CODEWITHSUNDEEP
pythonloopswhile-loopfibonacci
Tuning
Tuning

Reputation: 53

Python Fibonacci series - different while loops

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

Upvotes: 1

Views: 7959

Answers (2)

JD22
JD22

Reputation: 22

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.

Upvotes: 0

Eric Duminil
Eric Duminil

Reputation: 54223

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

Upvotes: 3

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