递归下降的表达式解析器(C#)
使用方法：10-2*3 得到结果 6。
ExpressionParser.cs
using System;namespace IBMS.Text.Script
{
Field#region Field
/**//// <summary> /// Parser of syntax
/// </summary> public class ExpressionParser { /**//// <summary> /// Threse are the token types /// </summary> public enum TokenTypes { /**//// <summary> /// Not defined type of token /// </summary> None, /**//// <summary> /// Delimiter token type /// </summary> Delimiter, /**//// <summary> /// Variable token type /// </summary> Variable, /**//// <summary> /// Number token type /// </summary> Number, } /**//// <summary> /// These are the types of syntax errors /// </summary> public enum SyntaxErrors { /**//// <summary> /// All syntax error /// </summary> Synatx, /**//// <summary> /// Bracket error /// </summary> UnBalparens, /**//// <summary> /// No expression error /// </summary> NoExpression, /**//// <summary> /// Divide by zero error /// </summary> DivByZero, } /**//// <summary> /// This token indicates end-of-expression /// </summary> readonly private string EOE; /**//// <summary> /// refers to expression string /// </summary> private string exp; /**//// <summary> /// Current index into the expression /// </summary> private int expIdx; /**//// <summary> /// Holds current token /// </summary> private string token; /**//// <summary> /// Holds current token's type /// </summary> private TokenTypes tokType; #endregion Public Mothord#region Public Mothord /**//// <summary> /// Constructor /// </summary> public ExpressionParser() { EOE= "\0"; } /**//// <summary> /// Parsor entry point /// </summary> /// <param name="expstr">expression string</param> /// <returns>result of the expression</returns> public double Evaluate(string expstr) { double result = 0.0; exp =expstr; expIdx = 0; GetToken(); if( token == EOE) { HandleError(SyntaxErrors.NoExpression); //No expression present } //Parse and evaluate the expression result = EvalExp2(); if( token != EOE) { HandleError(SyntaxErrors.Synatx); } return result; } #endregion Private Methord#region Private Methord /**//// <summary> /// Add or ssubtract two terms; /// </summary> /// <returns>Evaluated result</returns> private double EvalExp2() { char op; double result; double partialResult; result = EvalExp3(); while( ((op=token[0]) ) == '+' || op == '-') { GetToken(); partialResult = EvalExp3(); switch(op) { case '+': result += partialResult; break; case '-': result -= partialResult; break; } } return result; } /**//// <summary> /// Multiply or divide two factors. /// </summary> /// <returns>Evaluated result</returns> public double EvalExp3() { char op; double result; double partialResult; result = EvalExp4(); while( (op= token[0]) == '*'|| op == '/' || op =='%') { GetToken(); partialResult = EvalExp4(); switch(op) { case '*': result *=partialResult; break; case '/': if(partialResult == 0.0) { HandleError(SyntaxErrors.DivByZero); } result /= partialResult; break; case '%': if(partialResult == 0.0) { HandleError(SyntaxErrors.DivByZero); } result %= partialResult; break; } } return result; } /**//// <summary> /// Process an exponent /// </summary> /// <returns></returns> private double EvalExp4() { double result; double partialResult; double ex; int t; result = EvalExp5(); if(token =="^" ) { GetToken(); partialResult = EvalExp4(); ex = result; if(partialResult == 0.0 ) { result = 1.0; } else { for(t=(int)partialResult-1; t>0; t--) { result *= ex; } } } return result; } //Evaluate a unary + or - private double EvalExp5() { double result; string op; op=""; if( (tokType == TokenTypes.Delimiter) && ( token =="+" || token =="-") ) { op = token; GetToken(); } result = EvalExp6(); if(op == "-") result = - result; return result; } private double EvalExp6() { double result; if(token=="(") { GetToken(); result = EvalExp2(); if(token!=")") { HandleError(SyntaxErrors.UnBalparens); } GetToken(); } else { result = Atom(); } return result; } /**//// <summary> /// Get the value of a number /// </summary> /// <returns></returns> private double Atom() { double result = 0.0; switch(tokType) { case TokenTypes.Number: try { result = Double.Parse(token); } catch(InvalidCastException) { HandleError( SyntaxErrors.Synatx ); } GetToken(); break; default: HandleError( SyntaxErrors.Synatx ); break; } return result; } /**//// <summary> /// Handle an error /// </summary> /// <param name="error">index of error</param> void HandleError(SyntaxErrors error) { String [] err = new string[] { "Syntax Error", "Unabalanced Parentheses", "No Expression Present", "Division By Zero" }; throw new ExpressionParserException(err[(int)error]); } /**//// <summary> /// Obtain the next token /// </summary> private void GetToken() { tokType = TokenTypes.None; token = ""; //Check for end of expression. if( expIdx == exp.Length) { token = EOE; return; } //Skip over white space while( expIdx <exp.Length && char.IsWhiteSpace( exp[expIdx] ) ) ++expIdx; //Trailing whitespace ends expression. if( expIdx == exp.Length ) { token = EOE; return; } if( IsDelimiter(exp[expIdx] ) ) //Is operator { token+=exp[expIdx]; expIdx++; tokType = TokenTypes.Delimiter; } else if (Char.IsLetter(exp[expIdx])) //Is variable { while(!IsDelimiter( exp[expIdx]) ) { token += exp[expIdx]; expIdx ++; if(expIdx >= exp.Length ) break; } tokType = TokenTypes.Variable; } else if( char.IsDigit(exp[expIdx])) //Is a number { while( !IsDelimiter(exp[expIdx])) { token += exp[expIdx]; expIdx ++; if( expIdx >= exp.Length ) break; } tokType = TokenTypes.Number; } else { token = EOE; return; } } /**//// <summary> /// Check whether c is a delimiter /// </summary> /// <param name="c">the checked char</param> /// <returns>Return true if c is a delimiter, else return false</returns> private static bool IsDelimiter(char c) { if( (" +-/*%^=()".IndexOf(c) !=-1 )) { return true; } return false; } #endregion 
}}ExpressionParserException.cs
using System;namespace IBMS.Text.Script{ /**//// <summary> /// Exception class for parsor errors /// </summary> public class ExpressionParserException:Exception { /**//// <summary> /// Constructor /// </summary> /// <param name="str">error message</param> public ExpressionParserException(string str) :base(str) { } }}