18. Dictionaries

Suggested refs:

https://www.programiz.com/python-programming/dictionary

https://realpython.com/python-dicts/

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18.1. Dictionary structure

A Python dictionary is a collection of items.

Each item is a key: value pair.

Dictionary items are ordered, changeable, and do not allow duplicates.

All keys must be immutable (not able to be changed) such as integers, strings and tuples of integers or strings.

Values can be any data types such as string, int, float, boolean, tuple, list, dictionary.

dict_var = {key1 : value1, key2 : value2, …..}

Returns a dictionary with the specified key: value pairs.

An example of a dictionary of states and capitals is below.

eastern_state_capitals = {
                'Victoria': 'Melbourne',
                'New South Wales': 'Sydney',
                'Queensland': 'Brisbane'
                }
print(eastern_state_capitals)

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18.2. Getting a value from a dictionary

Values in a dictionary are retrieved by using the key as an index.

value_1 = dict_var[key1]

Returns a value in a dictionary with the specified key, key1.

In the dictionary below, the capital of Victoria can be found by indexing the dictionary: eastern_state_capitals['Victoria'].

THe key is ‘Victoria’. The value is ‘Melbourne’.

eastern_state_capitals = {
                'Victoria': 'Melbourne',
                'New South Wales': 'Sydney',
                'Queensland': 'Brisbane'
                }
capital = eastern_state_capitals['Victoria']
print(capital)
# Output is 'Melbourne'

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18.3. Case sensitive keys

Keys in a dictionary are case sensitive.

The keys ‘Vic’and ‘VIC’ are different keys.

eastern_state_capitals = {
                'Vic': 'Melbourne',
                'VIC': 'MELB',
                }
print(eastern_state_capitals['Vic'])
# Output is 'Melbourne'
print(eastern_state_capitals['VIC'])
# Output is 'MELB'

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18.4. Empty dictionary

An empty dictionary is needed as a starting point when a dictionary is built as the program runs.

An empty dictionary can be made using curly brackets:

empty_dict = {}

An empty dictionary can be made using the dict function:

empty_dict = dict()

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18.5. Making a dictionary

18.5.1. Making a dictionary: curly brackets

Make a dictionary by enclosing a comma-separated sequence of key-value pairs in curly braces {}.

The dictionary below has 3 items, each separated by a comma.

Each item is a key: value pair separated by a colon.

state_capitals = {
                'Victoria': "Melbourne",
                'Tasmania': "Hobart",
                'Queensland': "Brisbane"
                }
print(state_capitals)

Tasks

1. Create a dictionary using curly brackets such that it maps the names of three countries, Japan, France and England, to their capitals: Tokyo, Paris and London. Print the dictionary.

2. Create a dictionary using curly brackets such that it maps the names of three fruits, Apple, Banana, and Grapes, to their colors: Red, Yellow, and Purple. Print the dictionary.

Q1

Create a dictionary using curly brackets such that it maps the names of three countries, Japan, France and England, to their capitals: Tokyo, Paris and London. Print the dictionary.

country_capitals = {
    'Japan': 'Tokyo',
    'France': 'Paris',
    'England': 'London'
}
print(country_capitals)

Q2

Create a dictionary using curly brackets such that it maps the names of three fruits, Apple, Banana, and Grapes, to their colors: Red, Yellow, and Purple. Print the dictionary.

fruit_colors = {
    'Apple': 'Red',
    'Banana': 'Yellow',
    'Grapes': 'Purple'
}
print(fruit_colors)

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18.5.2. Making a dictionary from a list of lists

Make a dictionary using the dict function.

Pass in a list of lists, with each list made up 2 elements, e.g ["New South Wales", "Sydney"]

The first element becomes the key and the second element becomes the value. e.g "New South Wales": "Sydney"

state_capitals = dict([
    ["New South Wales", "Sydney"],
    ["Victoria", "Melbourne"],
    ["Queensland", "Brisbane"]
])

print(state_capitals)
# Output is {'New South Wales': 'Sydney', 'Victoria': 'Melbourne', 'Queensland': 'Brisbane'}

Tasks

1. Create a dictionary using the dict function and a list of lists such that it maps the names of three programming languages, Python, Java, and C++, to their creators: Guido van Rossum, James Gosling, and Bjarne Stroustrup. Print the dictionary.

2. Create a dictionary using the dict function and a list of lists such that it maps the names of three countries, China, India, and USA, to their populations in billions: 1.4, 1.4, and 0.3. Print the dictionary.

