关于Basic程序解释器及编译原理的简单化(1)--词法分析和代数式求值

关于Basic程序解释器及编译原理的简单化(1)--词法分析和代数式求值(7)     }
   
    if (isdigit(*prog))  {   /* number */
        while (!isdelim(*prog))  *temp++=*prog++;
        *temp = '\0';
        return (token_type = NUMBER);
    }

    if (isalpha(*prog))  {   /* var or command */
        while (!isdelim(*prog))  *temp++=*prog++;
        token_type = STRING;
    }

    *temp = '\0';

    /* see if a string is a command or a variable */
    if (token_type == STRING)  {
        tok = look_up(token);  /* convert to internal rep */
        if (!tok)  token_type = VARIABLE;
        else token_type = COMMAND;  /* is a command */
    }
    return token_type;
}

/* return a token to input stream */
void putback()
{
    char *t;
    t = token;
    for (;*t;t++)  prog--;
}

look_up(char *s)
{
    register int i,j;
    char *p;

    /* convert to lowercase */
    p = s;
    while (*p)  { *p = tolower(*p); p++;  }

    /* see if token is in table */
    for (i=0;*table[i].command;i++)
        if (!strcmp(table[i].command,s))  return table[i].tok;
    return 0;   /* unknown command */
}

/* return true if c is a delimiter */
isdelim(char c)
{
    if (strchr(";,+-<>/*%^=() ",c)c==9c=='\r'c==0)