Question

我正在尝试用C语言制作一个词法分析器，并且尝试逐步查看代码以查看问题所在，但我看不到它。假定第一行，并且文件中没有其他内容，此代码从文件中读取一行。所以我用"a = (b + 2) * c"进行了测试。

它可以正常工作，并打印出a，=，但是什么也没有。我发现switch函数中的lookup()语句存在问题，因为它似乎可以正常处理UNKNOWN中的lex()。任何见识都会有所帮助和赞赏。

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

int charClass;
char lexeme[100];
char nextChar;
int lexLen;
int token;
int nextToken;
FILE *fp;

void addChar();
void getChar();
void getNonBlank();
int lex();

#define LETTER 0
#define DIGIT 1
#define UNKNOWN 99

#define INT_LIT 10
#define IDENT 11
#define ASSIGN_OP 20
#define ADD_OP 21
#define SUB_OP 22
#define MULT_OP 23
#define DIV_OP 24
#define LEFT_PARENT 25
#define RIGHT_PARENT 26

int main(int argc, char *argv[])
{
    fp = fopen(argv[1], "r");

    if (fopen == NULL)
        printf("File can not be opened");
    else {
        getChar();

        while (nextToken != EOF) {
            lex();
        }
    }
    return 0;
}

int lookup(char ch) {
    switch (ch) {
    case '(':
        addChar();
        nextToken = LEFT_PARENT;
        break;
    case ')':
        addChar();
        nextToken = RIGHT_PARENT;
        break;
    case '+':
        addChar();
        nextToken = ADD_OP;
        break;
    case '-':
       addChar();
        nextToken = SUB_OP;
        break;
    case '*':
        addChar();
        nextToken = MULT_OP;
        break;
    case '/':
        addChar();
        nextToken = DIV_OP;
        break;
    default:
        addChar();
        nextToken = EOF;
    }
    return nextToken;
}

void addChar() {
    if (lexLen <= 98) {
        lexeme[lexLen++] = nextChar;
        lexeme[lexLen] = 0;
    } else
        printf("Error- lexele is too long...\n");
}

void getChar() {
    if ((nextChar = getc(fp)) != EOF) {
        if (isalpha(nextChar))
            charClass = LETTER;
        else if(isdigit(nextChar))
             charClass = DIGIT;
        else
            charClass = UNKNOWN;
    } else
        charClass =EOF;
}

void getNonBlank() {
    while (isspace(nextChar))
        getChar();
}

int lex() {
    lexLen = 0;
    getNonBlank();
    switch (charClass) {
    case LETTER:
        addChar();
        getChar();
        while (charClass == LETTER || charClass == DIGIT) {
            addChar();
            getChar();
        }
        nextToken = IDENT;
        break;
    case DIGIT:
        addChar();
        getChar();
        while (charClass == DIGIT) {
            addChar();
            getChar();
        }
        nextToken = INT_LIT;
        break;
    case UNKNOWN:
        lookup(nextChar);
        getChar();
        break;
    case EOF:
        nextToken = EOF;
        lexeme[0] = 'E';
        lexeme[1] = 'O';
        lexeme[2] = 'F';
        lexeme[3] = 0;
    }
    printf("Next token is :%d, next lexeme is %s\n", nextToken, lexeme);
    return nextToken;
}

Answer 1

首先要注意的是，当您获得一个字符并将其与EOF进行比较时，必须将字符保存在 int 中，而不是保存在字符中，所以

void getChar()
{
    if((nextChar=getc(fp))!=EOF)
        {
        if(isalpha(nextChar))
            charClass=LETTER;
            else if(isdigit(nextChar))
                charClass=DIGIT;
        else
            charClass=UNKNOWN;
        }   
    else
        charClass=EOF;
}

不能很好地处理 EOF 情况，因为 nextChar 是 char ，可以是：

void getChar()
{
    if((charClass=getc(fp))!=EOF)
        {
      nextChar = charClass;
        if(isalpha(nextChar))
            charClass=LETTER;
            else if(isdigit(nextChar))
                charClass=DIGIT;
        else
            charClass=UNKNOWN;
    }
}

第二点是您忘记管理大小写'='，所以我认为在lookup(char ch)中您必须添加大小写：

case '=':addChar();
       nextToken=ASSIGN_OP;
  break;

这就是为什么您读完'='后会停下来。

如果我做了这些更改：

pi@raspberrypi:/tmp $ gcc -g -Wextra q.c
pi@raspberrypi:/tmp $ cat in
a = (b + 2) * c
pi@raspberrypi:/tmp $ ./a.out in
Next token is :11, next lexeme is a
Next token is :20, next lexeme is =
Next token is :25, next lexeme is (
Next token is :11, next lexeme is b
Next token is :21, next lexeme is +
Next token is :10, next lexeme is 2
Next token is :26, next lexeme is )
Next token is :23, next lexeme is *
Next token is :11, next lexeme is c
^C

由于程序循环，我必须终止执行，这是因为在getNonBlank()中， EOF 情况没有得到管理，所以：

void getNonBlank()
{
    while((charClass != EOF) && isspace(nextChar))
        getChar();
}

更改之后：

pi@raspberrypi:/tmp $ ./a.out in
Next token is :11, next lexeme is a
Next token is :20, next lexeme is =
Next token is :25, next lexeme is (
Next token is :11, next lexeme is b
Next token is :21, next lexeme is +
Next token is :10, next lexeme is 2
Next token is :26, next lexeme is )
Next token is :23, next lexeme is *
Next token is :11, next lexeme is c
Next token is :-1, next lexeme is EOF

正如chqrlie在评论中所说，也将if(fopen == NULL)替换为if (fp == NULL)

词法分析器中的switch语句出现问题

1 个答案: