Fibonacci sequence in Ruby

Question

def fibonacci(n)
    n <= 1 ? n :  fibonacci( n - 1 ) + fibonacci( n - 2 ) 
 end
 puts fibonacci( 6)

Can somebody explain how this code works. I realise that it works due to recursion but i cannot figure it out gradually. Will be gratefull for step by step explanation.

Answer 1

Does this help you visualise it?:

def fibonacci(n, level = 0)
  puts ('|   ' * level) + 'n: ' + n.to_s
  res = n <= 1 ? n :  fibonacci(n - 1, level + 1) + fibonacci(n - 2, level + 1)
  puts ('|   ' * level) + 'res: ' + res.to_s
  res
end

irb(main):038:0> fibonacci(6)
n: 6
|   n: 5
|   |   n: 4
|   |   |   n: 3
|   |   |   |   n: 2
|   |   |   |   |   n: 1
|   |   |   |   |   res: 1
|   |   |   |   |   n: 0
|   |   |   |   |   res: 0
|   |   |   |   res: 1
|   |   |   |   n: 1
|   |   |   |   res: 1
|   |   |   res: 2
|   |   |   n: 2
|   |   |   |   n: 1
|   |   |   |   res: 1
|   |   |   |   n: 0
|   |   |   |   res: 0
|   |   |   res: 1
|   |   res: 3
|   |   n: 3
|   |   |   n: 2
|   |   |   |   n: 1
|   |   |   |   res: 1
|   |   |   |   n: 0
|   |   |   |   res: 0
|   |   |   res: 1
|   |   |   n: 1
|   |   |   res: 1
|   |   res: 2
|   res: 5
|   n: 4
|   |   n: 3
|   |   |   n: 2
|   |   |   |   n: 1
|   |   |   |   res: 1
|   |   |   |   n: 0
|   |   |   |   res: 0
|   |   |   res: 1
|   |   |   n: 1
|   |   |   res: 1
|   |   res: 2
|   |   n: 2
|   |   |   n: 1
|   |   |   res: 1
|   |   |   n: 0
|   |   |   res: 0
|   |   res: 1
|   res: 3
res: 8
=> 8