Python - Implementing Functions

Question

I am creating a function to check x is greater than y, and if not it switches the two values and returns them.

def xGreater(x, y):
    if(y > x):
        x, y = y, x
    return x, y

My query is what is the best way to go about using this function within another function, my current code is the following:

def gcd(x, y):
    x , y = xGreater(x, y)
    r = x % y
    while(r != 0):
        x, y = y, r
        r = x % y
    return y

Can I not simply call xGreater(x, y) to alter the values of x and y, without the x, y = in front? Or does this only work when there is a single variable being returned. Thanks!

By the way, there's no need for parentheses around expressions (in your if and while statements). — Tim Pietzcker
– Tim Pietzcker, Commented Mar 16, 2013 at 21:57
@thkang Integers being immutable has nothing to do with it. func(a) can never change what the local name a refers to in the calling scope, regardless of what kind of value that name refers to. — user395760
– user395760, Commented Mar 16, 2013 at 22:13

NPE · Accepted Answer · 2013-03-16 21:54:23Z

3

Can I not simply call xGreater(x, y) to alter the values of x and y, without the x, y = in front?

I am afraid you can't, since x and y are immutable and are passed into xGreater() by value.

It can be done in some special cases (for example, if x and y were two lists), but not generally.

To be totally honest, I'd get rid of xGreater() and just do the swap in gcd():

def gcd(x, y):
    if y > x:
        x, y = y, x
    r = x % y
    ...

I personally find the code more readable this way.

answered Mar 16, 2013 at 21:54

NPE

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2 Comments

h1h1 Over a year ago

thanks, this is how I originally had it, but my teacher is always saying we should try to split out our functions more.

NPE Over a year ago

@h1h1: What your teacher is saying is a good general guideline. It's just knowing when to stop that can be a little tricky at times, but that comes with experience.

Tim Pietzcker · Accepted Answer · 2013-03-16 21:56:19Z

2

No, integers are immutable. But hey, you can cut down on one instance of tuple packing/unpacking:

def xGreater(x, y):
    return (y, x) if y > x else (x, y)

answered Mar 16, 2013 at 21:56

Tim Pietzcker

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Comments

Dan · Accepted Answer · 2013-03-17 00:09:15Z

0

NPE has a better solution but I'm bored and wanted to write some bit hacks

def gcd(x,y):
    minVal = y ^((x ^ y) & -(x < y))

    if ( minVal == y ):
        y = x ^ y  # Swapping values without a intermediate variable
        y = y ^ y  # This is highly inefficient to use.
        x = x ^ y  # But still kinda cool.

   return x % y

answered Mar 17, 2013 at 0:09

Dan

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