Level 8 · Longest run of days on target

Two accumulators at once

A streak question needs a little more than a running total. You track two numbers as you walk through the list in order:

• current, the length of the run you are in right now;
• best, the longest run you have seen so far.

best = 0
current = 0
for revenue in daily_revenue:
    if revenue >= target:
        current += 1          # the run continues
    else:
        current = 0           # the run is broken
    best = max(best, current)

current += 1 is shorthand for current = current + 1. max(best, current) returns the larger of the two, so best only ever goes up. The loop has to visit the days in date order, which is why the list is already sorted from December 1 to 14. A streak on shuffled data means nothing.

Walking through December

Laid out day by day, the target test gives this pattern (✓ at or above $4,000):

Dec  1  2  3  4  5  6  7  8  9 10 11 12 13 14
     .  ✓  ✓  .  ✓  ✓  ✓  ✓  .  ✓  ✓  ✓  ✓  ✓

current climbs to 2, resets on Dec 4, climbs to 4, resets on Dec 9, and then climbs to 5 by the last day. best ends at 5. Notice that 11 of the 14 days were on target, but the question is about the longest unbroken run, so counting the ticks isn’t enough.

Where streak code usually breaks

The most tempting version updates best only when a run ends, inside the else. It looks tidy, but a run that is still going when the list ends never gets recorded. Here the longest run is exactly that one, Dec 10 to 14, so the tidy version answers 4. Updating best on every pass, after the if/else, handles the end of the list for free.

The other trap is the boundary. “At or above $4,000” means >=. On Dec 12 revenue was exactly 4,000, so with > the final run splits in two and the answer drops to 4 again.

Common mistakes

• Updating best only when a run breaks and losing the final run.
• Using > when the rule says “at or above”.
• Counting all on-target days (11) instead of the longest consecutive run.
• Forgetting to reset current to 0 on a miss, which turns the count into a plain total.