Checking vs. Savings: Where's the Money Moving?
Northline's product team is deciding whether to promote savings accounts harder, and wants aggregate cash-flow by account type first. They never look at the whole ledger at once: every promotion has a window, and the question is always about the money that has moved since that window opened.
Customers
| id | name | account_type | city |
|---|---|---|---|
| 1 | Farah Idris | checking | Chicago |
| 2 | Grant Boyle | savings | Miami |
| 3 | Hana Suzuki | checking | Chicago |
| 4 | Ibrahim Njoku | savings | Miami |
| 5 | Jade Wu | checking | Seattle |
Transactions
| id | customer_id | amount | type | txn_date |
|---|---|---|---|---|
| 1 | 1 | 500 | deposit | 2023-07-01 |
| 2 | 1 | -120 | withdrawal | 2023-07-03 |
| 3 | 2 | 1000 | deposit | 2023-07-02 |
| 4 | 2 | -300 | withdrawal | 2023-07-05 |
| 5 | 3 | 250 | deposit | 2023-07-04 |
| 6 | 3 | -600 | withdrawal | 2023-07-06 |
| 7 | 4 | 800 | deposit | 2023-07-07 |
| 8 | 4 | -450 | withdrawal | 2023-07-08 |
| 9 | 5 | 2000 | deposit | 2023-07-09 |
| 10 | 5 | -1500 | withdrawal | 2023-07-10 |
| 11 | 1 | -200 | transfer | 2023-07-11 |
| 12 | 3 | 400 | deposit | 2023-07-12 |
Input
Both tables above are already built for you — those rows are not read from input. What does arrive is a single line holding the first day of the window, written the same way txn_date writes its dates.
Task: Counting only transactions dated on or after that day, print a dict mapping each account_type → the total net amount moved by the customers holding that kind of account. An account type with nothing in the window does not appear at all, and a window with no transactions in it gives an empty dict rather than an error.
Example: if only savings accounts had moved in some window and they had netted two hundred and fifty dollars, you'd print {'savings': 250}.
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