, “Jacob” : 3}>>> values = list(nameToNumber.values())>>> print(values)[1, 2, 3]
In the next edition of the Zero to Hero Python series, we?ll be looking at writing functions in Python, as well as using iterables to process data. For any questions or suggestions, please reach out to me in the comments.
