pythonOff-by-oneEasy

Comparing each day with the next crashes at the end

Time: 0:00

BugFind the bug and fix the code

What it should do

count_rises(prices) counts the days a price went up from the day before. Example: count_rises([3, 5, 4, 6]) → 2.

What happens instead

count_rises([3, 5, 4, 6]) crashes: IndexError: list index out of range.

def count_rises(prices):
    rises = 0
    for i in range(len(prices)):
        if prices[i + 1] > prices[i]:
            rises += 1
    return rises

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

Hints

🏆 0 hints = Flawless Solve

Hint 2

Unlocks after Hint 1

Hint 3

Unlocks after Hint 2

This bug belongs to the off-by-one errors pattern

The loop that runs one time too many, the slice that drops the last item, the index that starts in the wrong place.

Learn the off-by-one errors pattern Step through your own code

Spoiler: why this bug happens(try it first; the hints get warmer one at a time)

What went wrong: on the last item, prices[i + 1] reads past the end of the list.

The fix: for i in range(len(prices) - 1):

Remember: when you read [i + 1], loop one fewer time.

The pattern behind it: off-by-one errors.

More off-by-one errors to practise

Seen this error in your own code?

IndexError: list index out of range

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