Q1

Create a dictionary using the dict function and a list of lists such that it maps the names of three programming languages, Python, Java, and C++, to their creators: Guido van Rossum, James Gosling, and Bjarne Stroustrup. Print the dictionary.

languages = dict([
    ['Python', 'Guido van Rossum'],
    ['Java', 'James Gosling'],
    ['C++', 'Bjarne Stroustrup']
])
print(languages)

Q2

Create a dictionary using the dict function and a list of lists such that it maps the names of three countries, China, India, and USA, to their populations in billions: 1.44, 1.39, and 0.33. Print the dictionary.

populations = dict([
    ['China', 1.44],
    ['India', 1.39],
    ['USA', 0.33]
])
print(populations)

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18.5.3. Making a dictionary from a list of tuples

Make a dictionary using the dict function.

Pass in a list of tuples, with each tuple made up 2 elements, e.g ("New South Wales", "Sydney")

The first element becomes the key and the second element becomes the value. e.g "New South Wales": "Sydney"

capitals = dict([
    ("South Australia", "Adelaide"),
    ("Western Australia", "Perth"),
    ("Australian Capital Territory", "Canberra")
])
print(capitals)
# Output is {'South Australia': 'Adelaide', 'Western Australia': 'Perth', 'Australian Capital Territory': 'Canberra'}

Tasks

1. Create a dictionary using the dict function and a list of tuples such that it maps the names of three car brands, Toyota, BMW, and Ford, to their countries of origin: Japan, Germany, and USA. Print the dictionary.

2. Create a dictionary using the dict function and a list of tuples such that it maps the names of three planets, Mercury, Venus, and Earth, to their average distances from the sun in million kilometers: 57.9, 108.2, and 149.6. Print the dictionary.

Q1

Create a dictionary using the dict function and a list of tuples such that it maps the names of three car brands, Toyota, BMW, and Ford, to their countries of origin: Japan, Germany, and USA. Print the dictionary.

car_brand_countries = dict([
    ('Toyota', 'Japan'),
    ('BMW', 'Germany'),
    ('Ford', 'USA')
])
print(car_brand_countries)

Q2

Create a dictionary using the dict function and a list of tuples such that it maps the names of three planets, Mercury, Venus, and Earth, to their average distances from the sun in million kilometers: 57.9, 108.2, and 149.6. Print the dictionary.

planet_distances_to_sun = dict([
    ('Mercury', 57.9),
    ('Venus', 108.2),
    ('Earth', 149.6)
])
print(planet_distances_to_sun)

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18.5.4. Making a dictionary from 2 lists

2 lists of keys and values can be combined and converted into a dictionary using several methods.

18.5.4.1. Making a dictionary from 2 lists –update

The update method is used to add each state key and city value.

states = ["Queensland", "South Australia", "Western Australia"]
cities = ["Brisbane", "Adelaide", "Perth"]

capitals = {}
for i in range(len(states)):
    capitals.update({states[i]: cities[i]})

print(capitals)
# Output is {'Queensland': 'Brisbane', 'South Australia': 'Adelaide', 'Western Australia': 'Perth'}

Tasks

1. Create a dictionary using the update method and two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

Q1

Create a dictionary using the update method and two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

names = ['Lockett', 'Coventry', 'Dunstall']
goals = [1360, 1299, 1254]
goal_kickers = {}
for i in range(len(names)):
    goal_kickers.update({names[i]: goals[i]})
print(my_dict)

18.5.4.2. Making a dictionary from 2 lists –set key value

Each state key gets its city value each time through the for loop.

states = ["Queensland", "South Australia", "Western Australia"]
cities = ["Brisbane", "Adelaide", "Perth"]

capitals = {}
for i in range(len(states)):
    capitals[states[i]] = cities[i]
print(capitals)
# Output is {'Queensland': 'Brisbane', 'South Australia': 'Adelaide', 'Western Australia': 'Perth'}

Tasks

1. Create a dictionary using the update method and two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

Q1

Create a dictionary using the update method and two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

names = ['Lockett', 'Coventry', 'Dunstall']
goals = [1360, 1299, 1254]
goal_kickers = {}
for i in range(len(names)):
    goal_kickers[names[i]] = goals[i]
print(goal_kickers)

18.5.4.3. Making a dictionary from 2 lists –dict and zip

The zip() function pairs each element from the states list with the corresponding element from the cities list.

The result is an iterator containing these tuples: ('Queensland', 'Brisbane'), ('South Australia', 'Adelaide'), ('Western Australia', 'Perth')

The dict function then converts the zip object into a dictionary.

states = ["Queensland", "South Australia", "Western Australia"]
cities = ["Brisbane", "Adelaide", "Perth"]

capitals = dict(zip(states, cities))
print(capitals)
# Output is {'Queensland': 'Brisbane', 'South Australia': 'Adelaide', 'Western Australia': 'Perth'}

Tasks

1. Create a dictionary using the zip function and two lists such that it maps the names of three animals, Elephant, Dog, and Cat, to their average lifespans in years: 70, 13, and 15. Print the dictionary.

