Lex-yacc程序显示语法错误 [英] Lex - yacc program showing syntax error

问题描述

为了对输入执行算术运算，已编写了以下两个代码，但是它始终使我出现语法错误 这是lex程序

The following two codes have been written in order to perform arithmetic operations on input , but it gives me syntax error all the time Here is the lex program

%{ 
#include "y.tab.h" 
#include <stdlib.h>
%}



%%

[0-9]+ {yylval =  atoi(yytext);return ID;}

[*-+/()] {return yytext[0];}

'\n'  {return END;}


. {return yytext[0];}

%%

这是yacc程序:

 %{
     #include <stdio.h>
     #include <stdlib.h>
     #include "y.tab.h"     
 %}
 %token ID END  
 %%
 S: expr END { printf("Answer is : %d\n",$$);   exit(1);  }   
 expr: ID
 | 
 | expr '+' expr { $$ = $1+$3;}
 | expr '-' expr { $$ = $1-$3;}
 | expr '*' expr { $$ = $1*$3;}
 | expr '/' expr { $$ = $1/$3;}
 | '(' expr ')' {$$ = $2;}
 ;
 %%
 int main()
 {
     yyparse();
 }

输出:

 2+3

 2+3
 Syntax error

可能是什么问题?

推荐答案

存在三个问题.

第一个问题是您的语法只能识别一个表达式:

The first problem is that your grammar only recognizes a single expression:

S: expr END { printf("Answer is : %d\n",$$);   exit(1);  }

因此，如果您输入两个表达式，则会出现语法错误.

So if you enter two expressions, there is a syntax error.

第二个问题是您的扫描仪实际上无法将换行符识别为END令牌:

The second problem is that your scanner does not actually recognize a newline character as an END token:

'\n'  {return END;}

'不是(f)lex的特殊字符. (有关可接受的模式字符的完整列表，请参见 Flex手册. )，因此该规则正在查找三个字符:撇号，换行符和另一个撇号.如果提供该输入，则将获得预期"输出:

' is not a special character for (f)lex. (See the Flex manual for a complete list of accepted pattern characters.) So that rule is looking for three characters: an apostrophe, a newline, and another apostrophe. If you provide that input, you will get the "expected" output:

$ ./badexpr
2+3'
'
Answer is : 5
$

你想写的是

\n    { return END; }

但这也不是很理想.这将在单个表达式后立即返回.一个更有趣的语法将允许使用任意数量的表达式:

But that's probably not ideal either. That will return immediately after a single expression. A more interesting grammar would allow for any number of expressions:

%{ 
#include <stdlib.h>
#include "y.tab.h" 
%}

%%

[0-9]+ {yylval =  atoi(yytext);return ID;}
.|\n   {return yytext[0];}

file.y

%{
     #include <stdio.h>
     #include <stdlib.h>
%}
%token ID
%%
S: /* %empty */
 | S expr '\n'   { printf("Answer is : %d\n",$2); }
 | S '\n'
expr: ID
 | expr '+' expr { $$ = $1+$3;}
 | expr '-' expr { $$ = $1-$3;}
 | expr '*' expr { $$ = $1*$3;}
 | expr '/' expr { $$ = $1/$3;}
 | '(' expr ')' {$$ = $2;}
 ;
 %%
 int main()
 {
     yyparse();
 }

更改:

• 我从flex输入中删除了所有单字符模式，因为它们都具有相同的效果:返回读取的单个字符.由于以下更改，其中包括换行符.
• 我更改了语法的开始形式，以便它显式查找换行符(而不是END)并接受任意数量的输入行.
• 我从野牛输入中删除了#include "y.tab.h"，因为没有必要在解析器文件中包含解析器标头.
• 我将expr的空生产移到了S生产中，这样它就不会尝试打印空表达式的值.

Changes:

• I removed all of the single-character patterns from the flex input because they all have the same effect: return the single character read. That includes the newline character, because of the following change.
• I changed the start production for the grammar so that it looks for a newline character explicitly (rather than END) and accepts any number of input lines.
• I removed the #include "y.tab.h" from the bison input because it is not necessary to include the parser header in the parser file.
• I moved the empty production for expr into the S production so that it does not attempt to print the value of an empty expression.

最后，您尝试在开始生产时明确识别END令牌.这可能有效，但不建议这样做.

Finally, you attempt to explicitly recognize an END token in your start production. That may work but it is not recommended.

Yacc/野牛通过添加看起来与您的规则S完全一样的外部规则来自动扩充语法:它可以识别开始的非终结符，后跟END.因此，无需添加自己的作品.

Yacc/bison automatically augment your grammar by adding an outer rule which looks precisely like your rule S: it recognizes the start non-terminal followed by END. So adding your own production is unnecessary.

这篇关于Lex-yacc程序显示语法错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！