如果其他条件，野牛减少/减少冲突

Question

我期待转移/减少对其他人的冲突，但它在线上减少/减少冲突＆＃34; | IF＆＃39;（＆＃39; boolean_statement＆＃39;）＆＃39;块＆＃34;

以下是一些可能有助于解释以下代码的信息：

• BOOL是在每行开头使用的关键字的标记，表示行是布尔操作

• BOOLEAN是＆＃34; true＆＃34;或＆＃34;假＆＃34;值

• 我使用此编译器将语言转换为C代码，该语言可以包含a,b,c=d+2之类的语句，相当于C中的a=b=c=d+2;和bool e = f * .N. g + h，相当于e = f && !g || h。

  statements:
          statements statement
          | statement
          ;
  
  statement:
          if_statement
          | BOOL variable_list '=' boolean_statement
          | variable_list '=' integer_statement
          ;
  
  if_statement:
          IF '(' boolean_statement ')' block ELSE block
          | IF '(' boolean_statement ')' block
          ;
  
  variable_list:
          variable_list ',' variable
          | variable
          ;
  
  variable:
          STRING 
          | STRING '[' INTEGER ']'
          | STRING '[' STRING ']'
          ;
  
  boolean_statement:
          '(' boolean_statement ')'
          | bval '*' boolean_statement
          | bval '+' boolean_statement
          | bval EQ boolean_statement
          | bval NEQ boolean_statement
          | NOT boolean_statement
          | bval
          ;
  
  bval:
          BOOLEAN   
          | variable
          ;
  
  integer_statement:
          '(' integer_statement ')'
          | value '+' integer_statement
          | value '*' integer_statement
          | value
          ;
  
  value:
          INTEGER        
          | variable
          ;
  
  block:
          statement
          | '{' statements '}'
          ;
  

这是完整的代码

   %{

    #include <cstdio>
    #include <iostream>
    using namespace std;

    //stuff from flex that bison needs to know about:
    extern "C" int yylex();
    extern "C" int yyparse();
    extern "C" FILE *yyin;
    extern int line_num;

    void yyerror(const char *s);

    %}

    //C union holding each of the types of tokens that Flex could return
    %union { 
            int ival;
            bool bval;
            char const *sval;
    }

    //symbol defination
    %token <sval> STRING;
    %token <sval> NOT
    %token CONSTANT_SECTION
    %token BOOLEAN_SECTION
    %token INTEGER_SECTION
    %token LOGIC_SECTION
    %token TIMER_SECTION
    %token <sval> BOOLEAN
    %token <ival> INTEGER
    %token <ival> HEX 
    %token ENDL 
    %token BOOL
    %token IF
    %token ELSE
    %token EQ NEQ
    %token AND
    %token OR
    %token SUBROUTINE_END
    %token SUBROUTINE_START
    %token DELAY SECONDS HOURS MINUTES MSEC
    %token GOTO
    %token LABEL
    %token CALL
    //end of declaration section
    %%

    logic:
            costants_declarations boolean_declarations integer_declarations timer_declarations logic_statements
            | boolean_declarations integer_declarations timer_declarations logic_statements
            | logic_statements
            ;

    costants_declarations:
            CONSTANT_SECTION constants  
            ;

    constants:
            constants STRING '=' INTEGER    { cout << "const int " << $2 << " = "  << $4 << ";" << endl; }
            | constants STRING '=' HEX      { cout << "const int " << $2 << " = "  << $4 << ";" << endl; }
            | STRING '=' INTEGER            { cout << "const int " << $1 << " = "  << $3 << ";" << endl; }
            | STRING '=' HEX                { cout << "const int " << $1 << " = "  << $3 << ";" << endl; }
            ;

    boolean_declarations:
            BOOLEAN_SECTION booleans       
            ;

    booleans:
            booleans ',' boolean             
            | booleans boolean               
            | boolean                       
            ;        

    boolean:
            STRING '[' INTEGER ']'          { cout << "bool " << $1 << "[" << $3 << "]" << ";" << endl; }
            | STRING '[' STRING ']'         { cout << "bool " << $1 << "[" << $3 << "]" << ";" << endl; }
            | STRING                        { cout << "bool " << $1 << " = true;" << endl; }
            ;

    integer_declarations:
            INTEGER_SECTION integers
            ;

    integers:
            integers ',' integer            
            | integers integer              
            | integer                      
            ;

    integer:
            STRING '[' INTEGER ']'          { cout << "int " << $1 << "[" << $3 << "]" << ";" << endl; }
            | STRING '[' STRING ']'         { cout << "int " << $1 << "[" << $3 << "]" << ";" << endl; }
            | STRING                        { cout << "int " << $1 << " = 0;" << endl; }
            ;

    timer_declarations:
            TIMER_SECTION timers
            ;

    timers:
            timers ',' timer
            | timers timer
            | timer
            ;

    timer:
            STRING                          { cout << "int " << $1 << ";" << endl; }
            ;