2. Create a dictionary using the zip function and two lists such that it maps the names of three cities, Tokyo, Delhi, and Shanghai, to their populations in millions: 37.4, 28.5, and 25.6. Print the dictionary.

Q1

Create a dictionary using the zip function and two lists such that it maps the names of three animals, Elephant, Dog, and Cat, to their average lifespans in years: 70, 13, and 15. Print the dictionary.

animals = ['Elephant', 'Dog', 'Cat']
lifespans = [70, 13, 15]
animal_lifespans = dict(zip(animals, lifespans))
print(animal_lifespans)

Q2

Create a dictionary using the zip function and two lists such that it maps the names of three cities, Tokyo, Delhi, and Shanghai, to their populations in millions: 37.4, 28.5, and 25.6. Print the dictionary.

cities = ['Tokyo', 'Delhi', 'Shanghai']
populations = [37.4, 28.5, 25.6]
city_populations = dict(zip(cities, populations))
print(city_populations)

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18.5.5. Making a dictionary by dictionary comprehension from 2 lists

The dictionary comprehension below creates a dictionary by iterating over length of the states list and using the index, i, to set the state key and city value.

states = ["Western Australia", "Tasmania", "Northern Territory"]
cities = ["Perth", "Hobart", "Darwin"]

capitals = {states[i]: cities[i] for i in range(len(states))}
print(capitals)

Tasks

1. Create a dictionary using a dictionary comprehension via the indexes of two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

Q1

Create a dictionary using a dictionary comprehension via the indexes of two lists such that it maps the names of ‘Lockett’, ‘Coventry’, and ‘Dunstall’, to their goals kicked: 1360, 1299, and 1254. Print the dictionary.

names = ['Lockett', 'Coventry', 'Dunstall']
goals = [1360, 1299, 1254]
goal_kickers = {names[i]: goals[i] for i in range(len(names))}
print(goal_kickers)

The dictionary comprehension below creates a dictionary by iterating over the tuples produced by zip().

For each tuple, the state becomes the key and city becomes the value.

states = ["Western Australia", "Tasmania", "Northern Territory"]
cities = ["Perth", "Hobart", "Darwin"]

capitals = {state: city for state, city in zip(states, cities)}
print(capitals)

Tasks

1. Create a dictionary using dictionary comprehension and two lists such that it maps the names of three sports, Soccer, Basketball, and Baseball, to the number of players in each team: 11, 5, and 9. Print the dictionary.

2. Create a dictionary using dictionary comprehension and two lists such that it maps the names of three countries, USA, China, and Japan, to their GDPs in trillion USD: 21.43, 14.34, and 5.08. Print the dictionary.

Q1

Create a dictionary using dictionary comprehension and two lists such that it maps the names of three sports, Soccer, Basketball, and Baseball, to the number of players in each team: 11, 5, and 9. Print the dictionary.

sports = ['Soccer', 'Basketball', 'Baseball']
players = [11, 5, 9]
sport_players = {sport: player for sport, player in zip(sports, players)}
print(sport_players)

Q2

Create a dictionary using dictionary comprehension and two lists such that it maps the names of three countries, USA, China, and Japan, to their GDPs in trillion USD: 21.43, 14.34, and 5.08. Print the dictionary.

countries = ['USA', 'China', 'Japan']
gdps = [21.43, 14.34, 5.08]
country_gdps = {country: gdp for country, gdp in zip(countries, gdps)}
print(country_gdps)

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18.5.6. Making a dictionary from key word arguments

Make a dictionary using the dict function and key word arguments.

a=1 will become the key value pair 'a': 1

simple_dict = dict(a=1, b=2, c=3, d=4)
print(simple_dict)
# Output is {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Tasks

1. Create a dictionary using keyword arguments such that it maps the names of three programming languages, Python, Java, and JavaScript, to their release years: 1991, 1995, and 1995. Print the dictionary.

2. Create a dictionary using keyword arguments such that it maps the names of three continents, Africa, Asia, and Europe, to their areas in million square kilometers: 30.37, 44.58, and 10.18. Print the dictionary.

Q1

Create a dictionary using keyword arguments such that it maps the names of three programming languages, Python, Java, and JavaScript, to their release years: 1991, 1995, and 1995. Print the dictionary.

languages_release_years = dict(Python=1991, Java=1995, JavaScript=1995)
print(languages_release_years)
# Output is {'Python': 1991, 'Java': 1995, 'JavaScript': 1995}

Q2

Create a dictionary using keyword arguments such that it maps the names of three continents, Africa, Asia, and Europe, to their areas in million square kilometers: 30.37, 44.58, and 10.18. Print the dictionary.

continents = dict(Africa=30.37, Asia=44.58, Europe=10.18)
print(continents)
# Output is {'Africa': 30.37, 'Asia': 44.58, 'Europe': 10.18}

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