Question

Hoping I can get an answer to this. For ps1 I wrote this code:

count = 0
n = 1
while count < 1000:
n = n + 1
divisors = 0
for testdiv in range(1,n):
testrem = n%testdiv
if testrem == 0:
divisors = divisors + 1
if divisors < 3:
count = count + 1
print ("the thousandth prime number is", n)

I know this is not great style (doesn't even limit test to odd numbers) but I was hoping to get it to work. When I run it I get 7699, which is the 977th prime number, not the 1000th. Not sure why this is. If anyone has an inkling please reply.

Answer 1

Run this program and see if it helps by reading the print lines in the shell...

It's only up to 30 though.

http://dpaste.com/563124/

Answer 2

http://primes.utm.edu/lists/small/1000.txt

a list of primes.

Answer 3

This is what I came up with. I'm not sure right now what you did.
http://dpaste.com/560941/

Answer 4

well i just figured it out..
you just need to replace the range(1,n) by range(1,n+1)
you intend to include n in the calculation..so you need to set the upper range to n+1 otherwise it will not include n

Another way to rectify is to replace 3 in " if divisors < 3:" by 2
because your code does not include the number itself while counting devisors.

i have uploaded the 2 rectified versions of your code. and both of the return the correct value 7919

http://pastebin.com/nWjPbyx1

http://pastebin.com/rWmZQfBX

Answer 5

sunu0000 .... Thanks! To quote Ren: "Stimpy, you're a genius!" I didn't realize you had to go one up from the upper limit on the range statement. I see now that my code above was returning "primes" when there was one other divisor besides 1 and the number itself (i.e.squares like 9 and 25). No wonder I came up just a bit short of the 1000th. Thanks again!!!

Answer 6

Glad i could help :D

Answer 7

Sunu, I understand the code, but something I am just not getting. Why does the divisors have to be less than 2. Wouldn't it always be equal to 2 if it's a prime considering it is dividing by 1 and then dividing by the number it is testing to get a remainder of 0? The code works, but I just don't understand, I feel like it should be if divisors == 2. Is there something I am missing? Thanks

Answer 8

well u r absolutely right...but your observation of the code was slightly wrong i think..
take another look at the code..
u can see the when we set the condition to divisors < 3 ..it includes the number itself..
but when the condition is divisors < 2...divisors do not include the number we are testing..so the only divisor is 1

Answer 9

Yes, I realized it like 5 minutes after I wrote that. I could have sworn that I posted a follow up stating I understood why you wrote it that way. Thanks.

Answer 10

LoL....i thought only i have to to post multiple times to make it shoe up here..