Week 07 · Tuesday: Dictionaries (Code Crew G4)

Tuesday: Change a value

The crew used two bottles of water at lunch. Then the supply ship dropped off more tape, and now there are seven rolls. "Let's update the inventory!" Rocket says.

He starts typing a whole new dictionary from the top. Raven stops him. "What do you notice? Only two numbers changed. The oats are the same."

Nova taps one glowing jar with her wing. "Would you like a hint? You can change just one value if you know its key."

Store a new value by key

Put the dictionary name and the key on the left of =. Put the new value on the right.

You can use the old value too, just like fuel = fuel + 15.

inventory = {"water": 12, "oats": 8, "tape": 3}
inventory["water"] = inventory["water"] - 2
inventory["tape"] = 7
print(inventory["water"])
print(inventory["tape"])
print(inventory["oats"])

The screen shows
10
7
8

Line	water	oats	tape
1	12	8	3
2	10	8	3
3	10	8	7

fuel = {"shuttle": 40, "rover": 15}
fuel["shuttle"] = fuel["shuttle"] + 25
fuel["rover"] = fuel["rover"] * 2
print(fuel)

shuttle at the end

rover at the end

WHICH LINE?

Read the question.
Tap your answer.

The crew opens 3 bags of oats. Which line updates the inventory?

scores = {"Rocket": 5, "Raven": 5}
scores["Rocket"] = scores["Rocket"] + 3
scores["Raven"] = scores["Rocket"]
print(scores["Raven"])

What number does the program above show?

WHY THIS EXERCISETrace one line at a time. The order of the lines matters.

Try it

Use your kitchen dictionary from Monday.
Pretend you used one thing. Write the line that updates it.

Practice lab: trace the air

air = {"deck1": 90, "deck2": 75}
air["deck1"] = air["deck1"] - 10
air["deck2"] = air["deck2"] + 5
air["deck1"] = air["deck1"] - 10
print(air)

deck1 after line 2

deck2 after line 3

deck1 at the end

UPDATE CHECK

Read the question.
Tap your answer.

Which line adds 4 to the rover value?

d = {"x": 2}. Then d["x"] = d["x"] * 3 runs. What is d["x"]?

Does changing one key change the other values?

Statement	True or false?
inventory["oats"] = 9 changes the oats value.	?
You must retype the whole dictionary to change one value.	?
The right side of = is worked out first.	?
After d["a"] = 1 and then d["a"] = 2, d["a"] is 1.	?

WHY THIS EXERCISEA key holds one value at a time, just like a variable.

pods = {"A": 4}. Then pods["A"] = pods["A"] + pods["A"] runs. What is pods["A"]?

WHY THIS EXERCISE4 plus 4 is 8, and that new value is stored back by key.

Write a dictionary for fuel with two keys. Write three lines that change it. Then keep a trace table.

Nice updating! Tomorrow you will add brand new keys.