Word Counting and Choosing a Container

What this session is for

• Get comfortable using dictionaries for counting and lookup tables
• Use sets for deduplication and membership tests
• Handle text with split, join and strip
• Build the instinct of choosing a container before writing anything

Problems

6-1 Invert a dictionary

Implement invert(d: dict[str, int]) -> dict[int, str], swapping keys and values.

invert({"a": 1, "b": 2})  →  {1: "a", 2: "b"}

In your submission notes, answer: what happens if two keys map to the same value? Give a concrete example.

6-2 Deduplicate, keeping order

Implement unique_ordered(items: list[int]) -> list[int], removing duplicates while preserving first-appearance order.

unique_ordered([3, 1, 3, 5, 1, 7])  →  [3, 1, 5, 7]

Use a set for the “have I seen this” test — searching a list with in is not acceptable here.

Explain in your notes why the set version is faster.

6-3 Names in common

Implement common(a: list[str], b: list[str]) -> list[str], returning the names that appear in both lists, sorted alphabetically.

common(["Alex","Blake","Chen"], ["Blake","Chen","Dana"])  →  ["Blake","Chen"]

6-4 Grouping

Implement group_by_first(words: list[str]) -> dict[str, list[str]], grouping words by their first letter.

group_by_first(["apple","avocado","banana"])
  →  {"a": ["apple","avocado"], "b": ["banana"]}

6-5 Parsing a results file

Given several lines of text, each formatted name,subject,score, implement parse(lines: list[str]) -> dict[str, dict[str, int]], returning a nested dictionary of {name: {subject: score}}.

parse(["Alex,maths,87", "Alex,physics,92", "Blake,maths,65"])
  →  {"Alex": {"maths": 87, "physics": 92}, "Blake": {"maths": 65}}

Note that lines may carry stray whitespace, which you must handle.

6-6 Word frequency

Implement top_words(text: str, n: int) -> list[tuple[str, int]], returning the n most frequent words as (word, count) pairs. Requirements:

• case-insensitive
• strip punctuation from the ends of words (handle at least .,!?;:)
• ties broken alphabetically

top_words("The cat. The DOG! the cat", 2)
  →  [("the", 3), ("cat", 2)]

6-7 Choosing a container (no code)

Pick a container for each scenario and write your choice plus a one-sentence reason in your submission notes:

1. a mapping from each of 116 student numbers to a name
2. every score from one exam, from which you later want the mean and the maximum
3. testing whether a word appears in a list of ten thousand common words
4. a point on a plane, to be used as a dictionary key
5. the set of courses a class is enrolled in, with no course listed twice
6. a student’s successive submissions, kept in the order they arrived

Before you submit

• Every function is annotated, with element types spelled out
• The questions in 6-1 and 6-2 are answered in your notes
• 6-4 handles a letter’s first appearance
• 6-5 handles stray whitespace
• 6-6 has been tested with ("The cat. The DOG! the cat", 2)
• 6-7 has a reason for every item