Programming, problem solving, and algorithms

CPSC 203, 2026 W1

October 13, 2026

Announcements

• Examlet 2 runs today through Sunday.
• Lab 5 (matplotlib and pandas) and POTW 5 (pandas and matplotlib) due Sunday.
  • If your scheduled lab was yesterday, you can still attend:
    • T1F: Tue, 18:00 - 19:30.
    • T1E: Thu, 9:30 - 11:00.
• Project 1 due Monday, October 26 (~two weeks).

There was a misconfiguration for examlet 2: Starts today, ends on Sunday. But you’ve had the material (dataclasses and lists) for more than a week, so no big deal.

Key-Value Lookup

Key-Value Pairs

• Given the key, we would like to quickly retrieve the corresponding value.

• Launch Class Activity: Grouping & Encoding in PrairieLearn.
• Open key_value_patterns.py.

Key	Value
name	Ian Mitchell
department	Computer Science
program	M.Sc.
start date	1995-09-01
terms	7
domestic student	yes
⋮	⋮

Representing Key-Value Pairs

• How could we represent this information? Don’t say “dictionary”.

  • What type for the whole set of data?
  • What type for each row of data?
  • What type for the keys?
  • What type for the values?

• Arbitrary size / list.
• Compound with two elements: key and value.
• String.
• Anything. Can use typing.Any to formally recognize that type, although it is essentially the Python default.

See alternative representation as two lists of the same length. Not recommended, but this approach was often used in older languages where compound data was syntactically difficult.

Efficiency of List Representations

If there are \(n\) key-value pairs in the store, how long does it take to:

• Look up the value associated with a key?
• Determine whether a key is in the store?
• Add a key-value pair?

• A Python dict can perform any of these operations in constant time for any \(n\).

• \(O(n)\).
• \(O(n)\) (even though you can write it without a loop using the in keyword).
• \(O(1)\).
• Note that using in on a dictionary is \(O(1)\).

You cannot tell how long an operation will take based solely on the operation; it will depend on the underlying representation and implementation. That is a feature not a bug: Generally speaking, we want to be able to first trust that the result of the operation is correct and not care about the representation / implementation. At some later date, we then want to be able to swap in a different representation / implementation if we need more efficiency.

Dictionary Patterns

Not Just a One-Trick Pony

Dictionaries / key-value lookup is useful for solving other common problems.

• Compound data.
• Tracking membership.
• Counting appearances / frequencies.
• Finding complements.
• Grouping by (discrete) category.
• Representing a map or translation over a discrete domain.

Today: Finish slides / class activity from last lecture for the first five.

Thursday: Translation / mapping.

Note that there is nothing special about dictionaries and compound data:

• Any key-value lookup representation could be used for compound data.
• Any compound data representation could be used for key-value lookup (although some might require you to specify all keys before running, or be inefficient).

Okay, all done discussing (bad) ways of implementing key-value lookup. Now let’s return to the ways we can use it.

If necessary, go back and complete any patterns that were not completed in the previous lecture.