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if else and if combination in bison

Heartly R (23)
if and if else conflicts in my Bison code. I have given here my sample code.

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   %token <lbls>    IF  /* For backpatching labels*/
   %token THEN ELSE FI

// ....
// ....

    stmts :
           stmt
          | ifstmt
          // ...
          // ...
    ;

    ifstmt :  ifonly
        | ifelsestmt
    ;

    ifonly  : IF exp {/* my code */ }
            THEN commands   
          FI    {/* my code */ }    
    ;

    ifelsestmt : IF exp          {/* my code */ }
               THEN commands {/* my code */ }
             ELSE            {/* my code */ }             
                  commands
             FI             {/* my code */ }
    ;

This is my sample language

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  if n != 5  then 
      write "equal"; 
    fi;

    if abc == bcd  then 
       else write "Str Not Equal";
    fi;

If I run the above, It works perfectly for If Statement. But It gives me error at the line of else Statement. Is there any error in the rules ?
mutexe (2372)
This doesnt look like c++ to me.
Little Bobby Tables (1079)
he said it was bison code...

edit: i think the error is because in your grammar, you have it that after then there must be commands, so its seeing the else as a command and places it in one level deeper of scope. because of this, the else technically doesnt have a preceding if, which is causing the error. how to fix this: put in a command between then second if's then statement and the else statement
Last edited on
mutexe (2372)
he said it was bison code...

Then why post in a C++ forum?
Zhuge (4664)
@TC: If you are trying to make an LL(1) parser (which is suppose may be a default), then you have a common prefix issue. I believe the technique used in this case is to left-factor out the common prefix (the if-statement).
Heartly R (23)
I tried this itself. 
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if n != 5  then 
      write "equal"; 
    fi;

    if abc == bcd  then 
       write "Str Equal ";
       else write "Str Not Equal";
    fi;


But Having the same Problem
Little Bobby Tables (1079)
Mutexe wrote:
Then why post in a C++ forum?
have you used bison?
@Heartly: did you try what zhuge said?
mutexe (2372)
Apologies OP. It does look relevant. Sort of :)
Heartly R (23)
I searched in some sites. It gives me the result that the problem is in Shift/reduce. So I had changed my code as Like this.
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 %token <lbls>    IF  /* For backpatching labels*/
  %token THEN

%nonassoc IFX
%nonassoc ELSE
%nonassoc FI

IF exp          { /* my code */ }
THEN commands   { /* my code */ }
%prec IFX
FI          { /* my code */ }

| IF exp        { /* my code */ }
 THEN commands  { /* my code */ }
 ELSE           { /* my code */ }                 
 commands
 FI             { /* my code */ }

If if Give the Instruction '%prec IFX' before 'FI' statement, It gives me compile time Error as

"sample.yy", line 168: ill-formed rule

"sample.yy", line 168: rule given for FI, which is a token

rather If I give the same Instruction '%prec IFX' after 'FI' statement, I am not getting the compiler time error. But Else statement Doesn't working. I Have got Parse error, When I run my below code.
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if n == 5  then write "equal"; fi;

if abc == bcd  then write "Equal"; 
else write "Str Not Equal";
fi;
I have got Parse error in the line of 'Else'.
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