A simple Reverse Polish Notation calculator written in Ruby. Equations can be entered interactively or as arguments.

rpn-calculator.rb

def update_stack(stack, arg)
    if /-?\d+(\.\d+)?/.match arg
        stack.push arg.to_f
    else
        i1 = stack.pop
        i2 = stack.pop

        begin
            result = i1.send(arg, i2)
            stack.push result
        rescue => exception
            if @interactive
                puts "Bad argument! Try again."                
                stack.push i2 if !i2.nil?
                stack.push i1 if !i1.nil?
            else
                puts "Bad arguments! Check your syntax."
                exit
            end
        end
    end
end

def interactive
    stack = []
    puts "Interactive mode: enter numbers or operators seperated by newlines, or x to exit."
    
    loop do
        cur = gets.chomp
        exit if cur == "x"
        update_stack(stack, cur)
        puts stack.inspect
    end
end

def all_at_once arr
    stack = []

    arr.each do |cur|
        update_stack(stack, cur)
    end

    if stack.size == 1
        puts "Answer = #{stack.pop}"
    else
        puts "Bad number of arguments, try again!"
    end
end

if ARGV.empty?
    @interactive = true
    interactive
else 
    @interactive = false
    all_at_once ARGV
end