文法分析与解释器模式（三）

这里把上两次内容作一个连接。
生成名为Parsing的类，它拥有一个名为Calculate（）的方法，传入表达式字符串，返回double结果。

 public class Parsing
    {
        private Context ctx;
        public Dictionary<string, double> _src;
        
        public Parsing(Dictionary<string,double> source)
        {            
            this._src = source;
            ctx = new Context();
            foreach (string key in _src.Keys)
            {
                ctx.Assign(new Variable(key), _src[key]);
            }
        }
        public void Update()
        {
            ctx.Clear();
            foreach (string key in _src.Keys)
            {
                ctx.Assign(new Variable(key), _src[key]);
            }
        }
        public double Calculate(string expression)
        {
            InToPost post = new InToPost(expression);
            Queue<string> q;
            int err=post.Trans(out q);
            if (err == 0) return double.MaxValue;
            Stack<Expression> se = new Stack<Expression>();
            string str;
            while (q.Count != 0)
            {
                str=q.Dequeue();
                switch (str)
                {
                    case"":
                    case"(":
                    case")":
                        break;
                    case"+":
                        se.Push(new Add(se.Pop(), se.Pop()));
                        break;
                    case "-":
                        se.Push(new Dec(se.Pop(), se.Pop()));
                        break;
                    case "*":
                        se.Push(new Mul(se.Pop(), se.Pop()));
                        break;
                    case "/":
                        se.Push(new Div(se.Pop(), se.Pop()));
                        break;
                    case "^":
                        se.Push(new Pow(se.Pop(), se.Pop()));
                        break;
                        default:
                        if (char.IsLetter(str[0]))
                            se.Push(new Variable(str));
                        else
                            se.Push(new Number(double.Parse(str)));
                        break;                        
                }
            }
            double result;
            if (se.Count == 1)
                result = se.Pop().Interpret(ctx);
            else
                throw new Exception("Expression is not correct");
            return result;
        }        
    }

大家可能注意到这个类中有一个Dictionary类型的变量和一条new Variable(str)的语句，这其实就是可以对支持变量进行扩展^_^.
有时间我会写一个近似于MathCAD的RichEditBox的表达式解析器发上来。
 一个类似简易的支持变量的表达式解析器例子