Building Dictionaries

Remeber: a dictionary is a map of keys to values:

a dictionary of meals with three key/value pairs (1) breakfast : yogurt, granola and fruit, (2) lunch : Cupbop, (3) dinner : lasagna and salad

So far we have shown you how to use dictionaries:

# a dictionary
meals = {
    'breakfast': 'yogurt, granola, and fruit',
    'lunch': 'Cupbop',
    'dinner': 'lasagna and salad'
}

# get the value for a key
food = meals['dinner']

In this guide we will show you how to build a dictionary.

Building a dictionary

There are two steps to building a dictionary, (1) creating an empty dictionary, and (2) adding key/value pairs to the dictionary.

Creating an empty dictionary

You can create an empty dictionary using a pair of curly braces:

meals = {}

Add key/value pairs to a dictionary

You can add key/value pairs to the dictionary one at a time using square brackets:

meals['lunch'] = 'crackers and cheese'

Dictionaries cannot have duplicate keys

This will over-write the value of the ‘lunch’ key:

meals['lunch'] = 'peanut butter and jelly sandwich'

Putting these together

If you want to create an entire dictionary from scratch:

meals = {}
meals['breakfast'] = 'ogurt, granola, and fruit'
meals['lunch'] = 'Cupbop'
meals['dinner'] = 'lasagna and salad'

You can do this in one step with:

meals = {
    'breakfast': 'yogurt, granola, and fruit',
    'lunch': 'Cupbop',
    'dinner': 'lasagna and salad',
}

But there will be time when you will need to build a dictionary one item at a time. We will see examples of this below.

Room assignments

You are helping to organize the next Women in Computer Science (WICS) Skillathon, where students can come hear various speakers in CS provide brief workshops on various tech skills. You have a list of speakers and a list of available rooms. You want to pair up speakers with a room. Write a function that takes a list of speakers and a list of rooms (both having the same length) and returns a dictionary of room assignments.

Let’s say that you have these two lists:

speakers = ['Page', 'Jones', 'Bean', 'Zappala', 'Clement', 'Fulda']
rooms = ['2233', '2228', '2230', '2242', '2220', '2238']

You want to write a function make_room_assignments() that creates a dictionary mapping each speaker to a room:

def make_room_assignments(speakers, rooms):
    """
    Return a dictionary mapping each speaker to a room
    """

To solve this problem, you need to iterate over two lists at a time, so that you can map “Page” to “2233, “Jones to “2228”, and so forth. Remember, zip helps you iterate over two lists:

zipping the two lists together results in a list of tuples

Zip creates a list of tuples. It makes these tuples by taking one element from each list at a time.

Here is how we can use this in a solution, which you can find in room_assignments.py:

def make_room_assignments(speakers, rooms):
    """
    Return a dictionary mapping each speaker to a room
    """
    # create an empty dictionary
    assignments = {}
    # zip the speakers and rooms, then loop through all of the tuples
    for speaker, room in zip(speakers, rooms):
        # map the speaker to the room in the dictionary
        assignments[speaker] = room
    # return the dictionary
    return assignments


if __name__ == '__main__':
    speakers = ['Page', 'Jones', 'Bean', 'Zappala', 'Clement', 'Fulda']
    rooms = ['2233', '2228', '2230', '2242', '2220', '2238']
    room_assignments = make_room_assignments(speakers, rooms)
    print(room_assignments)

Look through the comments in the code to see what each line is doing. Notice how we:

create an empty dictionary
zip the speaker and room lists together, then loop through the tuples
add each speaker to room mapping to the dictionary
return the dictionary at the end

If you run this program, you should see:

{'Page': '2233', 'Jones': '2228', 'Bean': '2230', 'Zappala': '2242', 'Clement': '2220', 'Fulda': '2238'}

Itineraries

This problem continues on from the previous one. Now that you have a list of speakers and room assignments, you need to assign each speaker to a time. You are given a list of speakers and their desired speaking times. You need to write a function that takes:

desired_speaker_times: a list of (speaker, time) tuples
room_assignments: a dictionary that maps speakers to rooms

and returns a list of speaker/room/time tuples.

def make_itinerary(desired_speaker_times, room_assignments):
    """
    desired_speaker_times: list of (speaker, time) tuples
    room_assignments: a dictionary that maps speakers to rooms
    Return a list of (speaker, room, time) tuples
    """

This picture illustrates what we are trying to do:

taking a list of speaker, time tuples and room assignments and creating a list of speaker, room, time tuples

Notice that this function takes a list of (speaker, time) tuples and returns (speaker, room, time) tuples. So you can solve this problem by iterating through the desired speaker times and creating a new list of tuples as you go:

def make_itinerary(desired_speaker_times, room_assignments):
    """
    Input: list of (speaker, time) tuples
    Return a list of (speaker, room, time) tuples
    """
    # create an empty list
    itinerary = []
    # iterate through the list of (speaker, time) tuples
    for speaker, time in desired_speaker_times:
        # append a new tuple to the list, using the dictionary to look up the room for the speaker
        itinerary.append((speaker, room_assignments[speaker], time))
    return itinerary


if __name__ == '__main__':
    desired_lineup = [('Fulda', '9am'), ('Page', '10am'), ('Jones', '11am'), ('Zappala', '12pm')]
    room_assignments = {'Page': '2233', 'Jones': '2228', 'Bean': '2230', 'Zappala': '2242', 'Clement': '2220', 'Fulda': '2238'}
    itinerary = make_itinerary(desired_lineup, room_assignments)
    print(itinerary)

The key piece of this is where we create the new list of tuples:

itinerary.append((speaker, room_assignments[speaker], time))

Notice that we create a new tuple of (speaker, room, time), and we get the middle piece of this — the room — by looking up the room in the room_assignments dictionary.

