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Platform: https://hack.levels.fyi/debug
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class NumbersService:
def is_present(self, numbers, value):
low, high = 0, len(numbers)
while low <= high:
mid = (low + high) // 2
if numbers[mid] == value:
return True
if numbers[mid] < value:
low = mid + 1
else:
high = mid - 1
return False
TLDR: The is_present method in the NumbersService class has an off-by-one error in its binary search implementation.
Summary: The original code initialized high to len(numbers), which is one index beyond the last valid index of the list. This could lead to an index out of range error when trying to access numbers[mid] if mid ever became equal to len(numbers).
By changing high to len(numbers) - 1, we ensure that high always points to a valid index in the list. This correction allows the binary search to work correctly, checking all elements in the list without risking an index out of range error.
class NumbersService:
def is_present(self, numbers, value):
low, high = 0, len(numbers) - 1 # Fix IndexError
while low <= high:
mid = (low + high) // 2
if numbers[mid] == value:
return True
if numbers[mid] < value:
low = mid + 1
else:
high = mid - 1
return False
See Similar Issue:
from collections import defaultdict
class VideoService:
def __init__(self):
self.views = defaultdict(int)
def add_views(self, video_id, views):
self.views[video_id] += views
def get_views(self, video_id):
return self.views[video_id]
def get_stats(self):
views = self.views.values()
return sum(views) / len(views)
TLDR: The get_stats method could potentially cause a runtime error due to division by zero. The method is inefficient as it recalculates the sum of views every time it's called.
Summary: We added a total attribute to keep track of the total number of views across all videos. This is updated in the add_views method, eliminating the need to recalculate the sum each time get_stats is called.
In the get_stats method, we now check if there are any videos before performing the division. If there are no videos (i.e., video_count is 0), we return 0 to avoid a division by zero error.
from collections import defaultdict
class VideoService:
def __init__(self):
self.views = defaultdict(int)
self.total = 0
def add_views(self, video_id, views):
self.views[video_id] += views
self.total += views # Fix performance
def get_views(self, video_id):
return self.views[video_id]
def get_stats(self):
videos = len(self.views.values())
return self.total / videos if videos else 0 # Fix DivisionByZero
History: A real-world example of the dangers of division by zero occurred in 1997 with the USS Yorktown, a guided missile cruiser of the United States Navy. The ship was paralyzed for about 2 hours and 45 minutes due to a division by zero error in its onboard computers. A crew member had entered a zero value into a database field, which was then used as a divisor in a calculation. This caused the ship's propulsion system to fail, leaving the warship dead in the water. This incident highlights the critical importance of handling potential division by zero errors in software, especially in systems where such failures can have severe consequences.
When Smart Ships Divide By Zer0 — Stranding the USS Yorktown
See Similar Issue: Integer overflow