Re: ibonacci sequence problem

Thanks, 

I think learning to programm is difficult without a sort of teacher. You never get checked if you are doing it the ruby way.
I'm now reading the Well Grounded Rubyist

But that one is without exercises so I can never check if I really understand it.
And the best way for me to learn something is to do it and not only to read about it.

Roelof
 


> Date: Sat, 29 Sep 2012 10:48:58 +0900
> From: [e-mail]
> Subject: Re: ibonacci sequence problem
> To: [e-mail]
> 
> 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".
> 
> -- 
> Posted via http://www.ruby-forum.com/.
>