Nested Collections

Collections hold other collections. A list of lists is a grid. A dictionary of lists groups things by category. Reading them is the same skill as always: one layer at a time.

EXAMPLE

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YOUR TURN

Your turn → grid[1][2] reads left to right: row 1, then column 2. Print the value 4.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

Grouping is the pattern that turns a flat list into an organized one, and it appears in real software constantly.

EXAMPLE

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YOUR TURN

Your turn → The check 'if this key does not exist yet, start an empty list' is the heart of grouping. Add two more businesses and rerun.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

◈ Graded Challenge

Write a function that takes a list of member records (each with a name and hours) and returns a list of just the names, sorted from most hours to fewest. The original list must not be modified.

Name it exactly: def top_volunteers(members):

3 visible + 2 hidden

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