Computing the 1000th prime

[Dave Angel]

Ray Holt wrote:
> I have an assigment to find the 1000th. prime using python. What's wrong
> with the following code:
> PrimeCount = 0
> PrimeCandidate = 1
> while PrimeCount < 2000:
>     IsPrime = True
>     PrimeCandidate = PrimeCandidate + 2
>     for x in range(2, PrimeCandidate):
>         if PrimeCandidate % x == 0:
> ##            print PrimeCandidate, 'equals', x, '*', PrimeCandidate/x
>               print PrimeCandidate  
>               IsPrime = False
>               break
>         if IsPrime:
>             PrimeCount = PrimeCount + 1
>             PrimeCandidate = PrimeCandidate + 2
>         print PrimeCandidate
> Thanks
>
>   
There are a bunch of things wrong here.  Did you write this code, or was 
it copied from somewhere else?  Because it looks like there are several 
typos, that you could catch by inspection.

First, at what point in the loop do you decide that you have a prime?  
Why not add a print there, printing the prime, instead of printing a 
value that's already been incremented beyond it.  And put labels on your 
prints, or you'll never be able to decipher the output.  Chnage the 
limit for PrimeCount to something small while you're debugging, because 
you can figure out small primes and composites by hand.

Second, you have a loop which divides by x.  But you change the 
PrimeCandidate within the loop, so it's not dividing the same value each 
time through.  Check your indentation.

Third, your limit value is too high.  You aren't looking for 2000 
primes, but 1000 of them.  Further, your count is off by 1, as this loop 
doesn't identify 2 as a prime.

I'd recommend making this whole thing a function, and have it actually 
build and return a list of primes.  Then the caller can check both the 
size of the list and do a double check of the primality of each member.  
And naturally you'll be able to more readily check it yourself, either 
by eye or by comparing it to one of the standard list of primes you can 
find on the internet.


The function should take a count, and loop until the len(primelist) 
matches the count.  Then just return the primelist.

DaveA