词法分析器c++代码

以下是一个简单的词法分析器的C++代码示例：

#include <iostream>
#include <string>
using namespace std;

enum TokenType {
    ID,
    INT,
    FLOAT,
    OPERATOR,
    PUNCTUATION,
    KEYWORD
};

struct Token {
    TokenType type;
    string value;
};

bool isNumber(char c) {
    return c >= '0' && c <= '9';
}

bool isAlpha(char c) {
    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
}

bool isOperator(char c) {
    string operators = "+-*/=";
    return (operators.find(c) != string::npos);
}

bool isPunctuation(char c) {
    string punctuations = "(){}[],.;:";
    return (punctuations.find(c) != string::npos);
}

bool isKeyword(string s) {
    string keywords[] = {"if", "else", "while", "for", "int", "float", "return"};
    for (int i = 0; i < 7; i++) {
        if (s == keywords[i]) {
            return true;
        }
    }
    return false;
}

Token getNextToken(string input, int& currentPos) {
    Token token;
    token.type = OPERATOR;
    while (currentPos < input.size() && input[currentPos] == ' ')
        currentPos++;
    if (currentPos == input.size()) {
        token.type = OPERATOR;
        token.value = "";
        return token;
    }
    if (isNumber(input[currentPos])) {
        token.type = INT;
        while (currentPos < input.size() && isNumber(input[currentPos])) {
            token.value += input[currentPos];
            currentPos++;
        }
        if (currentPos < input.size() && input[currentPos] == '.') {
            token.type = FLOAT;
            token.value += input[currentPos];
            currentPos++;
            while (currentPos < input.size() && isNumber(input[currentPos])) {
                token.value += input[currentPos];
                currentPos++;
            }
        }
    }
    else if (isAlpha(input[currentPos])) {
        token.type = ID;
        while (currentPos < input.size() && (isAlpha(input[currentPos]) || isNumber(input[currentPos]))) {
            token.value += input[currentPos];
            currentPos++;
        }
        if (isKeyword(token.value)) {
            token.type = KEYWORD;
        }
    }
    else if (isOperator(input[currentPos])) {
        token.type = OPERATOR;
        token.value += input[currentPos];
        currentPos++;
    }
    else if (isPunctuation(input[currentPos])) {
        token.type = PUNCTUATION;
        token.value += input[currentPos];
        currentPos++;
    }
    return token;
}

int main() {
    string input = "int a = 123; float b = 3.14; if (a > b) { return a; } else { return b; }";
    int currentPos = 0;
    while (currentPos < input.size()) {
        Token token = getNextToken(input, currentPos);
        cout << token.value << " ";
    }
    return 0;
}

该代码实现了一个简单的词法分析器，它将输入的字符串分解为多个token，每个token具有一种类型（如ID、INT、FLOAT、OPERATOR、PUNCTUATION、KEYWORD）和相应的值。getNextToken()函数根据当前位置的字符选择适当的类型，并返回包含类型和值的Token对象。在main()函数中，我们将字符串分解为多个token，并输出每个token的值。