07-maps.md

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7. Maps

A map (also called a dictionary) stores values under keys. Keys are strings; values can be anything.

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Creating maps

Two forms are available — a literal with : and a constructor with =:

user = {"name": "Sara", "age": 20}

person = map("name" = "Sara", "age" = 20, "city" = "Tehran")

The map(...) form is handy when keys are simple names.

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Reading and writing

Use square brackets to read a value by key, and the same syntax to create or update one:

user = {"name": "Sara"}

print(user["name"])     # Sara

user["age"] = 20        # add a new key
user["name"] = "Mina"   # update an existing key

print(user["age"])      # 20
print(user)             # {'name': 'Mina', 'age': 20}

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Map methods

Method	Description	Example	Result
has(key)	test whether a key exists	map("a"=1).has("a")	true
get(key, default)	value for key, or default if missing	map("a"=1).get("b", 0)	0
keys()	list of keys	map("a"=1).keys()	['a']
values()	list of values	map("a"=1).values()	[1]
items()	list of [key, value] pairs	map("a"=1).items()	[['a', 1]]
remove(key)	delete a key	map("a"=1).remove("a")	—
extend(other)	merge another map in	map("a"=1).extend(map("b"=2))	{'a': 1, 'b': 2}

person = map("name" = "Sara", "age" = 20, "city" = "Tehran")

print(person.has("name"))     # true
print(person.has("email"))    # false

print(person.get("name", "?"))      # Sara
print(person.get("email", "none"))  # none  (missing key -> default)

print(person.keys())          # ['name', 'age', 'city']
print(person.values())        # ['Sara', 20, 'Tehran']
print(person.items())         # [['name', 'Sara'], ['age', 20], ['city', 'Tehran']]

person.remove("city")
print(person.keys())          # ['name', 'age']

person.extend(map("country" = "Iran"))
print(person.keys())          # ['name', 'age', 'country']

Because maps have no direct len, count their keys instead:

print(len(person.keys()))

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Equality

Maps compare by value, regardless of insertion order:

print(map("a"=1, "b"=2) == map("b"=2, "a"=1))   # true

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Iterating

Looping over a map visits its keys:

person = map("name" = "Sara", "age" = 20)

repeat key -> person {
    print(key, person[key])
}

See Control Flow for more iteration forms.

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Next steps

Control Flow covers conditions and loops.

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