Capstone Brief: The Finance Core

The mission

A personal finance core — the computation and persistence heart of a ledger app, built from everything this course taught:

• a Txn record: id, cents amount, category
• operations: add, category totals, biggest expense, report line
• validation at every boundary

What the graders check

The capstone battery runs against four functions:

1. total_by_cat — category-filtered sums
2. biggest_idx — the index of the largest expense
3. after_balance — running balance from a start value
4. fmt_report — one exact summary line, built with snprintf

Why these four

They force every layer at once: structs (records), pointers (arrays and out buffers), const discipline (read-only inputs), algorithms (max scan), and formatting (report). Change any piece and the whole thing still must hold together — that is integration, the actual skill of engineering.

Beyond the battery

In a real project you would add: file persistence (module 16 — a save/load pair), a menu loop (module 5-6), and unit tests per function. The sandbox battery verifies the core logic; the architecture around it is yours to build in the project gallery.