Question

Hi guys, I'm trying to do the prime number generator thingy. I managed to write something that prints the prime numbers as well as about two hundred other random numbers. Anyone mind taking a look at it for me, see if you can tell where I messed up??

This is what I have so far
d=2
c=2
while c<7920:
if c%2==0:
c=c+1
else:
while (d<(c/2)):
if c%2==0:
c=c+1
elif c%d==0:
c=c+1
else:
d=d+1
d=2
print c
c=c+1

Answer 1

ausdan: It would be much better if you attached your code as well as post it. Copy / pastingint into the editor results in one very long line that would have to be formatted and indented.

Answer 2

ausdan: Notably missing in your solution is a variable to hold whether the number is a prime or not. You need to print it only if it is actually a prime. Another consideration is that if a number is not divisible by 2 than it is automatically not divisible by all even numbers. If you fact this into your logic your code runs faster (is more efficient).

At http://openstudy.com/users/chribonn#/updates/4f859edae4b0505bf085aced I posted my answers to this problem set.

Hope this helps.

Answer 3

Tricky, on first glance it appears to be working correctly.

I think Chribonn is correct, you need a variable to hold whether the number is prime. So you could set up a variable isPrime = True, but if you find c is divisible by d, then set that to False. Then you can set a condition so that you only print c if isPrime is true.

Have a look at the attached and see if that works. (I'm working through the same assignment so not an expert)

Answer 4

start with candidat 2 which you know is prime so
nbrOfFoundPrimes = 1
while nbrOfFoundPrimes is less than 1000
increment candidate
test candidate with divisors between 2 and candidate - 1
if it is evenly divisible by any divisor it is not prime
if it is not evenly divisible by any divisor it is prime
if it is prime, increment nbrOfFoundPrimes

Answer 5

How did you know that c should be less than 7920?

Answer 6

i kinda cheated with the c<7920, i just looked at the link to the list of primes on the sheet and saw which one was the 1000th one :P

Answer 7

treerock I was looking at your code, seems to work alright but what does the break command mean??

Answer 8

I think I figured it out, does it just mean that you skip the else bit?

Answer 9

http://pastebin.com/3F3SiqaR

Answer 10

Hey Ausdan, yep kinda. Break means you skip the rest of the processing within the **current loop**. So in this example, the while loop comes to an end when it finds the first value that breaks the primality test.