Python while loop. What is this construct called?

Question

On the Literate Programs site, I was looking at the Python code for the GCD algorithm.

def gcd(a,b):
        """ the euclidean algorithm """
        while a:
                a, b = b%a, a
        return b

What is going on in the body? Expression evaluation? Is it a compressed form of another structure?

[link](http://en.literateprograms.org/Euclidean_algorithm_(Python)) — mbubb, May 30 '14 at 20:38
There's no structure. it's just parallel assignment `x, y = 3, 4` is the same as doing `x = 3` and `y = 4` separately. — Marc B, May 30 '14 at 20:40
Do you mean the multiple assignment or the numeric evaluation as logical value? — Mephy, May 30 '14 at 20:40
Thank you - I thought it was something like the 'swap variables without a temp var' idea. Which uses tuples: a, b, c = b, c, a — mbubb, May 30 '14 at 20:42
Maybe it is Friday afternoon - but I just don't get it. "While a exists give me an 'a' and a 'b' which is modulus b over a ..." — mbubb, May 30 '14 at 20:44
OH - Thank you - it *is* like a, b = b, a . I was reading it as 3 expressions not 4. it is clear now. I get it — mbubb, May 30 '14 at 20:46
@mbubb - You would accept the most helpful answer by clicking the check next to it. Doing so lets people know that this problem is solved. — , May 30 '14 at 21:15

score 2 · Accepted Answer · answered May 30 '14 at 20:44

2

There are two things going on here:

            a, b = b%a, a

First, a tuple is created with the contents (b%a, a). Then the contents of that tuple are unpacked and assigned to the names a and b.

answered May 30 '14 at 20:44

bgporter

1

Thank you - I was reading it as 3 expressions seperated by commas - I was not seeing: x,y = y,x – mbubb May 30 '14 at 20:50

score 1 · Answer 2 · answered May 30 '14 at 20:41

1

Looks like shorthand for:

while a > 0:
    temp = a
    a = b%a
    b = temp
return b

answered May 30 '14 at 20:41

Andrew

Where does the temp come in? ;) – Dair May 30 '14 at 20:42
It stores the old value of `a` so that when `a` is overwritten, we still have its original value for storing into `b`. The Python shorthand in the question doesn't need it but if we split it up like this we need to introduce this separate variable. – Andrew May 30 '14 at 20:43
No, I mean look at your code... `temp = a`, you mutate `a` and then take the mutated `a` and assign it `b`. – Dair May 30 '14 at 20:44

score 0 · Answer 3 · edited May 23 '17 at 12:28

0

a is receiving the result of b%a while b is receiving the value of a

Works the same as:

while a > 0:
    tmp = a
    a = b%a
    b = tmp
return b

edited May 23 '17 at 12:28

Community

answered May 30 '14 at 20:41

julienc

3 Answers3