fibonacci works in python but fails in Java

Question

I have this code for calculating fibonacci number in python. It works and gives the expected result. but when I translated the same to Java, it fails. Any idea of what is going wrong here?

In python:

def fib3(n): 
  a,b=0,1
  while n>0:
      a,b=b,a+b
      n-=1
  return a

fib3(12) --> 144

In Java:

 public static int fib2(int n){
        int a = 0;
        int b =1;
        while(n-- >0){
            a=b;
            b=a+b;

        }
    return a;
}

fib2(12) --> 2048

Answer 1

In this section:

a=b;
b=a+b;

you're assigning b to a+b, but a is already b. So really you're doubling b

Easiest solution is a temp variable:

public static int fib2(int n){
    int a = 0;
    int b =1;
    while(n-- >0){
        int old_a;
        old_a = a;
        a=b;
        b=old_a+b;
    }
    return a;
}

In python, a, b = b, a + b stores an intermediate tuple automatically before assigning the new values to the variables, while in Java you need to be explicit about it

Breaking down Python's instructions, a, b = b, a + b is executing this disassembly:

  5          17 LOAD_FAST                1 (b)
             20 LOAD_FAST                0 (a)
             23 LOAD_FAST                1 (b)
             26 BINARY_ADD
             27 ROT_TWO
             28 STORE_FAST               0 (a)
             31 STORE_FAST               1 (b)

In a simpler sense, staying python, here's the process:

temp_tuple = (b, a + b)
a, b = temp_tuple