Ruby program to print first n Fibonacci Seqeunce-CodePudding

Newbie here! Trying to implement a program to print the first 20 Fibonacci numbers in Ruby. I've managed to create a program which generates the nth number, but I want to produce all from 0 through to 20.

Is there a simple way to do this or do I need to rewrite the whole program?

CURRENT CODE

def fib(n)
   if n < 1 
     return 0 
       elsif n == 1
     return 1 
    else fib(n-2)   fib(n-1)
  end 
end

puts fib(20)

CURRENT OUTPUT EXAMPLE

DESIRED OUTCOME

CodePudding user response：

At the moment you only print the last value returned by your method (fib(20)) but not the result of all intermediate steps.

An easy way would be to cache all intermediate results in a hash data structure. This would also improve performance for big n because you do not need to recalculate many values over and over again.

Then you can just print out all results from 0 to n:

def cached_fib(n)
  @cache ||= Hash.new do |cache, n| 
    @cache[n] = n < 2 ? n : cache[n-1]   cache[n-2]
  end

  @cache[n]
end

def fib(n)
  0.upto(n) { |i| puts cached_fib(i) }
end

fib(20)
#=> 0
    1
    1
    2
    3
    5
    8
    13
    21
    34
    55
    89
    144
    233
    377
    610
    987
    1597
    2584
    4181
    6765

CodePudding user response：

Printing each value is easier with a button-up approach where you start at 0 and 1 and calculate each following value based on its predecessors, e.g.:

i, j = 0, 1

puts i
puts j

21.times do
  k = i   j
  puts k
  i, j = j, k
end

You could turn the above into an Enumerator:

fib = Enumerator.new do |y|
  i, j = 0, 1

  y << i
  y << j

  loop do
    k = i   j
    y << k
    i, j = j, k
  end
end

Which will generate the sequence:

fib.take(21)
#=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144,
#    233, 377, 610, 987, 1597, 2584, 4181, 6765]