Dictionaries (records)

Store data by name

• A dictionary stores values under names called keys.
• Write pairs inside braces: {"name": "Sam", "age": 15}.
• Each key is unique and points to one value.

Get and set values

• Read a value with its key: person["name"].
• Add or change a value the same way: person["age"] = 16.
• A list uses a number index; a dictionary uses a key.

ages = {"Sam": 12, "Mia": 15}
print(ages["Sam"])
ages["Leo"] = 9
print(ages)

A dictionary as a record

• A record groups related facts about one thing.
• A dictionary is a neat way to hold a record.
• Each key is a field, like name or grade.

pupil = {"name": "Ada", "grade": "A"}
print(pupil["name"])
print(pupil["grade"])

Check and loop

• "name" in person is True if that key exists.
• for key in person: visits each key in turn.
• person.get("x", 0) returns a default if the key is missing.

menu = {"tea": 2, "cake": 5}
for item in menu:
    print(item, menu[item])
print("tea" in menu)

In Cambridge pseudocode

• A dictionary models the exam's TYPE record; each key is a field reached with a dot.

TYPE Student
    DECLARE Name : STRING
    DECLARE Age  : INTEGER
ENDTYPE

DECLARE pupil : Student
pupil.Name ← "Sam"
pupil.Age  ← 15

Now you try

• Build or update a dictionary in each task.
• Press Check answer to test your code.