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Running total, restarting per key · Python scenario · D8LooP

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Running total, restarting per key

The finance dashboard shows a running balance per account. The ledger table is written by the payments service in whatever order the events were processed, so the rows are neither grouped by account nor ordered by day. Write running_total(rows, key_field, value_field, order_field). Return the rows with a running_total field added, ordered by key then by the ordering field.

Function to write

running_total(rows: list[dict], key_field: str, value_field: str, order_field: str) -> list[dict]

Copies of the rows with a running_total field, ordered by key then by the ordering field.

How to approach it

Sort first, then keep the totals in a dict keyed by group rather than in one variable.

Sample cases

+ 2 held back until you submit

three accounts interleaved

The rows arrive ungrouped and out of order, one amount is negative and one is unusable.

Input

Argument 1

account	day	amount
a2	2026-03-02	40
a1	2026-03-01	10
a1	2026-03-03	5.5
a2	2026-03-01	100
a1	2026-03-02	-2.5
a3	2026-03-01	not a number

Argument 2

'account'

Argument 3

'amount'

Argument 4

'day'

Returns

account	day	amount	running_total
a1	2026-03-01	10	10
a1	2026-03-02	-2.5	7.5
a1	2026-03-03	5.5	13
a2	2026-03-01	100	100
a2	2026-03-02	40	140
a3	2026-03-01	not a number	0

no rows

An empty ledger returns an empty list rather than one row of zeroes.

Input

Argument 1

[] (empty list)

Argument 2

'account'

Argument 3

'amount'

Argument 4

'day'

Returns

[] (empty list)

one account only

A single account still has to be sorted before the total means anything.

Input

Argument 1

account	day	amount
solo	2026-03-02	3
solo	2026-03-01	7

Argument 2

'account'

Argument 3

'amount'

Argument 4

'day'

Returns

account	day	amount	running_total
solo	2026-03-01	7	7
solo	2026-03-02	3	10

Constraints

1The running total accumulates within one key and restarts at zero for the next. One account's balance never contributes to another's.
2Accumulate in order of order_field, ascending, compared as text.
3Return the rows ordered by key_field then order_field, both ascending.
4A value that is not a number — missing, null, a string or a boolean — contributes nothing, but the row still appears with the total so far.
5Negative values are real. A refund lowers the running total rather than being skipped.
6Round each running total to two decimal places for display, but accumulate the unrounded value so the rounding never compounds.
7Return copies with the extra field; do not modify the rows you were given.

Worked example

Account a1 has three rows spread through the file: 10.0 on the first, -2.5 on the second and 5.5 on the third. Sorted, its running total reads 10.0, then 7.5, then 13.0. Left in arrival order it reads 10.0, then 15.5, then 13.0 — which rises and then falls, and is exactly the kind of chart that gets escalated. The trap that survives testing longest is a single total variable declared outside the loop. It gives the right answer on a file containing one account, and every developer's first test file contains one account. On real data a2 starts from wherever a1 finished.

What this tests

Carrying per-group state through one pass, and knowing that sorting is a precondition for accumulation rather than a presentational nicety. The state has to live in a dict keyed by group, not in a variable.

PYTHONrunning_total(rows: list[dict], key_field: str, value_field: str, order_field: str) -> list[dict]

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