I'm trying to solve questions from Project Euler in Ruby one-liners, and I'm curious if there's a more elegant solution for question two:
Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
Here is my one line solution in Ruby:
(1..32).inject([0,1]) {|arr, i| (arr << arr[-1] + arr[-2] if arr[-1] + arr[-2] <= 4000000) || arr}.inject(0) {|total, i| total += i.even? ? i : 0}
My main concern here is that I'm using the range (1..32) only because I happen to know that that's all that's necessary until numbers in the Fibonacci sequence begin to exceed 4,000,000. I would prefer that this be built into the one-line somehow, but I haven't been able to figure it out.
Semi-colons are not allowed!
Answer
My favorite solution to this is to use a Hash, the values of which can be determined by an anonymous function:
fibonacci = Hash.new{ |h,k| h[k] = k < 2 ? k : h[k-1] + h[k-2] }
fibonacci[6] # => 8
fibonacci[50] # => 12586269025
It's a "genuine" one-liner and very Ruby-ish.