Is_prime


#1

I don't understand why there is the following error message :

Oops, try again. Your function fails on is_prime(2). It returns None when it should return True.

this is my code

def is_prime(x):
    if x < 2:
        return False
    else:
        for n in range(2, x - 1):
            if x / n != x and x / n != 1:
                return False
            else:
                return True

Can someone help me?


#2

The test for divisibility is modulo

x % n == 0

when x is evenly divisible by n. This is the indicator of a non-prime.

There should not be an else inside the loop.


#3

Ok for the else outside the loop

But for the rest of the code, I don't understand why it can't work. May be I am too tired for understanding it. I'm going to the bed and I will try to understand tommorow morning...


#4

When is x divided by n equal to x? When n is 1.

When is x divided by n equal to 1? When n is equal to x.

The above helps define what a Prime number is, but is not a test for primeness since it is not exhaustive (test for factors). That is why we need to cycle through numbers smaller than x starting with 2.

for n in range(2, x):
    if x % n == 0: return False

The only values that will survive this loop will be the primes.


#5

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