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Why are two similar bits of code giving different results? - Printable Version

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Why are two similar bits of code giving different results? - godmode - Dec-07-2016

Hi guys,

sorry for my novice question, but I can't really see why it gives a different result...
it's about a Fibonacci code...

a=1
b=1
c=1
while c < 11:
    a, b, c = b, a+b, c+1 #it gives the proper output

a=1
b=1
c=1
while c < 11:
    a=b
    b=a+b
    c=c+1 #this code gives the raising to the power of 2
before I red about it I thought it will give the same value to the variables, but I can't see that why it isn't...
Thanks in advance!

RE: Why is it differ for giving a value - nilamo - Dec-07-2016

In the second one...
a = b
b = a+b  # because of a's re-binding, this is the same as b=b+b
But in the first, that isn't the case, because you assign both at the same time, and thus b's new value still uses a's original binding. Does that make sense?

RE: Why is it differ for giving a value - godmode - Dec-08-2016

Thanks for your fast answer, you made it more clear!

So is it because they are in the same line? But how could they receive another value, being a same ones? Huh  like a=b and b=a+b? Which comes first?
Does they even change a value, or just after the next cycle?

RE: Why is it differ for giving a value - Larz60+ - Dec-08-2016

set b = 2
since a = b, a = 2

now set b = a + b, so b = 2 + 2 or 4

RE: Why is it differ for giving a value - godmode - Dec-08-2016

Sorry, I should reply more specific...

I understand that second example, when everything is in a new line, I have problem understanding the following:

a, b, c = b, a+b, c+1
So is it because they are in the same line? But how could they receive another value, being a same ones? [Image: huh.png]  like a=b and b=a+b? Which comes first?

Does they even change a value, or just after the next cycle?

RE: Why is it differ for giving a value - nilamo - Dec-08-2016

The right side of the equal sign is evaluated first.  So, to python, everything on the same line turns into roughly
a, b, c = b, a+b, c+1

# is roughly the same as:
old_a = a
old_b = b
old_c = c
a = old_b
b = old_a + old_b
c = old_c + 1
Depending on how it's easier for you to understand, you can think of all three variables being assigned at the same time, so b get's a+b before a has a new value yet.  Or, you can think of it like a bunch of temporary variables are created.  Either way you think of it, the left side of the equal sign has nothing to do with what's happening on the right.

RE: Why is it differ for giving a value - Kebap - Dec-09-2016

(Dec-08-2016, 11:01 AM)godmode Wrote: Which comes first?
Does they even change a value, or just after the next cycle?

No single value comes first. First all three values on the right side are calculated, only then the original values are replaced. So, yes just after the line, all values are changed at the same time. This is an easier way to write it without storing extra variables. Consider this:

a, b = b, a
When you want to split it in multiple lines, you need a third value c. Try it, maybe it's easier.

RE: Why are two similar bits of code giving different results? - Windspar - Dec-09-2016

Antoher way to look at it. It just unpacking the values from a tuple.
a, b, c = 1, 1, 1
pytuple = (a, a+b, c+1)
a, b, c = pytuple

print a, b, c, pytuple


RE: Why is it differ for giving a value - godmode - Dec-14-2016

(Dec-08-2016, 10:11 PM)nilamo Wrote: 
a, b, c = b, a+b, c+1

# is roughly the same as:
old_a = a
old_b = b
old_c = c
a = old_b
b = old_a + old_b
c = old_c + 1
Thanks for all of you, this example made it the clearest for me :) Don't know where to find the like button, but I would give you one ;)


RE: Why are two similar bits of code giving different results? - Kebap - Dec-14-2016

It will come to you naturally when you come more regularly here ;)