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    <p>As mientefuego pointed out you grammar has the classic "dangling else" problem. You could beat the problem by assigning precedence to the rules that causes conflict.</p> <p>The rule causing conflict is:</p> <pre><code>selection_stmt : IF '(' expression ')' statement | IF '(' expression ')' statement ELSE statement ; </code></pre> <p>First start by making ELSE and LOWER_THAN_ELSE ( a pseudo-token ) non associative:</p> <pre><code>%nonassoc LOWER_THAN_ELSE %nonassoc ELSE </code></pre> <p>This gives ELSE more precedence over LOWER_THAN_ELSE simply because LOWER_THAN_ELSE is declared first.</p> <p>Then in the conflicting rule you have to assign a precedence to either the shift or reduce action:</p> <pre><code>selection_stmt : IF '(' expression ')' statement %prec LOWER_THAN_ELSE ; | IF '(' expression ')' statement ELSE statement ; </code></pre> <p>Here, higher precedence is given to shifting. I have incorporated the above mentioned corrections and listed the complete grammar below:</p> <pre><code>/* C-Minus BNF Grammar */ %token ELSE %token IF %token INT %token RETURN %token VOID %token WHILE %token ID %token NUM %token LTE %token GTE %token EQUAL %token NOTEQUAL %nonassoc LOWER_THAN_ELSE %nonassoc ELSE %% program : declaration_list ; declaration_list : declaration_list declaration | declaration ; declaration : var_declaration | fun_declaration ; var_declaration : type_specifier ID ';' | type_specifier ID '[' NUM ']' ';' ; type_specifier : INT | VOID ; fun_declaration : type_specifier ID '(' params ')' compound_stmt ; params : param_list | VOID ; param_list : param_list ',' param | param ; param : type_specifier ID | type_specifier ID '[' ']' ; compound_stmt : '{' local_declarations statement_list '}' ; local_declarations : local_declarations var_declaration | /* empty */ ; statement_list : statement_list statement | /* empty */ ; statement : expression_stmt | compound_stmt | selection_stmt | iteration_stmt | return_stmt ; expression_stmt : expression ';' | ';' ; selection_stmt : IF '(' expression ')' statement %prec LOWER_THAN_ELSE ; | IF '(' expression ')' statement ELSE statement ; iteration_stmt : WHILE '(' expression ')' statement ; return_stmt : RETURN ';' | RETURN expression ';' ; expression : var '=' expression | simple_expression ; var : ID | ID '[' expression ']' ; simple_expression : additive_expression relop additive_expression | additive_expression ; relop : LTE | '&lt;' | '&gt;' | GTE | EQUAL | NOTEQUAL ; additive_expression : additive_expression addop term | term ; addop : '+' | '-' ; term : term mulop factor | factor ; mulop : '*' | '/' ; factor : '(' expression ')' | var | call | NUM ; call : ID '(' args ')' ; args : arg_list | /* empty */ ; arg_list : arg_list ',' expression | expression ; </code></pre>
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    1. POHow to find shift/reduce conflict in this yacc file?
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    1. USardsrk
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    1. POHow to find shift/reduce conflict in this yacc file?
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    1. COWhy should I make `ELSE and LOWER_THAN_ELSE ( a pseudo-token ) non associative` in the first place??
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    2. COOh. I had overlooked it. It does not seem necessary as ELSE and LOWER_THAN_ELSE don't have the same precedence.
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    3. CO%nonassoc is necessary. Using only %token won't tell bison anything about the precedence.
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