逆波兰表达式

最简单的解释器应该就是逆波兰表达式了，连括号都不需要处理，用出栈，入栈就可以写完解释器。这个例子对于那些总是被栈（Stack）搞糊涂的同学做思维练习很合适。用c#写了个实现。

 void Main()
 {
 var exps = new string[]{
 "3 4 +",
 "3 4 5 * -",
 "3 4 - 5 *",
 "5 1 2 + 4 * + 3 -"
 };
 foreach(var exp in exps){
 exp.Dump("calculate expression:");
 calculate(exp).Dump("result:");
 }
 }
 
 
// Define other methods and classes here
 double calculate(string exp){
 var chars = exp.Split(' ');
 var i=0;
 var statck = new Stack();
 while(i var v = chars[i++];
 if(v=="+"||v=="-"||v==""||v=="/"){
 if(statck.Count<2){
 throw new Exception("syntax error!");
 }
 var right =Convert.ToDouble(statck.Pop());
 var left = Convert.ToDouble(statck.Pop());
 Console.Write(" {0} ",v);
 switch(v){
 case "+":{
 var r = left+right;
 statck.Push(r.ToString());
 break;
 }
 case "-":{
 var r = left-right;
 statck.Push(r.ToString());
 break;
 }
 case "":{
 var r = left*right;
 statck.Push(r.ToString());
 break;
 }
 case "/":{
 var r = left/right;
 statck.Push(r.ToString());
 break;
 }
 default:
 throw new Exception("syntax error:unsupported operation!");
 }
 Console.WriteLine();
 foreach(var j in statck.ToArray().Reverse()){
 Console.Write(" {0} ",j);
 }
 }else{
 statck.Push(v.ToString());
 if(i>1)
 Console.WriteLine();
 foreach(var j in statck.ToArray().Reverse()){
 Console.Write(" {0} ",j);
 }
 }
 }
 if(statck.Count!=1)
 throw new Exception("syntax error:invalid expression!");
 return Convert.ToDouble(statck.Pop());
 }