Variable becomes undefined in recursive javascript function?

Question

I've got a simple script that I've written to find greatest common divisors (GCDs) and least common multiples (LCMs) using a recursive method- Euclid's algorithm.

Unfortunately if it needs to do this in more than one step the relevant variable to be returned from the function becomes undefined. I've tried following it in a debugger with breakpoints, which seems to follow the recursion adequately and go back to the original function appropriately, but it then mysteriously vanishes at the end of the function even though it's meant to be returned?

Not sure why this is happening or what I can do to fix it. My code is below:

function GCD(a, b) {
    if (a % b == 0) {
        return b;
    }
    else {
        GCD(b, (a % b));
    }
}

function LCM (a, b) {
    return (a*b)/GCD(a, b);
}

function makeDM (a, b) {
    return (GCD(a, b) + " " + LCM(a, b));
}

So if you use a & b such as 60, 20 it will give the correct answers of 20 and 60. However if you use numbers such as 20, 60 or 126, 35 it fails miserably.

I pasted you code to try in ideone.com, and it gave me the warning on line 2: Use '===' to compare with 0. — İsmet Alkan
– İsmet Alkan, Commented Mar 31, 2013 at 7:25
One of the paths in GCD doesn't have a return in it. That's fishy. — Mat
– Mat, Commented Mar 31, 2013 at 7:28
Thanks guys! Fixed now :). I knew I was missing something fundamental about this. Time to do more reading I guess :D — Tomcat
– Tomcat, Commented Mar 31, 2013 at 8:11

Guffa · Accepted Answer · 2013-03-31 07:32:12Z

7

You are not returning the value from the recursive part of the function, so the value returned from the function will be undefined. Add a return:

function GCD(a, b) {
    if (a % b == 0) {
        return b;
    }
    else {
        return GCD(b, (a % b));
    }
}

You can also write this with a single return:

function GCD(a, b) {
    return a % b == 0 ? b : GCD(b, (a % b));
}

answered Mar 31, 2013 at 7:32

Guffa

703k111 gold badges760 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Tomcat Over a year ago

Perfect! Thanks so much, I knew I had to have been missing something utterly fundamental hehe :)

Collectives™ on Stack Overflow

Variable becomes undefined in recursive javascript function?

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related