Duncan's blog

January 27, 2009

Project Euler: problem 25

Filed under: Project Euler,Python — duncan @ 11:58 pm
Tags: , , ,

Problem 25:

The Fibonacci sequence is defined by the recurrence relation:

Fn = Fn-1 + Fn-2, where F1 = 1 and F2 = 1.

Hence the first 12 terms will be:

F1 = 1
F2 = 1
F3 = 2
F4 = 3
F5 = 5
F6 = 8
F7 = 13
F8 = 21
F9 = 34
F10 = 55
F11 = 89
F12 = 144

The 12th term, F12, is the first term to contain three digits.

What is the first term in the Fibonacci sequence to contain 1000 digits?

In Python, reusing most of the logic from way back in Problem 2:

fibonacci = 1
old1 = 0
old2 = 1
limit = 1000

i = 1

while len(str(fibonacci)) < limit:
	fibonacci = old1 + old2
	old1 = old2
	old2 = fibonacci
	i = i + 1

print(i)
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1 Comment »

  1. […] previously blogged about this Project Euler puzzle nearly 6 years ago, using Python.  This is my approach using PHP as a simple practical exercise for myself, and I’d […]

    Pingback by Project Euler: problem 25 (PHP) | Duncan's blog — October 6, 2014 @ 8:00 am | Reply


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