--- title: "A map of rooms" subtitle: "Deeper Python in the console · Lists and dictionaries · Lesson 3" --- # A map of rooms **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A phone's contacts, a dictionary app and a game's map all do the same thing: look something up by its name. :::teacher-note **In one sentence:** Students store a map in dictionaries (rooms by name, and each room's exits), look values up by key, check keys with in, use a dictionary inside a dictionary, and count words with a dictionary. **Students should be able to say:** I can make a dictionary and look up a value by its key. I can check whether a key is in a dictionary before looking it up. I can use a dictionary to count things. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict `{"north": "kitchen", "east": "garden"}` pairs each **key** with a **value**. `exits["north"]` looks up the value for that key. What does it print? ```python exits = {"north": "kitchen", "east": "garden"} print(exits["north"]) print("west" in exits) ``` :::keep-together **What is the first line it prints?** ☐ kitchen ☐ {'north': 'kitchen'} ☐ garden ☐ north My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** kitchen: the value stored under the key "north". The second line is False, because "west" is not a key. ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate ```python rooms = { "hall": "A long hall with a creaky floor.", "kitchen": "A kitchen that smells of toast.", "garden": "A garden full of tall grass.", } exits = { "hall": {"north": "kitchen", "east": "garden"}, "kitchen": {"south": "hall"}, "garden": {"west": "hall"}, } here = "hall" while True: print(rooms[here]) print("Exits: " + ", ".join(exits[here])) command = input("> ").strip().lower() words = command.split() if command == "quit": break elif len(words) == 2 and words[0] == "go": if words[1] in exits[here]: here = exits[here][words[1]] else: print("You can't go that way.") else: print("Try: go north, or quit") ``` :::keep-together ### A dictionary in a dictionary **What is exits["hall"]["east"]?** ☐ "kitchen" ☐ "garden" ☐ "hall" ☐ {"north": "kitchen", "east": "garden"} ::: :::teacher-note **Answer:** "garden" exits["hall"] is the hall's own exits dictionary; ["east"] looks up east in it: the garden. ::: :::keep-together ### A wall **You are in the kitchen and type go west. What happens?** ☐ It stops with an error ☐ It prints You can't go that way. ☐ You go to the garden ☐ You go back to the hall ::: :::teacher-note **Answer:** It prints You can't go that way. "west" in exits["kitchen"] is False (the kitchen only has south), so the else replies instead of looking it up. ::: :::keep-together ### Spot the bug **This stops with KeyError: Hall. Click the line that causes it.** ``` 1 rooms = {"hall": "A long hall.", "kitchen": "A warm kitchen."} 2 here = "Hall" 3 print(rooms[here]) ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Keys must match exactly: the room is "hall", but here is "Hall" with a capital. Dictionaries do not guess. ::: :::typical ## Modify **An attic.** Add an **attic**, reached by `go up` from the hall (and `go down` back). **Count the moves.** Count each move, and say how many when the player quits. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. **An attic:** ```python rooms = { "hall": "A long hall with a creaky floor.", "kitchen": "A kitchen that smells of toast.", "garden": "A garden full of tall grass.", "attic": "A dusty attic full of old boxes.", } exits = { "hall": {"north": "kitchen", "east": "garden", "up": "attic"}, "kitchen": {"south": "hall"}, "garden": {"west": "hall"}, "attic": {"down": "hall"}, } here = "hall" while True: print(rooms[here]) print("Exits: " + ", ".join(exits[here])) command = input("> ").strip().lower() words = command.split() if command == "quit": break elif len(words) == 2 and words[0] == "go": if words[1] in exits[here]: here = exits[here][words[1]] else: print("You can't go that way.") else: print("Try: go north, or quit") ``` **Count the moves:** ```python rooms = { "hall": "A long hall with a creaky floor.", "kitchen": "A kitchen that smells of toast.", "garden": "A garden full of tall grass.", } exits = { "hall": {"north": "kitchen", "east": "garden"}, "kitchen": {"south": "hall"}, "garden": {"west": "hall"}, } here = "hall" moves = 0 while True: print(rooms[here]) print("Exits: " + ", ".join(exits[here])) command = input("> ").strip().lower() words = command.split() if command == "quit": print("You made " + str(moves) + " moves.") break elif len(words) == 2 and words[0] == "go": if words[1] in exits[here]: here = exits[here][words[1]] moves += 1 else: print("You can't go that way.") else: print("Try: go north, or quit") ``` ::: ::: ## Make a word counter Ask for a **sentence** and print how many times each word appears, such as `the: 3`. Start with an empty dictionary, `counts = {}`. For each word: if it is already a key, add 1; if not, set it to 1. Tidy the sentence first so that `The` and `the` count as the same word. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ```python sentence = input("Type a sentence: ").strip().lower() counts = {} for word in sentence.split(): if word in counts: counts[word] += 1 else: counts[word] = 1 for word in counts: print(word + ": " + str(counts[word])) ``` ::: :::wider ## Where this leads You now have the parts of a text adventure: a command parser, a bag and a map. Still to come in this course: handling errors with `try`, saving and loading a game in a file, and the adventure itself. ::: ## Exit ticket :::keep-together **ages = {"Ada": 12}. What do ages["Ada"] and "Bo" in ages give?** {{lines-3}} ::: :::teacher-note **Answer:** 12, and False: Bo is not a key. :::