Code that looks right

Fast, confident, and sometimes wrong

AI tools write code in seconds, and it usually looks clean and sure of itself. Most of the time it's right. Sometimes it's wrong in a way that only shows up on one kind of input.

In this course you're the detective: read code you didn't write, find the input that breaks it, fix it, and judge whether it's fast enough. Every program here is a real one: what you're told about it was checked by running it.

The average

n = int(input())
nums = list(map(int, input().split()))
print(sum(nums) // n)

This should print the average of the numbers (it can be a decimal, like 2.5). Is it right?

• Yes, it's correct
• No: it drops the decimals, so 1 2 gives 1 instead of 1.5
• No: it crashes when the numbers are negative

The biggest number

n = int(input())
nums = list(map(int, input().split()))
best = 0
for x in nums:
    if x > best:
        best = x
print(best)

This should print the biggest number. When does it go wrong?

• Never: it's correct
• When every number is negative
• When the list has repeats

Counting evens

n = int(input())
nums = list(map(int, input().split()))
count = 0
for x in nums:
    if x % 2 == 0:
        count += 1
print(count)

This should count the even numbers (negatives and zero included). Is there a bug?

• It's correct
• Wrong for negatives: -3 % 2 is negative in Python
• Wrong for zero