转移/减少野牛冲突 [英] Shift/reduce conflicts in bison

问题描述

我是Bison的新手，但遇到了shift/reduce冲突的问题...我正在尝试从文件加载到array data[]:

I'm new to Bison and I'm having trouble with shift/reduce conflicts... I'm trying to load from file to array data[]:

struct  _data
{
  char name[50]; 
  char surname[50]; 
  int year;
} data[1000];

这是我的野牛代码的一部分:

Here is part of my bison code:

%token ID NUM NL EOF 

%%

File   : List EOF
       ;
List   : Record
       | List Record
       ;
Record : Name Surname Year NL  { count++; }
       | NL                    { count++; }
       | /*empty*/
       ;
Name   : ID                    { strcpy(data[count].name, yytext); }
       ;
Surname: ID                    { strcpy(data[count].surname, yytext); }
       ;
Year   : NUM                   { data[count].year= atoi(yytext); }
       ;

%%            

我收到此错误:

conflicts: 5 shift/reduce

知道我哪里出错了吗?

推荐答案

您可以使用-v选项获取bison来生成包含更多信息的.output文件，这些信息可以帮助您诊断移位/减少冲突.特别是，它将显示每个解析器状态，包括项目列表，并指出哪些状态有冲突.

You can use the -v option to get bison to produce an .output file containing a lot more information which can help you diagnose shift/reduce conflicts. In particular, it will show you every parser state, including the list of items, and also indicate which states have conflicts.

但是在这种情况下，问题非常简单.剥夺其要点，您将拥有:

But in this case, the problem is pretty simple. Stripped to its essentials you have:

List: Record
    | List Record
    ;

Record: Something
      | /* Nothing */
      ;

忽略Something的定义是什么，问题是List可以由任意数量的Records组成，一个接一个，而Record可以为空.这意味着没有什么可以像任何数量的空Records一样被解析，这完全是模棱两可的.输入中的任何两个连续Somethings可以用0、1、2、42或273个空Records分隔.由于解析器不知道是开始解析新的Something(移位)还是发出空的Record(减少)，因此它抱怨存在移位/减少冲突.

Ignoring what the definition of Something is, the problem is that a List can consist of any number of Records, one after another, and a Record can be empty. That means that nothing can be parsed as any number of empty Records, which is totally ambiguous. Any two consecutive Somethings in the input could be separated by 0, 1, 2, 42, or 273 empty Records. Since the parser can't know whether to start parsing a new Something (shift) or to emit an empty Record (reduce), it complains that there is a shift/reduce conflict.

在这种情况下，解决方案非常简单.我们可以看到，非空的Something必须以NL结尾；大概的目的是File由任意数量的Records组成，每个Records都在其自己的行上.这样我们可以重写:

In this case the solution is pretty simple. We can see that a non-empty Something must end with a NL; presumably the intent was that the File consists of any number of Records, each on its own line. So we can rewrite:

File:    List EOF
    ;

List:    Record
    |    List NL Record
    ;

Record:  Name Surname Year
      |  /* Empty */
      ;

现在，Record(是否为空)之后必须紧跟EOF或NL.不能直接跟随另一个Record.

Now a Record, empty or not, must be followed by either an EOF or a NL. It cannot be directly followed by another Record.

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