表驱动词法分析器是一种常用的词法分析器实现方法,它通过使用预定义的有限状态机来识别和提取源代码中的词法单元。下面是使用C#编写表驱动词法分析器的步骤:
NextToken()
,它读取源代码字符并根据状态转换表进行状态转换,直到识别出一个完整的词法单元。以下是一个简单的示例代码,演示了如何使用C#编写表驱动词法分析器:
using System;
using System.Collections.Generic;
public class LexicalAnalyzer
{
private enum State
{
Start,
Identifier,
Number,
Operator,
Delimiter
}
private static readonly int[,] TransitionTable = {
// Letter Digit Operator Delimiter
{ 1, 2, 3, 4 }, // Start
{ 1, 1, 5, 5 }, // Identifier
{ 5, 2, 5, 5 }, // Number
{ 5, 5, 3, 5 }, // Operator
{ 5, 5, 5, 4 } // Delimiter
};
private static readonly HashSet<string> Keywords = new HashSet<string> {
"if", "else", "while", "for", "int", "float"
};
private string sourceCode;
private int currentPosition;
public LexicalAnalyzer(string sourceCode)
{
this.sourceCode = sourceCode;
currentPosition = 0;
}
public Token NextToken()
{
State currentState = State.Start;
string lexeme = "";
while (currentPosition < sourceCode.Length)
{
char currentChar = sourceCode[currentPosition];
int column = GetColumn(currentChar);
if (column == -1)
{
// Invalid character
break;
}
currentState = (State)TransitionTable[(int)currentState, column];
if (currentState == State.Start)
{
// Reset lexeme
lexeme = "";
}
else
{
lexeme += currentChar;
currentPosition++;
if (currentState == State.Identifier && currentPosition < sourceCode.Length)
{
// Check if the lexeme is a keyword
string nextChar = sourceCode[currentPosition].ToString();
if (!char.IsLetterOrDigit(nextChar[0]))
{
if (Keywords.Contains(lexeme))
{
return new Token(TokenType.Keyword, lexeme);
}
}
}
if (currentState == State.Number && currentPosition < sourceCode.Length)
{
// Check if the lexeme is a floating-point number
string nextChar = sourceCode[currentPosition].ToString();
if (nextChar == ".")
{
currentState = State.Start;
}
}
if (currentState == State.Operator || currentState == State.Delimiter)
{
return new Token(GetTokenType(lexeme), lexeme);
}
}
}
return null;
}
private int GetColumn(char c)
{
if (char.IsLetter(c))
{
return 0; // Letter
}
else if (char.IsDigit(c))
{
return 1; // Digit
}
else if (IsOperator(c))
{
return 2; // Operator
}
else if (IsDelimiter(c))
{
return 3; // Delimiter
}
return -1; // Invalid character
}
private bool IsOperator(char c)
{
return "+-*/=".Contains(c);
}
private bool IsDelimiter(char c)
{
return "(){}[];,.".Contains(c);
}
private TokenType GetTokenType(string lexeme)
{
if (Keywords.Contains(lexeme))
{
return TokenType.Keyword;
}
else if (int.TryParse(lexeme, out _))
{
return TokenType.Number;
}
else if (lexeme.Length == 1 && IsOperator(lexeme[0]))
{
return TokenType.Operator;
}
else if (lexeme.Length == 1 && IsDelimiter(lexeme[0]))
{
return TokenType.Delimiter;
}
return TokenType.Identifier;
}
}
public class Token
{
public TokenType Type { get; }
public string Value { get; }
public Token(TokenType type, string value)
{
Type = type;
Value = value;
}
}
public enum TokenType
{
Identifier,
Number,
Operator,
Delimiter,
Keyword
}
public class Program
{
public static void Main()
{
string sourceCode = "int x = 10; float y = 3.14; if (x > y) { Console.WriteLine(\"Hello, World!\"); }";
LexicalAnalyzer analyzer = new LexicalAnalyzer(sourceCode);
Token token;
while ((token = analyzer.NextToken()) != null)
{
Console.WriteLine($"Type: {token.Type}, Value: {token.Value}");
}
}
}
这个示例代码实现了一个简单的词法分析器,可以识别标识符、关键字、运算符、分隔符和常量。你可以根据需要扩展和修改代码,以适应更复杂的词法分析需求。
请注意,这个示例代码仅用于演示如何使用C#编写表驱动词法分析器,不涉及任何特定的云计算或腾讯云产品。如果你需要了解更多关于云计算或腾讯云的信息,请参考腾讯云官方文档或咨询腾讯云的技术支持团队。
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