Question

I've written some code to calculate the 'n'th prime number. There are issues when calculating the 25th prime and above (n=25 gets 95). Here's my code, please help me out if you can see my problem!

Answer 1

n=int(raw_input("This program will give you the nth prime number. Please enter n:"))
if n == 1:
print "the 1st prime is: 2" #the first prime is 2!
else:
if n<= 0: #if user inputs a negative n
print "n must be >= 1" # can't calculate a -nth prime!
else:
counter = 1
prime = 1 # 2 is the first prime number
while counter<n: #to stop at the desired nth prime number
prime += 2
for divisor in range(2,prime/2): #too check for divisors up to 1/2 number
if prime%divisor == 0: #a remainder means devisor means not a prime
prime += 2 #check next prime
print prime
counter += 1 #next counter to continue the loop
print"The ",n,"th prime is: ", prime

Answer 2

In python btw. Thanks!

Answer 3

95 is correct. next is 101. problem is?

Answer 4

95 isn't a prime number!

Answer 5

I recomend you to use the Sieve of Eratosthenes: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

Answer 6

ah yes sorry, i missread 95 and 97, sleepyness ;)

Answer 7

I really don't know Python, but I think the error may be in this part:

for divisor in range(2,prime/2):
if prime%divisor == 0:
prime += 2 #when you do that you MUST reset divisor variable
print prime

You can put some like that:

for divisor in range(2,prime/2):
if prime%divisor == 0:
prime += 2
divisor = 2
print prime

Answer 8

A modulus of zero means that the divisor divided into prime evenly, so whatever the variable prime contains isn't a prime number. And if one number doesn't divide with modulus of zero, they all have to. I took a much different approach:

primes = [ 2, 3 ]
testNum = 5
numberOfPrimes = 10000

while( len( primes ) < numberOfPrimes ):
for i in xrange( 0, len( primes ) ):
if( testNum % primes[i] ):
if( i == ( len( primes ) - 1 )):
primes.append( testNum )
else:
break
testNum += 2

lastPrime = primes[len( primes ) - 1]
print "The", str( numberOfPrimes ) + "th prime is:", lastPrime

Answer 9

I don't know Python, but here is an algorithm in pseudo code that will work if you appy it no matter what language you are programming in.

maxValue = 50 //The maximum value to be checked
isPrime = true //Is true if we find a prime

//Check all values from 2 to maxValue
OuterLoop
for i = 2 to maxValue
isPrime=true //Assume the current i is prime

//Try dividing by all integers from 2 to i-1
for j = 2 to i - 1
if i modulus j == 0 //This is true if j divides exactly
continue with OuterLoop //so exit loop
//We can only get here if wwe have a prime, so . . .
print i

This is pretty much what rsmith6559 said, just using pseudo code instead.