Maps in Scheme

Maps#

Maps are Python’s built-in associative data type, commonly known as dictionaries.

To create an empty map, use the built-in dict function:

m = {}

Set key/value pairs using typical name[key] = val syntax.

m["k1"] = 7
m["k2"] = 13

Printing a map with, e.g., print will show all its key/value pairs.

print("map:", m)

Get a value for a key with name[key].

v1 = m["k1"]
print("v1:", v1)

If the key doesn’t exist, a KeyError is raised by default. However, you can use the get method to return a default value if the key doesn’t exist.

v3 = m.get("k3", 0)
print("v3:", v3)

The built-in len function returns the number of key/value pairs when called on a map.

print("len:", len(m))

To remove key/value pairs from a map, use the del statement.

del m["k2"]
print("map:", m)

To remove all key/value pairs from a map, use the clear method.

m.clear()
print("map:", m)

The in operator checks if a key is present in the map. This can be used to disambiguate between missing keys and keys with zero values like 0 or "".

prs = "k2" in m
print("prs:", prs)

You can also declare and initialize a new map in the same line with this syntax.

n = {"foo": 1, "bar": 2}
print("map:", n)

There are also a number of useful utility functions for dictionaries in Python.

n2 = {"foo": 1, "bar": 2}

if n == n2:
    print("n == n2")

Note that dictionaries appear in the form {'k': 'v', 'k': 'v'} when printed with print.

$ python maps.py 
map: {'k1': 7, 'k2': 13}
v1: 7
v3: 0
len: 2
map: {'k1': 7}
map: {}
prs: False
map: {'foo': 1, 'bar': 2}
n == n2

Now that we understand basic operations on dictionaries in Python, let’s move on to learn more about the language in the next example.

Next example: Functions.