Lecture 22 — Dictionaries, Part 1

Overview

  • Dictionaries and dictionary operations

  • Solutions to the problem of counting how many items are associated with each category in Customers.txt files

  • Other applications

Introduction to Dictionaries

  • Association between “keys” (like words in an English dictionary) and “values” (like definitions in an English dictionary). The values can be anything.

  • Examples:

    >>> children = dict()
>>> children = {}    # either of these works
>>> children['Sam'] = 11
>>> children['Peter'] = 12
>>> children['Alice'] = 10
>>> children['Jon'] = 13
>>> children
{'Sam': 11, 'Peter': 12, 'Alice': 10, 'Jon': 13}
>>> 'Sam' in children
True
>>> 'Luke' in children
False
>>> 13 in children
False
>>> list(children.keys())
['Sam', 'Peter', 'Alice', 'Jon']
>>> sorted(children.keys())
['Alice', 'Jon', 'Peter', 'Sam']
>>> children.values()
dict_values([11, 12, 10, 13])
>>> list(children.values())
[11, 12, 10, 13]

  • Details:

    • Two initializations; either would work.

    • Syntax is very much like the subscripting syntax for lists, except dictionary subscripting/indexing uses keys instead of integers!

    • The keys, in this example, are children names and the values are their age (integers).

    • The in method tests only for the presence of the key, like looking up a word in the dictionary without checking its definition.

    • The keys are NOT ordered.

  • Just as in sets, the implementation uses hashing of keys.

    • Conceptually, sets are dictionaries without values.