在函数"yylex"中:未声明“变量"

在函数

问题描述：

我正在使用词法分析.为此，我正在使用Flex并获取以下问题.

I am working with Lexical Analysis. For this I am using Flex and I fetch following Problems.

int cnt = 0,num_lines=0,num_chars=0; // Problem here.
%%
[" "]+[a-zA-Z0-9]+      {++cnt;}
\n  {++num_lines; ++num_chars;}
.   {++num_chars;}
%%
int yywrap()
{
    return 1;
}
int main()
{   yyin = freopen("in.txt", "r", stdin);
    yylex();
    printf("%d %d %d\n", cnt, num_lines,num_chars);
    return 0;
}

然后，我使用以下命令，它可以正常工作并创建lex.yy.c.

then, I use following command and it work properly and create lex.yy.c.

Rezwans-iMac:laqb-2 rezwan $ flex work.l

Rezwans-iMac:laqb-2 rezwan$ flex work.l

然后，我使用以下命令.

then, I use following command.

Rezwans-iMac:laqb-2 rezwan $ gcc lex.yy.c -o b

Rezwans-iMac:laqb-2 rezwan$ gcc lex.yy.c -o b

并关注error:

work.l: In function ‘yylex’:
work.l:3:4: error: ‘cnt’ undeclared (first use in this function); did you mean int’?
 [" "]+[a-zA-Z0-9]+  {++cnt;}
    ^~~
    int
work.l:3:4: note: each undeclared identifier is reported only once for each function it appears in
work.l:4:4: error: ‘num_lines’ undeclared (first use in this function)
 \n {++num_lines; ++num_chars;}
    ^~~~~~~~~
work.l:4:17: error: ‘num_chars’ undeclared (first use in this function); did you mean ‘num_lines’?
 \n {++num_lines; ++num_chars;}
                 ^~~~~~~~~
                 num_lines
work.l: In function ‘main’:
work.l:15:23: error: ‘cnt’ undeclared (first use in this function); did you mean ‘int’?
  return 0;
                       ^  
                       int
work.l:15:28: error: ‘num_lines’ undeclared (first use in this function)
  return 0;
                            ^        
work.l:15:38: error: ‘num_chars’ undeclared (first use in this function); did you mean ‘num_lines’?
  return 0;
                                      ^        
                                      num_lines

如果我这样修改work.l文件，我就不会超越error.

I am not getting above error, if I modify work.l file like this .

    int cnt = 0,num_lines=0,num_chars=0; // then work properly above command.
%%
[" "]+[a-zA-Z0-9]+      {++cnt;}
\n  {++num_lines; ++num_chars;}
.   {++num_chars;}
%%
int yywrap()
{
    return 1;
}
int main()
{   yyin = freopen("in.txt", "r", stdin);
    yylex();
    printf("%d %d %d\n", cnt, num_lines,num_chars);
    return 0;
}

也就是说，如果我在int cnt = 0,num_lines=0,num_chars=0;行之前使用1 tab，它将正常工作.

That is to say, If I use 1 tab before this line int cnt = 0,num_lines=0,num_chars=0;, it works properly.

现在我有两个问题:

是否必须在该行int cnt = 0,num_lines=0,num_chars=0;之前使用1 tab?为什么?进行逻辑解释.

Is necessary use 1 tab before this line int cnt = 0,num_lines=0,num_chars=0;? why? explain logically.

是解决此错误的另一种方法?

Is another solution to solve this error ?

答

我对制表符问题不太确定，但一个解释是，如果您不放置制表符并在第一部分中写类似以下内容的话:>

I'm not very sure about the tab problem but one explanation is that if you don't place tab and write in the first section something like:

int cnt = 0;

然后请注意，在第一部分中，您还可以编写快捷方式"，例如:

Then note that in the first section you may also write "shortcuts" like:

Digit  [0-9]

这是一个正则表达式，用于定义数字是什么，而不是一直写[0-9]来表示数字.

which is a regular expression that defines what digit is instead of writing all the time [0-9] to denote digit.

因此，在不使用制表符的情况下在第一列中编写int cnt = 0;时，就像定义关键字int一样(这可能就是错误告诉您did you mean int’?的原因).因此，tab是一种区分上述两种情况的方法.

So when writing int cnt = 0; in the first column without using tab is like defining the keyword int (that's probably why the error tells you did you mean int’?). So tab is a way to distinguish the two above cases.

根据flex ，要编写c/c ++代码，它必须位于内部:%{ ... c/c++ code... %}，因此对于您的示例:

According to flex in order to write c/c++ code it needs to be inside: %{ ... c/c++ code... %} so for your example:

%{
   int cnt = 0,num_lines=0,num_chars=0;
%}

所以我认为最好的方法是在%{ %}中编写您的c代码.

So I thing the best way is to write your c-code inside %{ %}.

在函数"yylex"中:未声明“变量"

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