词法分析器c语言编写

词法分析器主要是将输入的程序代码转换成单词流，单词流中包含了程序中的各种关键字、变量名、符号等信息。下面是一个简单的词法分析器的C语言实现。

#include <stdio.h>
#include <ctype.h>
#include <string.h>

#define MAX_TOKEN_LEN 100

typedef enum {
    IDENTIFIER,
    KEYWORD,
    NUMBER,
    OPERATOR,
    DELIMITER,
    ERROR
} TokenType;

const char* keywords[] = {"auto", "break", "case", "char", "const", "continue",
                          "default", "do", "double", "else", "enum", "extern",
                          "float", "for", "goto", "if", "int", "long", "register",
                          "return", "short", "signed", "sizeof", "static", "struct",
                          "switch", "typedef", "union", "unsigned", "void", "volatile",
                          "while", NULL};

typedef struct {
    char lexeme[MAX_TOKEN_LEN];
    TokenType type;
} Token;

int isKeyword(char* token) {
    int i = 0;
    while(keywords[i] != NULL) {
        if(strcmp(keywords[i], token) == 0)
            return 1;
        i++;
    }
    return 0;
}

TokenType getTokenType(char* token) {
    if(isalpha(token[0])) {
        if(isKeyword(token))
            return KEYWORD;
        else
            return IDENTIFIER;
    }
    else if(isdigit(token[0]))
        return NUMBER;
    else if(token[0] == '+' || token[0] == '-' || token[0] == '*' || token[0] == '/' || token[0] == '=')
        return OPERATOR;
    else if(token[0] == '(' || token[0] == ')' || token[0] == '{' || token[0] == '}' || token[0] == ';')
        return DELIMITER;
    else
        return ERROR;
}

Token getNextToken(char* program, int* pos) {
    Token token;
    token.lexeme[0] = '\0';
    token.type = ERROR;

    while(isspace(program[*pos]))
        (*pos)++;

    int i = 0;
    while(isalnum(program[*pos]) || program[*pos] == '_') {
        if(i >= MAX_TOKEN_LEN - 1) {
            printf("Error: token too long\n");
            return token;
        }
        token.lexeme[i++] = program[(*pos)++];
    }
    token.lexeme[i] = '\0';

    token.type = getTokenType(token.lexeme);

    return token;
}

int main() {
    char program[] = "int main() { int x = 5 + 3; printf(\"%d\\n\", x); return 0; }";
    int pos = 0;

    Token token = getNextToken(program, &pos);
    while(token.type != ERROR) {
        printf("%s\t%d\n", token.lexeme, token.type);
        token = getNextToken(program, &pos);
    }

    return 0;
}

这个词法分析器可以识别C语言中的关键字、标识符、数字、运算符和分隔符，并将它们输出到控制台。当识别到错误的时候，会输出一个错误提示。