质数判断中的if条件语句始终返回false问题求助

Hey there! Sorry to hear your prime-checking code is stuck always returning false with that if condition—let’s dig into this and fix it up.

First, it’d be way easier to spot the exact issue if you can share the full code you’ve written, especially the if statement and the surrounding logic. That said, there are a handful of super common mistakes that cause this exact problem, so let’s run through them:

• Starting your divisor check at 1: Every integer is divisible by 1, so if your loop starts at 1 and checks n % i == 0, it’ll immediately trigger the false return, no matter what number you test.
• Accidental assignment instead of comparison: If you wrote if n = i instead of if n == i (easy typo!), that’ll cause unexpected behavior (and in most languages, a syntax error or unintended variable overwrite).
• Forgetting edge cases: Numbers less than 2 (0, 1, negative numbers) aren’t primes, but if your code doesn’t handle these first, it might fall into the main check incorrectly.
• Incorrect loop bounds: For example, looping all the way to n instead of sqrt(n) (though this usually just makes code slow, not always false) or stopping the loop too early before verifying all possible divisors.

Here’s an example of a flawed prime checker that would always return false, thanks to starting the loop at 1:

def is_prime(n):
    for i in range(1, n):
        if n % i == 0:
            return False
    return True

Fixing the range to start at 2 (range(2, int(n**0.5) + 1)) would fix this issue right up.

Share your code snippet, and we’ll get this sorted in no time!

内容的提问来源于stack exchange，提问作者Brendan