Hash Maps in Python

Hash Tables, better known as Hash Maps, are just collections of key-value pairs. In other words, they are pieces of data (values) mapped to unique identifiers called properties (keys).

Hash Maps were designed to give us a way of, given a key, associating a value with it for very quick lookups.

In Python, the Hash Map is implemented as a dictionary. This is why adding and lookup in a dictionary is so fast.

If you've worked with dictionaries in the past, you were working with Hash Maps without knowing it.

Creation:

You create an empty Hash Map just by creating an empty dictionary, like this:

hashMap = {}

Creating an empty dictionary takes O(1) time.

Initialization:

You can also add some key-value pairs in your dictionary at creation:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown'
}

Accessing values:

A value is retrieved from a dictionary by specifying its corresponding key in square brackets ([]):

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown'
}

# Accessing values:
print(person['name']); # "Andy"
print(person['age']); # 30

# Accessing non-existing key:
print(person['job']); # raises a KeyError exception

As you can see, if you refer to a key that is not in the dictionary, Python raises a KeyError exception.

Accessing a value from a dictionary takes O(1) time.

Adding an entry:

Adding an entry to an existing dictionary is simply a matter of assigning a new key and value, via the assignment operator:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown'
}

# Adding new entries:
person['job'] = 'Teacher'

str = 'hobby'
person[str] = 'Fishing'

print(person)
# The new Hash Map:
# {
#	'name': 'Andy',
#	'age': 30,
#	'hair color': 'brown',
#	'job': 'Teacher',
#	'hobby': 'Fishing'
# }

Adding a new entry to a dictionary takes O(1) time.

Updating an entry:

If you want to update an entry, you can just assign a new value to an existing key:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown'
}

# Updating entries:
person['name'] = 'Andrew'
person['age'] = 29

print(person)
# The new Hash Map:
# {
#	'name': 'Andrew',
#	'age': 29,
#	'hair color': 'brown'
# }

Updating an entry from a dictionary takes O(1) time.

Deleting entries:

The del operator allows you to remove a key from a dictionary:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown',
	'hobby': 'Fishing'
}

# Deleting entries:
del person['name']

key = 'age'
del person[key]

# Deleting a non-existent entry:
# del person['job'] -> this would raise a KeyError exception

print(person)
# The new Hash Map:
# {
#	'hair color': 'brown',
#	'hobby': 'Fishing'
# }

If you try to delete a key that is not in the dictionary, Python raises a KeyError exception.

Deleting an entry from a dictionary takes O(1) time.

Checking if a key exists:

You can check if a key exists in a dictionary using the in operator:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown',
	'hobby': 'Fishing'
}

key = 'age'
print(key in person) # True

print('job' in person) # False

Iterating over the dictionary keys:

If we want to iterate over keys of the dictionary, we can use the for loop along with the keys() method:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown',
	'hobby': 'Fishing'
}

for key in person.keys():
    print(key + ' is ' + str(person[key]))

# This will print the following:
# hobby is Fishing
# age is 30
# hair color is brown
# name is Andy

Notice the order is not the same as initiated. Dictionary keeps the data in random order

Also Notice that we used str() to cast / convert the value into a string since we have the integer 30 which needs casting. If we don't use it, we will get a runtime error

Iterating over a dictionary takes O(n) time.

Iterating over the dictionary values:

If we want to iterate over values of the dictionary, we can use the for loop along with the values() method:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown',
	'hobby': 'Fishing'
}

for val in person.values():
    print(val)

# This will print the following:
# Fishing
# 30
# brown
# Andy

Notice the order is not the same as initiated. Dictionary keeps the data in random order

Iterating over a dictionary takes O(n) time.

Iterating using both the key and the value for simplicity:

If we want to iterate over both keys and values of the dictionary, we can use the for loop along with the items() method:

person = {
	'name': 'Andy',
	'age': 30,
	'hair color': 'brown',
	'hobby': 'Fishing'
}

for key, val in person.items():
    print(key + ' is ' +  str(val))

# This will print the following:
# hobby is Fishing
# age is 30
# hair color is brown
# name is Andy

Notice the order is not the same as initiated. Dictionary keeps the data in random order

Also Notice that we used str() to cast / convert the value into a string since we have the integer 30 which needs casting. If we don't use it, we will get a runtime error

Iterating over a dictionary takes O(n) time.

Assignment
Follow the Coding Tutorial and let's play with some dictionaries.

Hint
Look at the examples above if you get stuck.