Discrepancy between function printing and print function output in Python [duplicate]

Question

I have a (perhaps clumsy) function to find the largest prime factor of a number n (below).

def largest_prime_factor(n,factor=2):
    if n % factor == 0:
        if n == factor:
            return factor
        else:
            largest_prime_factor(n/factor,factor)
    else:
        largest_prime_factor(n,factor+1)

#Printing function output
print largest_prime_factor(600)

But when I try to print the function's returned output it simply says None. However, when I make the function print its output (below) it prints the correct answer (5). What am I doing wrong in the snippet above?

def largest_prime_factor(n,factor=2):
    if n % factor == 0:
        if n == factor:
            print factor #Printing, not returning
        else:
            largest_prime_factor(n/factor,factor)
    else:
        largest_prime_factor(n,factor+1)

#Calling the function
largest_prime_factor(600)

Hint: since you're using recursion - the print statement will only see the outer most return... — Jon Clements
– Jon Clements, Commented Oct 18, 2013 at 7:37

Vincent · Accepted Answer · 2013-10-18 07:38:39Z

2

You missed some return calls :

def largest_prime_factor(n,factor=2):
    if n % factor == 0:
        if n == factor:
            return factor
        else:
            return largest_prime_factor(n/factor,factor)
    else:
        return largest_prime_factor(n,factor+1)

answered Oct 18, 2013 at 7:38

Vincent

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