    logic_statements:
            LOGIC_SECTION subroutines statements
            ;

    subroutines:
            /* empty */
            | SUBROUTINE_START STRING statements SUBROUTINE_END STRING
            ;

    statements:
            statements statement
            | statement
            ;

    statement:
            if_statement
            | delay_statement
            | GOTO STRING
            | LABEL
            | CALL STRING
            | BOOL variable_list '=' { cout << " = "; } boolean_statement { cout << ";\n"; } 
            | variable_list '=' { cout << " = "; } integer_statement { cout << ";\n"; }
            ;

    if_statement:
            IF '(' { cout << "if("; } boolean_statement ')' { cout << ")" << endl; } block
            | IF '(' { cout << "if("; } boolean_statement ')' { cout << ")" << endl; } block ELSE block
            ;

    delay_statement:
            DELAY '=' INTEGER SECONDS statement 
            ;

    variable_list:
            variable_list ',' { cout << " = "; } variable
            | variable
            ;

    variable:
            STRING                          { cout << $1; }
            | STRING '[' INTEGER ']'        { cout << $1 << "[" << $3 << "]"; }
            | STRING '[' STRING ']'         { cout << $1 << "[" << $3 << "]"; }
            ;

    boolean_statement:
            '('{ cout << "("; } boolean_statement ')'{ cout << ")"; }
            | bval '+' { cout << " || "; } boolean_statement
            | bval OR { cout << " || "; } boolean_statement
            | bval '*' { cout << " && "; } boolean_statement
            | bval AND { cout << " && "; } boolean_statement
            | bval EQ { cout << " == "; } boolean_statement
            | bval NEQ { cout << " != "; } boolean_statement
            | NOT { cout << $1; } boolean_statement
            | bval
            ;

    bval:
            BOOLEAN                          { cout << $1; }
            | variable
            ;

    integer_statement:
            '('{ cout << "("; } integer_statement ')'{ cout << ")"; }
            | value '+'{ cout << " + "; } integer_statement
            | value '*'{ cout << " * "; } integer_statement
            | value
            ;

    value:
            INTEGER                       { cout << $1; }
            | variable
            ;

    block:
            { cout << "{" << endl; } statement { cout << "}" << endl; }
            | '{' { cout << "{" << endl; } statements '}' { cout << "}" << endl; }
            ;

    //end of grammer section
    %%

    int main(int argc, char *argv[]) {

            // default input is stdin
            // if file is given read from it
            if(argc == 2)
            {
                    // open a file handle to a particular file:
                    FILE *myfile = fopen(argv[1], "r");
                    // make sure it's valid:
                    if (!myfile) {
                            cout << "Can't open "<< argv[1] <<" file" << endl;
                            cout << "Usage: " << argv[0] << " <filename>\n";
                            return -1;
                    }
                    // set lex to read from it instead of defaulting to STDIN:
                    yyin = myfile;
            }
            else if(argc != 1)
            {
                    cout << "Usage: " << argv[0] << " <filename>\n";
                    cout << "Usage: " << argv[0] << endl;
                    return -1;
            }

            // parse through the input until there is no more:
            do 
            {
                    yyparse();
            } while (!feof(yyin));
    }

    void yyerror(const char *s) {
        cout << "Parse error on line " << line_num << "!  Message: " << s << endl;
        // might as well halt now:
        exit(-1);
    }

Answer 1

问题不在于IF语句。这是if_statement的两个作品中的中规则动作（MRAs）：

if_statement:
          IF '(' { cout << "if("; } boolean_statement ')' { cout << ")" << endl; } block
        | IF '(' { cout << "if("; } boolean_statement ')' { cout << ")" << endl; } block ELSE block
        ;

中等规则操作（如{ cout << "if("; }）将转换为唯一命名的空非终端。实际上，上述作品转化为以下内容：

if_statement:
          IF '(' @3 boolean_statement ')' @4 block
        | IF '(' @5 boolean_statement ')' @6 block ELSE block
        ;

@3: %empty { cout << "if("; }        ;
@4: %empty { cout << ")" << endl; }  ;
@5: %empty { cout << "if("; }        ;
@6: %empty { cout << ")" << endl; }  ;

在上文中，@3和@5相同（如@4和@6），但野牛不会检查;每个MRA都被认为是独一无二的这会导致减少/减少冲突，因为一旦解析器读取 if （，它将需要减少@3或{{1}中的一个在它可以移动以下标记之前，无论该标记是什么，但下一个标记都不会提示是否最终会出现 else 。（两个作品都继续{{1}在任何一种情况下，以下标记都可以是@5中的任何标记。）

冲突得到解决，有利于boolean_statement（文本上较早的非终端）的事实意味着FIRST(boolean_statement)永远无法减少，而野牛会对此提出警告。 （至少，我的野牛版本确实如此。）

这是MRA的一个典型问题，足以让它保证bison manual中的某个部分。

在这种情况下，您只需通过左因子分析即可解决问题：

@3