Trouble with return in python

Question

####
# GCD calculator using euclidean algorithm
####

def euclid_gcd(x,y) :
    new_gcd = y
    remainder = x % y
    print x,y, new_gcd, remainder
    if(remainder != 0) :
        euclid_gcd(y,remainder)   
    else :
        print x,y, new_gcd, remainder
        return new_gcd

print 'x | y | new_gcd | remainder'
print euclid_gcd(252,198)

However, when I run this code it returns this...

x | y | new_gcd | remainder
252 198 198 54
198 54 54 36
54 36 36 18
36 18 18 0
36 18 18 0
None

It should return 18 in this case yet it returns none where did I go wrong everything seems to be following the logical steps??

Marcus · Accepted Answer · 2012-10-30 03:43:14Z

4

You should do return euclid_gcd(y,remainder).

You forgot to return the recursion result here:

if(remainder != 0) :
    euclid_gcd(y,remainder)

answered Oct 30, 2012 at 3:43

Marcus

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5 Comments

cat Over a year ago

Oh yea? where should I place that please?

cat Over a year ago

Oh wow that fixed it! could I bother you for one more second and ask why that fixed it?

Makoto Over a year ago

@cat: Most compiled languages wouldn't let you get away (as in won't compile) with returning something in one part, but not another. The same thing applies here - you have to return the value you get back from the method call, or it won't give you back an answer.

Marcus Over a year ago

@cat, I believe Makoto has described this better than I can :)

cat Over a year ago

Brilliant I can't believe I didn't notice that thank you guys!

John La Rooy · Accepted Answer · 2012-10-30 03:53:25Z

1

Another way to solve your problem

def euclid_gcd(x, y):
    new_gcd = y
    remainder = x % y
    print x, y, new_gcd, remainder
    if remainder != 0:
        new_gcd = euclid_gcd(y, remainder)   
    print x, y, new_gcd, remainder
    return new_gcd

I suggested this because it seems strange otherwise to have the variable new_gcd if you only ever initialise it to y

The reason that you were getting None is because Python implicitly returns None if the function doesn't explicitly return anything

answered Oct 30, 2012 at 3:53

John La Rooy

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