If you run the program, which is in itinerary.py, you should see:

[('Fulda', '2238', '9am'), ('Page', '2233', '10am'), ('Jones', '2228', '11am'), ('Zappala', '2242', '12pm')]

Inverting a dictionary

The speaker-to-room dictionary was helpful for making individual itineraries. Now the building scheduling crew wants a dictionary that maps rooms to speakers. Because the room-assignments dictionary is 1-to-1 (the keys and the values are all unique), we can do this.

You should write a function called invert_assignments that takes the room assignments dictionary:

room_assignments: a dictionary that maps speakers to rooms

and returns a new dictionary that maps rooms to speakers.

This picture illustrates what we are trying to do:

inverting a dictionary

To do this, you need to iterate through all of the (speaker, room) tuples in the room_assignments dictionary, which you can do using items():

def invert_assignments(speaker_rooms):
    """
    Return a new dictionary mapping room to speaker
    """
    # create an empty dictionary
    room_speakers = {}
    # loop through all of the (speaker, room) tuples in the dictionary
    for speaker, room in speaker_rooms.items():
        # create a new entry in the new dictionary, mapping room to speaker
        room_speakers[room] = speaker
    # return the new dictionary
    return room_speakers


if __name__ == '__main__':
    room_assignments = {'Page': '2233', 'Jones': '2228', 'Bean': '2230', 'Zappala': '2242', 'Clement': '2220', 'Fulda': '2238'}
    assigned_rooms = invert_assignments(room_assignments)
    print(assigned_rooms)

Notice that we are making room to speaker, when the original dictionary mapped speaker to room:

room_speakers[room] = speaker

When you run this code, it should show:

{'2233': 'Page', '2228': 'Jones', '2230': 'Bean', '2242': 'Zappala', '2220': 'Clement', '2238': 'Fulda'}

Everything all at once

We can put all of these pieces into one program that:

makes room assignments
makes an itinerary
makes the assigned rooms

You can see this program in skillathon.py:

def make_room_assignments(speakers, rooms):
    """
    Return a dictionary mapping each speaker to a room
    """
    assignments = {}
    for speaker, room in zip(speakers, rooms):
        assignments[speaker] = room
    return assignments

def make_itinerary(desired_speaker_times, room_assignments):
    """
    Input: list of (speaker, time) tuples
    Return a list of (speaker, room, time) tuples
    """
    # create an empty list
    itinerary = []
    # iterate through the list of (speaker, time) tuples
    for speaker, time in desired_speaker_times:
        # append a new tuple to the list, using the dictionary to look up the room for the speaker
        itinerary.append((speaker, room_assignments[speaker], time))
    return itinerary


def invert_assignments(speaker_rooms):
    """
    Return a new dictionary mapping room to speaker
    """
    # create an empty dictionary
    room_speakers = {}
    # loop through all of the (speaker, room) tuples in the dictionary
    for speaker, room in speaker_rooms.items():
        # create a new entry in the new dictionary, mapping room to speaker
        room_speakers[room] = speaker
    # return the new dictionary
    return room_speakers


def print_room_assignments(assignments):
    print("Room Assignments:")
    for speaker, room in assignments.items():
        print(f"* {speaker}: {room}")


def print_itinerary(itinerary):
    print("The itinerary:")
    for speaker, room, time in itinerary:
        print(f"* {speaker}: {room}, {time}")


def print_assigned_rooms(assigned_rooms):
    print("Assigned rooms:")
    for room, speaker in assigned_rooms.items():
        print(f"* {room}: {speaker}")

if __name__ == '__main__':
    # Make the room assignments and print them out
    speakers = ['Page', 'Jones', 'Bean', 'Zappala', 'Clement', 'Fulda']
    rooms = ['2233', '2228', '2230', '2242', '2220', '2238']
    room_assignments = make_room_assignments(speakers, rooms)
    print_room_assignments(room_assignments)
    print()

    # Make an itinerary and print it out
    desired_lineup = [('Fulda', '9am'), ('Page', '10am'), ('Jones', '11am'), ('Zappala', '12pm')]
    the_itinerary = make_itinerary(desired_lineup, room_assignments)
    print_itinerary(the_itinerary)
    print()

    # Make a list of assigned rooms and print it out
    rooms = invert_assignments(room_assignments)
    print_assigned_rooms(rooms)

We have also added some nicer printing by iterating through the dictionaries and lists and printing them out with bullets. If you run this program, you get:

Room Assignments:
* Page: 2233
* Jones: 2228
* Bean: 2230
* Zappala: 2242
* Clement: 2220
* Fulda: 2238

The itinerary:
* Fulda: 2238, 9am
* Page: 2233, 10am
* Jones: 2228, 11am
* Zappala: 2242, 12pm

Assigned rooms:
* 2233: Page
* 2228: Jones
* 2230: Bean
* 2242: Zappala
* 2220: Clement
* 2238: Fulda