Python总理检查器

Question

I recently started learning python (by which I mean, 35 minutes ago at the time of posting...) and I've wrote a few things eg square root generator, factorial generator, Fibonacci number generator, prime checker etc. After I wrote the prime checker, I thought I'd try modifying it so that instead of checking every number in the specified range, it would accept an input and check that one specifically.

DISCLAIMER: If I were better at Python then I would only check numbers up to sqrt(p) and I would add an option to check if it is even and not 2 then it automatically returns that it's not prime but let me walk before I run! :)

CODE:

p = input("Enter a potential prime.")

for n in range (2,p):
    if p % n == 0:
        print(p, "equals", n, "x", p//n, "so it isn't a prime number")
        break
else:
    print(p, "is a prime number.")

It works for p = 2 but that's it...

NB - Obviously the code is indented accordingly, it's just not formatted properly here.

Answer 1

I see some errors: You need to convert your user input to an int, plus you need to move the else: clause to beneath your for-loop instead of beneath your if-statement. The following code works for what you want:

p = int(input("Enter a potential prime."))
for n in range (2,p):
    if p % n == 0:
        print(p, "equals", n, "x", p//n, "so it isn't a prime number")
        break
else:
    print(p, "is a prime number.")

Yes, this is correct - the else: is NOT part of the if-statement, it is part of your for-loop. This syntax means that if your for-loop runs to a break, then it'll break as normally. If there is no break, then the else: clause will be executed. Thus, it'll do the basic trial division, and if the number passes the test, it'll print "is a prime number" only once. The code you posted will print "is a prime number" for every iteration of your loop.

---

Edit: Try the following code for your followup question.

def primeChecker():
    # Function that keeps prompting for input, and checks if it's prime. Enter "quit"
    # to exit:
    user_input = input("Enter a potential prime (or type 'quit' to exit): ")
    if user_input == 'quit':
        return
    else:
        p = int(user_input)

    # Trial division algorithm:
    for n in range (2,p):
        if p % n == 0:
            print(p, "equals", n, "x", p//n, "so it isn't a prime number")
            break
    else:
        print(p, "is a prime number.")

    # Recursive function call:
    primeChecker()

# Start by calling the main function:   
primeChecker()