Building Dictionaries Dynamically — From Input and Loops

"Every dictionary you've built so far was hardcoded. But real applications build their dictionaries at runtime — from what the user types, from data in a file, from calculations on existing data. Building dictionaries dynamically is a core skill. Today you practice every important pattern: building from input, building from loops, building by transforming existing data. These patterns go directly into the expense tracker's data collection layer."