beginner question fibonacci

[Joon]

# Fibonacci series: .... # the sum of two elements defines the next
.... a, b = 0, 1 while b < 10: .... print b
.... a, b = b, a+b
....
1
1
2
3
5
8
a, b = 0, 1
while b < 10:

print b
a = b
b = a+b
1
2
4
8

Why a, b = b, a+b isn't a = b; b = a+b ?

[Michael Hoffman]

Joon wrote:

>>> a, b = 0, 1
>>> while b < 10:

print b
a = b
b = a+b
1
2
4
8

Why a, b = b, a+b isn't a = b; b = a+b ?

Because you changed a before you added it to b.

Let's call your existing a and b "a0" and "b0", and the next a and b
"a1" and "b1". When you do a, b = b, a+b you are assigning:

a1 = b0
b1 = a0 + b0

But when you use separate statements, you are assigning:

a1 = b0
b1 = a1 + b0 = b0 + b0 = 2*b0
--
Michael Hoffman

[Robert Kern]

Joon wrote:

>>> # Fibonacci series: ... # the sum of two elements defines the next
... a, b = 0, 1 >>> while b < 10: ... print b
... a, b = b, a+b
...
1
1
2
3
5
8
>>> a, b = 0, 1
>>> while b < 10:

print b
a = b
b = a+b

1
2
4
8

Why a, b = b, a+b isn't a = b; b = a+b ?

It's actually equivalent to:

temp = (b, a+b)
a = temp[0]
b = temp[1]

The temporary tuple object is created first, with the old values of a
and b. Then a and b are reassigned. The value of a doesn't change until
*after* a+b is calculated.

--
Robert Kern
rk***@ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

[ralobao]

The case is that Python in attribution commands solves first the right
side, so he atributes the vars.

So the a+b expression is executed first.

Joon escreveu:

# Fibonacci series: ... # the sum of two elements defines the next
... a, b = 0, 1 >>> while b < 10: ... print b
... a, b = b, a+b
...
1
1
2
3
5
8
>>> a, b = 0, 1
>>> while b < 10:

print b
a = b
b = a+b
1
2
4
8

Why a, b = b, a+b isn't a = b; b = a+b ?

[Joon]

Yes, i see.
Thank you very much for the fast help!