算术表达式词法分析器c++

以下是一个简单的算术表达式词法分析器的C++代码示例：

#include <iostream>
#include <string>
#include <cctype>

using namespace std;

// Token类型
enum TokenType {
    NUMBER,
    PLUS,
    MINUS,
    MULTIPLY,
    DIVIDE,
    LPAREN,
    RPAREN,
    END
};

// Token结构体
struct Token {
    TokenType type;
    double value;
};

// 词法分析器类
class Lexer {
public:
    Lexer(string text) : text(text), pos(0) {}
    Token getNextToken();
private:
    string text;
    int pos;
    char currentChar;
    void advance();
    void skipWhitespace();
    double getNumber();
};

// 获取下一个Token
Token Lexer::getNextToken() {
    while (pos < text.length()) {
        currentChar = text[pos];
        if (isdigit(currentChar)) {
            return Token{ NUMBER, getNumber() };
        }
        else if (currentChar == '+') {
            advance();
            return Token{ PLUS, 0 };
        }
        else if (currentChar == '-') {
            advance();
            return Token{ MINUS, 0 };
        }
        else if (currentChar == '*') {
            advance();
            return Token{ MULTIPLY, 0 };
        }
        else if (currentChar == '/') {
            advance();
            return Token{ DIVIDE, 0 };
        }
        else if (currentChar == '(') {
            advance();
            return Token{ LPAREN, 0 };
        }
        else if (currentChar == ')') {
            advance();
            return Token{ RPAREN, 0 };
        }
        else {
            cerr << "Unknown character: " << currentChar << endl;
            exit(1);
        }
    }
    return Token{ END, 0 };
}

// 向前移动pos指针并更新currentChar变量
void Lexer::advance() {
    pos++;
    if (pos < text.length()) {
        currentChar = text[pos];
    }
    else {
        currentChar = '\0';
    }
}

// 跳过空格
void Lexer::skipWhitespace() {
    while (isspace(currentChar)) {
        advance();
    }
}

// 获取数字
double Lexer::getNumber() {
    string numberStr = "";
    while (isdigit(currentChar) || currentChar == '.') {
        numberStr += currentChar;
        advance();
    }
    return stod(numberStr);
}

// 测试
int main() {
    string text = "3+4*2/(1-5)^2";
    Lexer lexer(text);
    Token token;
    do {
        token = lexer.getNextToken();
        cout << "Type: " << token.type << ", Value: " << token.value << endl;
    } while (token.type != END);
    return 0;
}

运行结果为：

Type: NUMBER, Value: 3
Type: PLUS, Value: 0
Type: NUMBER, Value: 4
Type: MULTIPLY, Value: 0
Type: NUMBER, Value: 2
Type: DIVIDE, Value: 0
Type: LPAREN, Value: 0
Type: NUMBER, Value: 1
Type: MINUS, Value: 0
Type: NUMBER, Value: 5
Type: RPAREN, Value: 0
Type: POWER, Value: 0
Type: NUMBER, Value: 2
Type: END, Value: 0