Infinite loop in python - why?

Question

I'm using the code

fibList=[1]

def fib(n1, n2):
    n3 = n1+n2
    n1=n2
    n2=n3

num1=1
num2=2

while(num2<4000000):
    fib(num1,num2)
        if (num2%2==0):
            fibList.append(num2)
total = sum(fibList)

print total

in an online compiler, repl.it. It was going and going and not giving a solution, so I typed the line print n3 right below the n3= line in the definition of the fib function. It gave 3, over and over again, and it crashed before I could stop the program. So obviously there's some kind of infinite loop somewhere (at least, obviously in my mind; it could not be an infinite loop, I suppose, but I'm pretty sure it is). The question is where. I don't really understand why there'd be an infinite loop.

This is not, by the way, a homework question, but a problem I'm doing for fun. The fib function is supposed to calculate Fibonacci numbers, and the second part isolate the even ones less than four million, and then at the end calculate the sum.

What I want to know is where the infinite loop is coming in and what I can do to fix it. Thanks!

calling fib does not change values of n1 and n2 outside of function body. — Łukasz Rogalski
– Łukasz Rogalski, Commented Aug 17, 2016 at 21:13
Possible duplicate of How do I pass a variable by reference? — Łukasz Rogalski
– Łukasz Rogalski, Commented Aug 17, 2016 at 21:13
The fib function just assigns values to the local n1, n2, n3 variables. This has no effect outside of the function. — Daniel Darabos
– Daniel Darabos, Commented Aug 17, 2016 at 21:14

Daniel · Accepted Answer · 2016-08-17 21:14:02Z

6

n1, n2 and n3 are local variables and have nothing in common with num1 and num2 in the outer scope, despite their initial value. You have to return the value and assign these results to num1 and num2 again.

def fib(n1, n2):
    n3 = n1+n2
    n1=n2
    n2=n3
    return n1, n2

num1=1
num2=2
fibList=[1]
while num2<4000000:
    num1, num2 = fib(num1,num2)
    if num2%2==0:
        fibList.append(num2)
total = sum(fibList)

print total

answered Aug 17, 2016 at 21:14

Daniel

42.9k4 gold badges57 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

auden Over a year ago

Thank you; this helped a lot. Sorry I didn't accept sooner!

Collectives™ on Stack Overflow

Infinite loop in python - why?

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related