Forward-fill sparse inventory snapshots to a daily series

Question

Inventory is only logged when it changes, but the dashboard needs a value for every day.

Write `daily_stock(df)` that returns a Series of stock on hand for every calendar day between the first and last snapshot, carrying the last known value forward.

Accepted Answer

Step 1: `pd.date_range(min, max, freq='D')` builds the complete calendar the dashboard expects, with no gaps. Step 2: `reindex(full)` inserts the missing days as `NaN` rather than dropping them — that is the step people forget. Step 3: `ffill()` carries the last known reading forward, which is the correct semantics for a stock level (unlike interpolation, which invents movement). Step 4: `astype(int)` restores the integer dtype that `NaN` had forced to float.

Forward-fill sparse inventory snapshots to a daily series

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