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Sep 21st, 2009

Problem with Python Loop

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Hello,

This is driving me crazy so here it goes.

I'm trying to create a program that finds the 1000th prime number. This is the code I started with to check if a number is prime:

Python Syntax (Toggle Plain Text)
numberofprimesfound = 0
numbertotest = 303
divisor = 2
 
 
if numbertotest%divisor == 0:
  print numbertotest, 'Is not a prime (broke at stage 1)'
 
else:
   while divisor < numbertotest:
    divisor = divisor +1
    if numbertotest%divisor == 0:
     print numbertotest, 'Is not a prime (broke at stage 2)'
     numbertotest = numbertotest +2
     break
    elif numbertotest%divisor <> 0:
           print numbertotest, 'Is a prime'
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           break
numberofprimesfound = 0
numbertotest = 303
divisor = 2


if numbertotest%divisor == 0:
  print numbertotest, 'Is not a prime (broke at stage 1)'
  
else:
   while divisor < numbertotest:
    divisor = divisor +1
    if numbertotest%divisor == 0:
     print numbertotest, 'Is not a prime (broke at stage 2)'
     numbertotest = numbertotest +2
     break
    elif numbertotest%divisor <> 0:
           print numbertotest, 'Is a prime'
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           break

This works correctly to check if any number is a prime above 3.

However when I add a while loop to find the 1000th prime it all goes wrong.

Python Syntax (Toggle Plain Text)
numberofprimesfound = 0
numbertotest = 5
divisor = 2
 
while numberofprimesfound < 1000:
 
 if numbertotest%divisor == 0:
  numbertotest = numbertotest +2
  divisor = 2
 
 else:
 
   while divisor < numbertotest:
    divisor = divisor +1
 
    if numbertotest%divisor == 0:
     numbertotest = numbertotest +2
     divisor = 2
     break
 
    elif numbertotest%divisor <> 0:
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           divisor = 2
           break
 
print 'The', numberofprimesfound,'th prime is', numbertotest
numberofprimesfound = 0
numbertotest = 5
divisor = 2

while numberofprimesfound < 1000:

 if numbertotest%divisor == 0:
  numbertotest = numbertotest +2
  divisor = 2
 
 else:
    
   while divisor < numbertotest:
    divisor = divisor +1
    
    if numbertotest%divisor == 0:
     numbertotest = numbertotest +2
     divisor = 2
     break
    
    elif numbertotest%divisor <> 0:
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           divisor = 2
           break

print 'The', numberofprimesfound,'th prime is', numbertotest

The loop reports the 1000th prime to be 3003 which obviously isnt right. But I cant figure out why when the prime check works fine it all breaks when I simply add a while loop.

Any ideas?

Thanks

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racshot65

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since Dec 2007

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Sep 21st, 2009

Re: Problem with Python Loop

At first, I would adopt consistent indentations! Very important in Python. Most people use 4 spaces.

sneekula

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Nearly a Posting Maven

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since Oct 2006

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Sep 21st, 2009

Re: Problem with Python Loop

Done

Python Syntax (Toggle Plain Text)
numberofprimesfound = 0
numbertotest = 5
divisor = 2
 
while numberofprimesfound < 1000:
 
    if numbertotest%divisor == 0:
        numbertotest = numbertotest +2
        divisor = 2
 
    else:
 
        while divisor < numbertotest:
            divisor = divisor +1
 
            if numbertotest%divisor == 0:
                numbertotest = numbertotest +2
                divisor = 2
                break
 
            elif numbertotest%divisor <> 0:
                numbertotest = numbertotest +2
                numberofprimesfound = numberofprimesfound +1
                divisor = 2
                break
 
print 'The', numberofprimesfound,'th prime is', numbertotest
numberofprimesfound = 0
numbertotest = 5
divisor = 2

while numberofprimesfound < 1000:

    if numbertotest%divisor == 0:
        numbertotest = numbertotest +2
        divisor = 2

    else:
    
        while divisor < numbertotest:
            divisor = divisor +1
    
            if numbertotest%divisor == 0:
                numbertotest = numbertotest +2
                divisor = 2
                break

            elif numbertotest%divisor <> 0:
                numbertotest = numbertotest +2
                numberofprimesfound = numberofprimesfound +1
                divisor = 2
                break

print 'The', numberofprimesfound,'th prime is', numbertotest

racshot65

Reputation Points: 10

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since Dec 2007

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Sep 21st, 2009

Re: Problem with Python Loop

Note that you add 2 to numbertotest before you break, so the prime would have been 3001 (don't know if that really is a prime).

Python Syntax (Toggle Plain Text)
## assume numberofprimesfound = 999
while numberofprimesfound < 1000:
   .
   .
   .
   elif numbertotest%divisor <> 0:
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           divisor = 2
           break
## assume numberofprimesfound = 999
while numberofprimesfound < 1000:
   .
   .
   .
   elif numbertotest%divisor <> 0:
           numbertotest = numbertotest +2
           numberofprimesfound = numberofprimesfound +1
           divisor = 2
           break

woooee

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Nearly a Posting Maven

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2,305 posts
since Dec 2006

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Sep 21st, 2009

Re: Problem with Python Loop

Your method of finding primes is flawed. Try 25, 35 and 49. None of those numbers are prime and it says that they are. Fix your indentation and your algorithm and then try to loop it again. I suggest using a function.

jlm699

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