From: James Edward Gray II <james@...>
Date: 2004-09-30T07:08:31+09:00
Subject: Math Tricks

Quick question.  When I run:

#!/usr/bin/env ruby

class RPNCalc
	def initialize(stack = [ ])
		@stack = stack
	end
	
	def push(number)
		if number.kind_of? Numeric
			@stack.push(number)
			return @stack[-1]
		else
			return nil
		end
	end
	
	def add; return binary { |l, r| l + r } end
	def sub; return binary { |l, r| l - r } end
	def mul; return binary { |l, r| l * r } end
	def div; return binary { |l, r| l / r } end
	
	private
	
	def unary(&op)
		if @stack.size < 1
			raise "Insufficient elements on stack for operation."
		end
		
		return push( op.call( @stack.pop ) )
	end
	
	def binary(&op)
		if @stack.size < 2
			raise "Insufficient elements on stack for operation."
		end
		
		return push( op.call( *@stack.slice!(-2, 2) ) )
	end
end

calc = RPNCalc.new
puts calc.push(3)
puts calc.push(5)
puts calc.add
puts calc.push(2)
puts calc.mul
puts calc.push(2)
puts calc.push(3)
puts calc.add
puts calc.div

__END__

I see 3 as the last output when I expect to see 3.2.  Why is that?

James Edward Gray II

P.S.  I wanted to collapse those math defs to one line for easy reading 
and found the semicolon trick used above.  Is that the only way to do 
it, or is there a method similar to if ... then ... end?