Fibonacci sequence works in python, but not in c?

Question

I have the following python code:

a, b = 1, 1
for i in range(0, 100):
    print a
    a, b = b, a + b

It generates this: 1 1 2 3 5 8 etc

I wrote the same in c:

#include <stdio.h>
long long unsigned int a = 1, b = 1;
void main(){
    for(int i = 0; i < 100; i++){
        printf("%llu \n", a);
        a = b, b = a + b;
    }
}

It generates this: 1 1 2 4 8 16 32 etc

Why does the c program generate powers of 2 when it is using the exact same operations?

Answer 1

a, b = b, a + b

in python packs the values b and a + b into a tuple, then unpacks it back into a and b.

C does not supports that feature, but rather use the comma to separate between assignments, so a = b, b = a + b get translated as

a = b;
b = a + b;

where b gets doubled every time because the assignment is not simultaneous.

To fix that you'd have to assign each variable separately:

b = a + b;
a = b - a; // a + b - a = b