Syntax error while running lex and yacc

Question

I am pretty new to lex and yacc. I was trying out a grammar that creates matrices and performs some operations like adition,multiplication etc.

Here is the grammar I am using

 program:     block ENDS
 block:       stmt | block ENDS stmt
 stmt:        defn_stmt | init_stmt | print_stmt | oper_stmt
 defn_stmt:   DEF definitions
 definitions: MATID OPENSQ NUMBER COMMA NUMBER CLOSEQ ENDS | 
              definitions COMMA MATID OPENSQ NUMBER COMMA NUMBER CLOSEQ ENDS
 init_stmt:   INIT MATID [mat_contents]
 mat_contents: NUMBER | mat_contents COMMA mat_contents | mat_contents RANGE mat_contents
 print_stmt:  PRINT MATID
 oper_stmt:   MATID mat_operator inputs | MATID CMUL inputs
 mat_operator: ADD | MMUL
 inputs: MATID COMMA MATID

But for the following input am getting syntax error at second DEF.

 DEF A [2,3]; // defines a 2X3 Matrix A.
 DEF I [2,3]; // defines a 2X3 Matrix A.
 DEF BA[2,3],C[2,3], D[3,4], E[2,4];

ALso while running yacc its showing conflicts: 4 shift/reduce.

What causes the syntaxerror here? How can I correct it?

There's not really enough information there to tell - the definitions for all your terminals are missing. However, it does seem that your grammer doesn't include anything at all for dealing with the semicolons in your input... — twalberg
In that case, your definitions expects to end with an ENDS, but then you also expect your stmts to be separated by an ENDS... Maybe DEF A [2,3];; DEF I [2,3];; would avoid the syntax error... — twalberg

rici rici · Accepted Answer · 2014-04-07T17:27:54

definitions must end with ENDS (;, apparently), which means that defn_stmt must end with ;. However, a stmt must always be followed by a ;. So the grammar insists that a definition be followed by two semicolons.

You should remove the ENDS from the defn_stmt productions. Also, you might want to factor out the two occurrences of MATID OPENSQ NUMBER COMMA NUMBER CLOSEQ; that's not the cause of your problems, though.

One obvious ambiguity (actually, two) is in the productions:

mat_contents: NUMBER
            | mat_contents COMMA mat_contents
            | mat_contents RANGE mat_contents

since it is (exponentially) ambiguous how to parse:

NUMBER COMMA NUMBER COMMA NUMBER RANGE NUMBER COMMA NUMBER

You could solve this with precedence declarations, or you could be more precise in your grammar.

Syntax error while running lex and yacc

2 Answers