第10回 :: ruby_fifth

!Mac OS X-10.15.7 !ruby-2.7.1p83$

お題:Fibonacci数列

fib(n) = fib(n-1)+fib(n-2)
0 1 1 2 3 5 8 13 21 ...

をrecursion(再帰)で求める．

解説

前回の講義で作成した作ったassert_equal.rbをcodes(作業directory)にcopyしておくこと．

fib(0) = 0

Fibonacci数列の初項は0．

def fib(n)
  if n==0
    return 0
  end
end

p fib(0)

前回作成したassert_equalを用いて出力ために

require './assert_equal'

def fib(n)
  if n==0
    return 0
  end
end

assert_equal(0, fib(0))

実行すると

expected :: 0
result   :: 0
succeeded in assert_equal

ちゃんとできてる

fib(1) =1

次はfib(1)=1

require './assert_equal'

def fib(n)
  if n==0
    return 0
  end
  if n==1
    return 1
  end
end

assert_equal(0, fib(0))
assert_equal(1, fib(1))

これで実行すると上手く出力される．

テストの部分が多くなってきたので，配列にする．

require './assert_equal'

def fib(n)
  if n==0
    return 0
  end
  if n==1
    return 1
  end
end

[[0,0],[1,1]].each do |pair|
  puts assert_equal(pair[0], fib(pair[1]))
end

fib(2) = fib(1) + fib(0) = 1

条件分岐を少し変更する

require './assert_equal'

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

[[0,0],[1,1],[2,1]].each do |pair|
  puts assert_equal(pair[1], fib(pair[0]))
end

これが正解．

refactoring

もう少し配列の受け取りを明示的にすると，

index, expected = pair

と修正できて

require './assert_equal'
[[0,0],[1,1],[2,1]].each do |index, expected|
  puts assert_equal(expected, fib(index))
end

の方がいいらしい．

最終的に

 require './assert_equal'

 def fib(n)
   if n==0
     return 0
   end
   if n<=2
     return 1
   end
 end
[[0,0],[1,1],[2,1]].each do |index, expected|
  puts assert_equal(expected, fib(index))
end

あとはフィボナッチ数列の計算を繰りかえす

フィボナッチ数列は第2項からfin(n) = fin(n-1) + fin(n-2)になるから最終形は

require './assert_equal'

def fib(n)
  return 0 if n==0
  return 1 if n==1
  return fib(n-1) + fib(n-2)
end

[[0,0],[1,1],[2,1],[3,2],[4,3],
[5,5],[6,8],[7,13],[8,21]].each do |index, expected|
  puts assert_equal(expected, fib(index))
end

これを書いて実行すると

> ruby fobonacci.rb
expected :: 0
result   :: 0
succeeded in assert_equal
expected :: 1
result   :: 1
succeeded in assert_equal
expected :: 1
result   :: 1
succeeded in assert_equal
expected :: 2
result   :: 2
succeeded in assert_equal
expected :: 3
result   :: 3
succeeded in assert_equal
expected :: 5
result   :: 5
succeeded in assert_equal
expected :: 8
result   :: 8
succeeded in assert_equal
expected :: 13
result   :: 13
succeeded in assert_equal
expected :: 21
result   :: 21
succeeded in assert_equal
expected :: 34
result   :: 34
succeeded in assert_equal

ちゃんとフィボナッチ数列が実行されてる．

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• source ~/grad_members_20f/members/okamoto0910/article/c10.org