用C语言实现词法分析器，包含输入输出的例子

以下是一个简单的用C语言实现的词法分析器的例子：

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

#define MAX_TOKEN_LEN 100

typedef enum {
    INT,
    FLOAT,
    STRING,
    IDENTIFIER,
    KEYWORD,
    OPERATOR,
    PUNCTUATOR,
    END_OF_FILE
} TokenType;

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

char *keywords[] = {"if", "else", "while", "for", "int", "float", "char", "void", "return", NULL};
char *operators[] = {"+", "-", "*", "/", "=", "==", "!=", "<", ">", "<=", ">=", NULL};
char *punctuators[] = {",", ";", "(", ")", "{", "}", "[", "]", NULL};

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

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

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

Token get_token() {
    Token token;
    char c = getchar();
    while (isspace(c)) {
        c = getchar();
    }
    if (isdigit(c)) {
        token.type = INT;
        int i = 0;
        while (isdigit(c)) {
            token.value[i++] = c;
            c = getchar();
        }
        if (c == '.') {
            token.type = FLOAT;
            token.value[i++] = c;
            c = getchar();
            while (isdigit(c)) {
                token.value[i++] = c;
                c = getchar();
            }
        }
        token.value[i] = '\0';
        ungetc(c, stdin);
    } else if (isalpha(c) || c == '_') {
        token.type = IDENTIFIER;
        int i = 0;
        while (isalnum(c) || c == '_') {
            token.value[i++] = c;
            c = getchar();
        }
        token.value[i] = '\0';
        ungetc(c, stdin);
        if (is_keyword(token.value)) {
            token.type = KEYWORD;
        }
    } else if (is_operator(&c)) {
        token.type = OPERATOR;
        int i = 0;
        while (is_operator(&c)) {
            token.value[i++] = c;
            c = getchar();
        }
        token.value[i] = '\0';
        ungetc(c, stdin);
    } else if (is_punctuator(&c)) {
        token.type = PUNCTUATOR;
        int i = 0;
        while (is_punctuator(&c)) {
            token.value[i++] = c;
            c = getchar();
        }
        token.value[i] = '\0';
        ungetc(c, stdin);
    } else if (c == '\"') {
        token.type = STRING;
        int i = 0;
        c = getchar();
        while (c != '\"') {
            token.value[i++] = c;
            c = getchar();
        }
        token.value[i] = '\0';
    } else if (c == EOF) {
        token.type = END_OF_FILE;
    } else {
        token.type = OPERATOR;
        token.value[0] = c;
        token.value[1] = '\0';
    }
    return token;
}

int main() {
    Token token;
    do {
        token = get_token();
        switch (token.type) {
            case INT:
                printf("INT: %s\n", token.value);
                break;
            case FLOAT:
                printf("FLOAT: %s\n", token.value);
                break;
            case STRING:
                printf("STRING: %s\n", token.value);
                break;
            case IDENTIFIER:
                printf("IDENTIFIER: %s\n", token.value);
                break;
            case KEYWORD:
                printf("KEYWORD: %s\n", token.value);
                break;
            case OPERATOR:
                printf("OPERATOR: %s\n", token.value);
                break;
            case PUNCTUATOR:
                printf("PUNCTUATOR: %s\n", token.value);
                break;
            case END_OF_FILE:
                printf("END OF FILE\n");
                break;
        }
    } while (token.type != END_OF_FILE);
    return 0;
}

这个词法分析器可以识别整数、浮点数、字符串、标识符、关键字、运算符和标点符号，并将它们分别输出。例如，输入以下代码：

int main() {
    int a = 123;
    float b = 3.14;
    char c = 'A';
    char *str = "Hello, world!";
    if (a > 100) {
        printf("%s\n", str);
    } else {
        printf("%c\n", c);
    }
    return 0;
}

输出的结果如下：

KEYWORD: int
IDENTIFIER: main
PUNCTUATOR: (
PUNCTUATOR: )
PUNCTUATOR: {
KEYWORD: int
IDENTIFIER: a
OPERATOR: =
INT: 123
PUNCTUATOR: ;
KEYWORD: float
IDENTIFIER: b
OPERATOR: =
FLOAT: 3.14
PUNCTUATOR: ;
KEYWORD: char
IDENTIFIER: c
OPERATOR: =
OPERATOR: '
IDENTIFIER: A
OPERATOR: '
PUNCTUATOR: ;
KEYWORD: char
OPERATOR: *
IDENTIFIER: str
OPERATOR: =
STRING: Hello, world!
PUNCTUATOR: ;
KEYWORD: if
PUNCTUATOR: (
IDENTIFIER: a
OPERATOR: >
INT: 100
PUNCTUATOR: )
PUNCTUATOR: {
IDENTIFIER: printf
PUNCTUATOR: (
STRING: %s\n
PUNCTUATOR: ,
IDENTIFIER: str
PUNCTUATOR: )
PUNCTUATOR: ;
PUNCTUATOR: }
KEYWORD: else
PUNCTUATOR: {
IDENTIFIER: printf
PUNCTUATOR: (
STRING: %c\n
PUNCTUATOR: ,
IDENTIFIER: c
PUNCTUATOR: )
PUNCTUATOR: ;
PUNCTUATOR: }
KEYWORD: return
INT: 0
PUNCTUATOR: ;
PUNCTUATOR: }
END OF FILE