Question

problem set 1: i thought i figured out a test to see if a number is prime but now i'm having problems with the script. If I enter a prime number, I get the correct answer (I included a print statement to indicate when a number is prime.) If I enter any other number, the script either runs indefinitely until I force it to quit or I get a false positive. Here's a link to my current script: http://dpaste.com/524568/
I most recently ran the test with the number 8 and got the statement: "8 is a prime number." Does anyone see what I'm doing wrong here?

Answer 1

Well, what happens when you've given it a number like 8?

It will execute the first line, assigning divisor the value 3. Then it will ask "Is 3 < 8/2?", and since it is it will execute the block for the loop. The loop block first asks, "Is 8%3 != 0?". Since 8%3 is 2 and therefore not 0, it will execute assign divisor the value 3 + 1, or 4. Then it will ask "Is 4 < 8/2?". Since 4 is 8/2 this will fail and the loop will end. Then it will claim 8 is prime.

I think it would be helpful and informative to insert some print statements to see the value of x and divisor at each step along the way.Something like:

print 'In loop: x=', x, ', divisor=', divisor # between lines 3 & 4.
and also
print 'x%divisor was', x%divisor, ' so incrementing divisor' #between lines 4 & 5.

Answer 2

polpak,
i will try this technique. i keep thinking i need to create a print statement for when the isprime test fails and exits the loop. but, i don't know where to put it in the program. i'll give your suggestions a go and hope this will spark some insight.

Answer 3

Sure thing. Let me know if you get stuck.