Ruby 代码说明 : Numbers to String using Recursion

Question

有人分享了一些 Ruby 代码，讲述了他们如何完成将数字转换为文字的挑战。这个解决方案是迄今为止最容易阅读的，但我并不完全理解它的递归方面。具体来说，在下面一行...

elsif int.to_s.length == 1 && int/num > 0
  return str + "#{name}" 

如果 return 语句将代码从循环中中断，为什么此代码不执行并在它为真时中断循环？例如，当 int = 4 且 num = 1 时，这将被证明为真并触发 return 语句，但代码会继续执行直到 num = 4。

对代码这方面的任何澄清，以及对理解此解决方案的任何一般性建议，都会有所帮助。仍在尝试理解递归。谢谢

def in_words(int)
  numbers_to_name = {
      1000000 => "million",
      1000 => "thousand",
      100 => "hundred",
      90 => "ninety",
      80 => "eighty",
      70 => "seventy",
      60 => "sixty",
      50 => "fifty",
      40 => "forty",
      30 => "thirty",
      20 => "twenty",
      19=>"nineteen",
      18=>"eighteen",
      17=>"seventeen", 
      16=>"sixteen",
      15=>"fifteen",
      14=>"fourteen",
      13=>"thirteen",              
      12=>"twelve",
      11 => "eleven",
      10 => "ten",
      9 => "nine",
      8 => "eight",
      7 => "seven",
      6 => "six",
      5 => "five",
      4 => "four",
      3 => "three",
      2 => "two",
      1 => "one"
    }

  str = ""
  numbers_to_name.each do |num, name|
    if int == 0
      return str
    elsif int.to_s.length == 1 && int/num > 0
      return str + "#{name}"      
    elsif int < 100 && int/num > 0
      return str + "#{name}" if int%num == 0
      return str + "#{name} " + in_words(int%num)
    elsif int/num > 0
      return str + in_words(int/num) + " #{name} " + in_words(int%num)
    end
  end
end

puts in_words(4) == "four"
puts in_words(27) == "twenty seven"
puts in_words(102) == "one hundred two"
puts in_words(38_079) == "thirty eight thousand seventy nine"
puts in_words(82102713) == "eighty two million one hundred two thousand seven hundred thirteen"

Answer 1

调试代码

这是一个在每个分支中都有调试行的修改版本:

def in_words(int, indent="")
  puts "#{indent}in_words(#{int})"
  numbers_to_name = {
      1000000 => "million",
      1000 => "thousand",
      100 => "hundred",
      90 => "ninety",
      80 => "eighty",
      70 => "seventy",
      60 => "sixty",
      50 => "fifty",
      40 => "forty",
      30 => "thirty",
      20 => "twenty",
      19=>"nineteen",
      18=>"eighteen",
      17=>"seventeen", 
      16=>"sixteen",
      15=>"fifteen",
      14=>"fourteen",
      13=>"thirteen",              
      12=>"twelve",
      11 => "eleven",
      10 => "ten",
      9 => "nine",
      8 => "eight",
      7 => "seven",
      6 => "six",
      5 => "five",
      4 => "four",
      3 => "three",
      2 => "two",
      1 => "one"
    }

  str = ""
  numbers_to_name.each do |num, name|
    puts "#{indent}  testing #{num}"
    if int == 0
      puts "#{indent}   Empty string"
      return str
    elsif int.to_s.length == 1 && int/num > 0
      puts "#{indent}  Single digit found (#{name})!"
      return str + "#{name}"      
    elsif int < 100 && int/num > 0
      puts "#{indent}  Double digits found (#{name})!"
      return str + "#{name}" if int%num == 0
      puts "#{indent}  Recursive call with #{int%num} :"
      return str + "#{name} " + in_words(int%num, indent+"  ")
    elsif int/num > 0
      puts "#{indent}  Recursive call with #{int/num} and #{int%num} :"
      indent += "  "
      return str + in_words(int/num, indent) + " #{name} " + in_words(int%num,indent)
    end
  end
end

对于 in_words(4)，它输出:

in_words(4)
  testing 1000000
  testing 1000
  testing 100
  testing 90
  testing 80
  testing 70
  testing 60
  testing 50
  testing 40
  testing 30
  testing 20
  testing 19
  testing 18
  testing 17
  testing 16
  testing 15
  testing 14
  testing 13
  testing 12
  testing 11
  testing 10
  testing 9
  testing 8
  testing 7
  testing 6
  testing 5
  testing 4
  Single digit found (four)!
four

对于 27:

in_words(27)
  testing 1000000
  testing 1000
  testing 100
  testing 90
  testing 80
  testing 70
  testing 60
  testing 50
  testing 40
  testing 30
  testing 20
  Double digits found (twenty)!
  Recursive call with 7 :
  in_words(7)
    testing 1000000
    testing 1000
    testing 100
    testing 90
    testing 80
    testing 70
    testing 60
    testing 50
    testing 40
    testing 30
    testing 20
    testing 19
    testing 18
    testing 17
    testing 16
    testing 15
    testing 14
    testing 13
    testing 12
    testing 11
    testing 10
    testing 9
    testing 8
    testing 7
    Single digit found (seven)!
twenty seven

回答

For example, when int = 4 and num = 1, this would prove to be true and trigger the return statement, yet the code continues until num = 4.

将 4 作为输入，num = 1 永远不会发生。 numbers_to_name 是递减顺序的，所以返回语句将发生在 num=4 之后，该方法不会执行任何其他代码。