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I am relatively new to Python, so pardon my ignorance.

These two implementation of while loop for generating a Fib series is resulting in very different outputs.

The first one is returning the power series of 2, though I feel it is should be doing exactly what the latter is; which is returning the expected series.

The second while loop is obviously doing something right. I am guessing it has to do with the way the variables as being assigned while swapping values.

What is driving this difference?

Appreciate your inputs and help,

First while loop:

def fib(n):
x=0 
y=1
while y < n:
    print(y)
    x = y
    y = x + y

The second while loop:

x,y=0,1
while y < 100:
    print(y)
    x,y = y,x+y
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3
  • 2
    Run through it line by line (or add more print statements) - you'll soon see what's happening. Commented Dec 20, 2017 at 14:48
  • x is modified before the new value of y is calculated. Commented Dec 20, 2017 at 14:52
  • Thanks, good suggestion Commented Dec 20, 2017 at 14:56

2 Answers 2

Reset to default
3

Parallel assignments

x,y = y,x+y

This is a parallel assignment. The x on the right side is still the old x, it hasn't been set to y yet. This is the desired behaviour, for example in order to swap two variables without needing a 3rd one:

>>> a = 1
>>> b = 2
>>> a, b = b, a
>>> a
2
>>> b
1

As explained by @Alfe, x, y = y, x + y is actually one single tuple assignment:

(x, y) = (y, x + y)

The tuple on the right is completely defined before the assignment happens.

Successive assignments

x = y
y = x + y

The x on the right side of the second line has been set to y on the previous line, so the second line is actually y = y + y.

If you try to swap two variables this way you'll get the same value twice:

>>> a = 1
>>> b = 2
>>> a = b
>>> b = a
>>> a
2
>>> b
2

You need a third variable:

>>> a = 1
>>> b = 2
>>> temp = b
>>> b = a
>>> a = temp
>>> a
2
>>> b
1
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3 Comments

Thanks for the quick response Eric. One follow up query - how does writing the code in the same line (parallel assignment) not allow for immediate assignment. Is this just how the coding standard was built?
@user9020348: It's the desired behaviour, e.g. to switch two variables. If you want immediate assignment, you can write x = y; y= x instead of x, y = y, x.
Actually, writing something like a, b = b, a does not two separate assignments. The rvalue (b, a) builds a tuple of two values, and that tuple is built completely before any assignment takes place. Then the tuple is assigned to another tuple, consisting of two variables, so it gets unpacked and each variable is given one element from the recently built tuple. Then the tuple is forgotten again.
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Edited Answer:

No prob. Following the sequence of your first loop, so x=0, y=1. The first line in your loop makes x = y = 1.

So now y=1.

Then, your next line creates y=x+y. Which means y=1+1=2. Now x=1 and y=2.

For the next iteration:

x = y = 2

so x = 2

then:

y= 2 + 2 = 4

As was explained, your second loop is a parallel assignment. So following the logic starting with x=0 and y=1:

x,y=y,x+y

causes:

x=1 and y= 0+1 = 1 simultaneously

So now x=1 and y=1. Then for the next iteration:

x=1 and y= 1+1=2 simultaneously

So now x=1 and y=2. My professor for my computer science class taught me that following code line by line on paper helps understand the process the computer follows. I also found it was good to help build an ability to read code. Hope this helped.

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1 Comment

Yes, tried and figured. But would still appreciate your inputs.

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