Dictionaries

A dictionary is a data structure used to store mappings from unique keys to values.

• Unlike lists, dictionaries are not a sequence type: elements are not typically accessed by sequential index.

• Instead, given a key, you can look up its associated value. However, given a value, you cannot look up its key.

• Keys must be unique (no duplicate keys) and must be an immutable data type, such as int, float, or str.

• Values do not need to be unique and can be of any data type.

fav_color = {"akap1204": "lavender", "ktgower": "pink", "aronacho": "fuchsia", "ksuhas16": "green"}
squares = {2: 4, 3: 9, 5: 25, -5: 25, -2: 4, -3: 9}
atomic_number = {"H": 1, "Fe": 26, "Au": 79}
food_price = {"Taco": 3.25, "Burrito": 7.5, "Chips": 2.5, "Guac": 4.75}
empty_dict = {} # An empty dictionary

Retrieve and modify information in a dictionary using square bracket syntax:

• my_dict[key] retrieves the value associated with the given key.

• my_dict[key] = new_value adds or updates a key-value pair in the dictionary.

• del my_dict[key] removes a key and its associated value from the dictionary.

Practice: Food prices¶

Given the dictionary food_price = {"Taco": 3.25, "Burrito": 7.5, "Chips": 2.5, "Guac": 4.75}, which statement will result in an error?

food_price[2.5]
food_price["Guac"] = 3
del food_price["taco"]
food_price[2.5] = "Chips"

Iteration¶

By default, if we attempt to loop over a dictionary, we loop over only its keys in the order they were initially added to the dictionary.

for uwnetid in fav_color:
    print(uwnetid, "fav color is", fav_color[uwnetid])

Avoid iterating over a dictionary. A very common mistake is to use a loop when you only needed to look up a single value. Only iterate over a dictionary if you need to modify or access the values associated with every key. To find a single person’s favorite color, just look it up directly: fav_color["aronacho"].

Dictionaries also have methods to retrieve collections of their keys, values, or key-value pairs:

• my_dict.keys() returns all the keys.

• my_dict.values() returns all the values.

• my_dict.items() returns all the (key, value) pairs, where the key is index 0 and value is index 1.

Practice: Counting words¶

Write code that uses a dictionary to count how many times each unique word appears in the phrase below:

phrase = "how much wood could a woodchuck chuck if a woodchuck could chuck wood"