From: "Jan E." <lists@...>
Date: 2012-09-29T10:48:58+09:00
Subject: Re: ibonacci sequence problem

Hi,

Roelof Wobben wrote in post #1077938:
> By considering the terms in the Fibonacci sequence whose values do not
> exceed four million, find the sum of the even-valued terms.

The problem is that you (unintentionally) start the sum with 1 instead 
of 0. When you don't set a start value for inject, the aggregate value 
is set to the first element. In this case it's 1, so the result will be 
1 too big.

Also your code is rather "naive" in the sense that you're treating the 
numbers as if they were physical objects and actually need to be 
collected. This is very inefficient and completely unnessary. You only 
need to track the last two numbers.

You should also get rid of some habits you seem to have adopted from 
other programming languages (like Java or so). Things like "Array.new 
[1,2]" and "number % 2 == 0" are useless in Ruby. The literal "[1,2]" 
already *is* an array. And checking if an integer is even can be done by 
simply calling "even?".

A low-level solution might look something like this:

previous, current =
  0, 1
sum = 0
while current <= 4_000_000
  sum += current if current.even?
  previous, current =
    current, previous + current
end
puts sum

A more high-level approach could consist of defining an Enumerator for 
the fibonacci sequence and then apply "take_while", "select" and 
"reduce" subsequently:

fibonacci = Enumerator.new do |yielder|
  previous, current =
    0, 1
  yielder << previous << current
  loop do
    previous, current =
      current, previous + current
    yielder << current
  end
end

sum = fibonacci.take_while{|e| e <= 4_000_000}.select(&:even?).reduce :+
puts sum

This actually works similar to your code, so it's not efficient. But 
it's very readable and kind of "the Ruby way".

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