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6.G. While: Fibonacci index
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pole55

Statement
"Fibonacci numbers are the numbers in the integer sequence starting with 1, 1 where every number after the first two is the sum of the two preceding ones: 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

Given a positive integer, determine if it's the nth Fibonacci number for some n. If it is, print such n, otherwise print -1."

Example input: 8
Example output: 6

=============================

Have a problem with the digit 1134903170 (see the code below).
It is the 45th Fibonacci number, but the code did not work.

Also had an easier way:

x = int(input())
lst = [1, 1]

for i in range(2, x+1):
--lst.append(lst[-1] + lst[-2])

if lst.count(x) >= 1:
--print(lst.index(x) + 1)
else:
--print(-1)

But the same problem with 1134903170 here.
The program took too long to execute.

Who knows where things went wrong?

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pole55
Comments
hotnewtop
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darwin1012

prev = 1
next = 1
index = 2
check_fib = int(input())

while check_fib > next:
new_next = prev + next
prev = next
next = new_next
index += 1

if check_fib == next:
print(index)
else:
print(-1)

it says to use a while loop

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ash15khng

There doesn't seem to be anything wrong with typing that number in?

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pole55

@ash15khng
the code gives -1 instead of 45. and the easier version does not calculate it, freezes, because calculations go very long.

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jiuyuan3

try this:
prev, next = 1, 1
index = 2
possible_fib = int(input())
while possible_fib > next:
prev, next = next, prev + next
index += 1
if possible_fib == next:
print(index)
else:
print(-1)