CODEWITHSUNDEEP
substringbisontokenizeoperator-precedence
mooly
mooly

Reputation: 113

operator precedence in bison

I have a flex-bison project in which I need to support a few string operators, and operator '^' means reverse a string and operator [i] means return index i in the string. correct input and output for example :

input : ^"abc"[0]      ---> correct output: "c", my output: "a"

that's because first I want to reverse it("cba") and then take the 0 index ("cba"[0] is c). Now, I don't know how to do that precedence, so my code outputs "a" since it first takes "abc"[0]--> "a" and then reverses it-->"a". as of now I have in my bison file:

%left STR MINI
%left '^'         
substring:

STR MINI          {    //THIS IS DONE FIRST, SUBSTRING    
                   $$ = substringFind($1,$2,$2,temp);
                  }
| '^' substring   {   //BUT I WANT THIS (REVERSING) TO BE FIRST    
                  $$ = reverseStrings($2,temp);
                  }
;

how do I change that precedence? I don't really understand the precedence rules, it was very easy with plus (+) before multiple (*) but with those operators I don't really know how to work with it. ANY HELP...?

Upvotes: 0

Views: 225

Answers (1)

user207421
user207421

Reputation: 310980

You need separate productions, not alternates within the same production, something like:

string
    : substring
    ;

substring
    : reverse MINI { ... }
    | reverse
    ;

reverse
    : "^" reverse { ... }
    | STR
    ;

Upvotes: 1

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