q4b.py

#!/usr/bin/python

import sys
from array import *

# returns true if x is prime, false is not
def is_prime(x):
	if x < 2:
		return True
	if x >= 2:
        	for y in range(2,x):
            		if not (x % y):
                		return False
    	else:
		return False
	return True

# returns the next prime number after x
def next_prime(x):
	for i in range(x+1, x+50):
		if is_prime(i): 
			return i
	return -1	

# helper for the build_chain function
def helper(n, p, arr):
	print arr
	if n == 1:
		# printing the chain here with ./q4b 3 gives [1,3,7]
		# chain is None after returning..?
		return arr
	else:
		p1 = 2*p + 1
		if is_prime(p1) == False:
			return -1 
		else:
			arr.append(p1)
			return helper(n-1, p1, arr)

# builds x terms of the cunningham chain
def build_chain(n):
	p = 1
	arr = []
	arr.append(p)
	chain = helper(n, p, arr)
	# printing out the chain here gives None
	if chain == -1:
		arr = []
		next_p = next_prime(p)
		arr.append(next_p)
		chain = helper(n, next_p, arr)
	print "chain: " + str(chain)
		
build_chain(int(sys.argv[1]))