为什么我的素数函数不起作用？

Question

I am new to programming and I face an issue while trying to write a program in finding out prime number. Here is my code:

def is_prime(x):
    if x < 2:
        return False
    elif x == 2:
        return True
    else:
        for n in range (2,x-1):
            if x % n == 0:
                return False
            else:
                return True

I received an error stating "Your function fails on is_prime(3). It returns None when it should return True."

Can someone please explain the flaw in this code?

Thank you!

Answer 1

range() has an exclusive upper bound , so it's trying to get the range between 2 and 2 (3 - 1), which is no elements. Since you can't iterate over nothing, the for loop never runs, so None is returned (this is the default return type of a function if none is specified).

The solution to your immediate problem would be to use range(2, x) rather than range(2, x - 1) . You'll find that you'll have problems at x > 3 though because as @khelwood said, you're returning True or False immediately after checking the first value. Instead, only return True after checking all values in the range.

Answer 2

def is_prime(x):
    if x < 2:
        return False
    elif x == 2:
        return True
    else:
        for n in range (2,x): # range function will iterate till x-1
            if x % n == 0:
                return False
        # return true only at the end after making sure it is not divisible by any number in the middle
        return True