Ruby程序设计语言

让我们写个函数计算阶乘。数学上阶乘的定义为：

n 的阶乘:

n! = 1                (当 n==0)
   = n * (n-1)!       (否则)

在ruby里, 这个可以写为:

def fact(n)
  if n == 0
    1
  else
    n * fact(n-1)
  end
end

你会注意到在结尾重复的end。Ruby被称作"Algol-like"就是因为这个。(实际上ruby的语法更像Eiffel语言。)你可能注意到函数缺少一个返回声明。它是不需要的，因为ruby总是返回其最后计算的东西。在这里使用一个return声明是允许的但不是必须的。

让我们来测试这个函数，在添加一行代码来运行程序：

# Program to find the factorial of a number
# Save this as fact.rb

def fact(n)
  if n == 0
    1
  else
    n * fact(n-1)
  end
end

puts fact(ARGV[0].to_i)

这里, ARGV是一个包含命令行参数的数组，to_i 转换一个字符串到一个整数。

% ruby fact.rb 1
1
% ruby fact.rb 5
120

使用40做参数可以运行吗？它将会使你的计算器溢出...

% ruby fact.rb 40
815915283247897734345611269596115894272000000000

它可以运行。实际上,ruby可以处理你计算机内存允许的任意一个整数。因此 400!也可以被计算出来：

% ruby fact.rb 400
64034522846623895262347970319503005850702583026002959458684
44594280239716918683143627847864746326467629435057503585681
08482981628835174352289619886468029979373416541508381624264
61942352307046244325015114448670890662773914918117331955996
44070954967134529047702032243491121079759328079510154537266
72516278778900093497637657103263503315339653498683868313393
52024373788157786791506311858702618270169819740062983025308
59129834616227230455833952075961150530223608681043329725519
48526744322324386699484224042325998055516106359423769613992
31917134063858996537970147827206606320217379472010321356624
61380907794230459736069956759583609615871512991382228657857
95493616176544804532220078258184008484364155912294542753848
03558374518022675900061399560145595206127211192918105032491
00800000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000

看一眼并不能判断上面的结果是正确的，但是它应该是正确的，哈哈。

输入/求值 环路

当你不带参数运行ruby的时候，它从标准输入中读取命令，在输入的结尾计算它们：

% ruby
puts "hello world"
puts "good-bye world"
^D
hello world
good-bye world

上面的^D代表 Ctrl-D, 在Unix环境下结束输入的简便方法。在DOS/Windows下, 尝试用 F6 或者 ^Z 代替.

Ruby也附带了一个名为eval.rb的程序，允许从键盘交互式输入ruby代码并显示结果。我们将在这个指南的后面大量地使用它。

如果你有一个ANSI兼容的终端(如果你正在使用UNIX，基本都是ANSI兼容终端；在某些旧版本的DOS，你需要安装ANSI.SYS 或 ANSI.COM； 如果是Windows XP，很不幸，这几乎就不可能了)，你应该使用这个增强的 eval.rb 来增加可视化的辅助，错误警告，和颜色加亮。另外，你可以在ruby发布里的例子子目录找到适合任何终端的非ANSI兼容的版本。下面是一个简短的eval.rb会话:

% ruby eval.rb
ruby> puts "Hello, world."
Hello, world.
   nil
ruby> exit

hello world 是 puts 生成的。下一行, 在这里是nil, 报告哪里求值结束；ruby不能够区分声明和表达式，因此求值一个代码片段和执行它是一回事。这里，nil表示puts没有返回一个有意义的值。我们可以输入exit退出解释器，虽然你也可以使用 ^D 。

贯穿这个指南，"ruby>"用来指示我们的游泳的小eval.rb程序的提示。

eval.rb内容：

#!/usr/local/bin/ruby

#######################################################
#
# Ruby interactive input/eval loop
# Written by matz           (matz@netlab.co.jp)
# Modified by Mark Slagell  (slagell@ruby-lang.org)
#    with suggestions for improvement from Dave Thomas
#                           (Dave@Thomases.com)
#
#######################################################
#
# NOTE - this file has been renamed with a .txt extension to
# allow you to view or download it without the rubyist.net
# web server trying to run it as a CGI script.  You will
# probably want to rename it back to eval.rb.
#
#######################################################

module EvalWrapper

  # Constants for ANSI screen interaction.  Adjust to your liking.
  Norm = "\033[0m"
  PCol = Norm           # Prompt color
  Code = "\033[1;32m"   # yellow 
  Eval = "\033[0;36m"   # cyan 
  Prompt = PCol+"ruby> "+Norm
  PrMore = PCol+"    | "+Norm
  Ispace = "   "        # Adjust length of this for indentation.
  Wipe = "\033[A\033[K" # Move cursor up and erase line
  
  # Return a pair of indentation deltas. The first applies before
  # the current line is printed, the second after.
  def EvalWrapper.indentation( code )
    case code
    when /^\s*(class|module|def|if|case|while|for|begin)\b[^_]/
      [0,1]      # increase indentation because of keyword
    when /^\s*end\b[^_]/
      [-1,0]     # decrease because of end
    when /\{\s*(\|.*\|)?\s*$/
      [0,1]      # increase because of '{'
    when /^\s*\}/
      [-1,0]     # decrease because of '}'
    when /^\s*(rescue|ensure|elsif|else)\b[^_]/
      [-1,1]     # decrease for this line, then come back
    else
      [0,0]      # we see no reason to change anything
    end
  end
  
  # On exit, restore normal screen colors.
  END {  print Norm,"\n" }
  
  
  ##############################################################
  # Execution starts here.
  ##############################################################
  
  indent=0
  while TRUE   # Top of main loop.
    
    # Print prompt, move cursor to tentative indentation level, and get
    # a line of input from the user.
    if( indent == 0 )
      expr = ''; print Prompt  # (expecting a fresh expression)
    else
      print PrMore             # (appending to previous lines)
    end
    print Ispace * indent,Code
    line = gets
    print Norm
    
    if not line
      # end of input (^D) - if there is no expression, exit, else
      # reset cursor to the beginning of this line.
      if expr == '' then break else print "\r" end
    else
      
      # Append the input to whatever we had.
      expr << line
      
      # Determine changes in indentation, reposition this line if
      # necessary, and adjust indentation for the next prompt.
      begin
	ind1,ind2 = indentation( line )
	if( ind1 != 0 )
	  indent += ind1
	  print Wipe,PrMore,(Ispace*indent),Code,line,Norm
	end
	indent += ind2
      rescue      # On error, restart the main loop.
	print Eval,"ERR: Nesting violation\n",Norm
	indent = 0
	redo
      end
      
      # Okay, do we have something worth evaulating?
      if (indent == 0) && (expr.chop =~ /[^; \t\n\r\f]+/)
	begin
	  result = eval(expr, TOPLEVEL_BINDING).inspect
	  if $!  # no exception, but $! non-nil, means a warning
	    print Eval,$!,Norm,"\n"
	    $!=nil
	  end
	  print Eval,"   ",result,Norm,"\n"
	rescue ScriptError,StandardError
	  $! = 'exception raised' if not $!
	  print Eval,"ERR: ",$!,Norm,"\n"
	end
	break if not line
      end
    end
  end  # Bottom of main loop
  print "\n"

end