I wrote the following program to prime factorize a number:
import math
def prime_factorize(x,li=[]):
until = int(math.sqrt(x))+1
for i in xrange(2,until):
if not x%i:
li.append(i)
break
else: #This else belongs to for
li.append(x)
print li #First print statement; This is what is returned
return li
prime_factorize(x/i,li)
if __name__=='__main__':
print prime_factorize(300) #Second print statement, WTF. why is this None
Following is the output I get:
[2, 2, 3, 5, 5]
None
Altho', the returned value is printed properly, the after returned value seems to be printing none, all the time. What am I missing?
Also, how can I improve the program (continuing to use the recursion)
Your prime_factorize function doesn't have a return statement in the recursive case -- you want to invoke "return prime_factorize(x/i,li)" on its last line. Try it with a prime number (so the recursive call isn't needed) to see that it works in that case.
Also you probably want to make the signature something like:
def prime_factorize(x,li=None):
if li is None: li = []
otherwise you get wrong results when calling it two or more times:
>>> prime_factorize(10)
[2, 5]
>>> prime_factorize(4)
[2, 5, 2, 2]
>>> prime_factorize(19)
[2, 5, 2, 2, 19]