1-
def fib1(n):
a = 0
b = 1
while a < n:
print b
a = b
b = a+b
2-
def fib2(n):
a, b = 0,1
while a < n:
print b
a,b = b, b+a
On execution:
fib1(10) I got the wrong answer: 0 1 2 4 8
fib2(10) I got the right answer: 0 1 1 2 3 5 8
In fib 1
a = b
overwrites the value of a,
which means a is no longer the right value for the statement
b = a+b
However, in your second example both those things happen at the same time on the line a,b = a, b+a which means a is the right value still.
a+b is computed and stored somewhere tempory in memory. After that computation the value is moved in to be. The same thing happens in this case a,b = b, b+a first, b+a is computed and stored somewhere temporary. Then b is computed and stored somewhere temporary. Finally b = the computed value of a+b) and a = the computed value of bhere's a quick answer:
the basic difference is the way the values of a and b are reassigned.
In fib1(), you have
a = b
b = a + b
while in fib2(), you have
a, b = b, b + a
Now, these two looks equal statements but they are not. Here's why:
In fib2(), you assign the values of the tuple (b, b + a) to the tuple (a, b). Hence, reassignment of values are simultaneous.
However, with fib1(), you first assign value of b to a using a = b and then assign the value a + b to b. Since you have already changed value of a, you are in effect doing
b = a + b = b + b = 2b
In other words, you are doing a, b = b, 2b and that is why you are getting multiples of 2 rather than the fibonacci sequence.
fib1 contains a classic bug. It is in the same realm as that of swapping values of two variables. Think about how you would have done that in C or C++.
int a = 3;
int b = 5;
int temp;
temp = a; /* 3 */
a = b; /* 5 */
b = temp; /* 3, hence swapped */
There is a way to do without temp, although there are intermediate calculations involved. Now in Python, if you are not going to exploit the tuple unpacking feature, you have to involve a temp variable.
a = 3
b = 5
temp = a
a = b
b = temp
OR
a = 3
b = 5
a_ = (a+b)/2 - (a-b)/2 # 5.0
b_ = (a+b)/2 + (a-b)/2 # 3.0
Better use the tuple unpacking as in fib2.
