YACC LEX IF AND ELSE calculator - c

Question

I have a question about this code I would like to create a IF / ELSE in YACC and LEX. but I have difficulties to detect the condition and execute the right block (because it's depend on the condition!)
I have to make a calculator that takes into account the IF and ELSE I have added TOKEN.
expr allows to evaluate an expression
stmt and stmtlist allows nesting.
%%
// US : liste de ... (commande, assignation, expression)
liste :
| liste error RC { yyerrok; yyclearin; code (STOP); return 1;}
| liste stmtList RC { printf("stmtList\n"); code((instr_t)printExprCode); code (STOP); return 3;}
| liste cmd RC { code (STOP); return 6;}
;
// US : Commande
cmd :
opCmd { code ((instr_t)*($1->U.pFct));}
| DBG sym { dbgSymbol($2); }
;
// US : Expression algébrique
expr : ENTIER { code2((instr_t)intPush, (instr_t)$1); }
| REEL { code2((instr_t)floPush, (instr_t)$1);}
| IVAR { code3((instr_t)varPush, (instr_t)$1, (instr_t)varEval);}
| FVAR { code3((instr_t)varPush, (instr_t)$1, (instr_t)varEval);}
| UNDEF
| PO expr PF { printf("test&\n, $$=%f"); $$=$2; }
| expr opAlg expr { code ((instr_t)*($2->U.pFct)); }
| SUB expr { code((instr_t)negate); } %prec UNARY_MINUS
| PREDEF PO expr PF { code2((instr_t)predef, (instr_t)$1); }
| PO ENTIER LT ENTIER PF { code3((instr_t)intPush, (instr_t)$1, (instr_t)intPush);}
| expr opCOMP expr { code ((instr_t)*($2->U.pFct)); }
;
assgn :
IVAR AFF expr { code3((instr_t)varPush, (instr_t)$1, (instr_t)varAssign); }
| FVAR AFF expr { code3((instr_t)varPush, (instr_t)$1, (instr_t)varAssign); }
| UNDEF AFF expr { code3((instr_t)varPush, (instr_t)$1, (instr_t)varAssign); }
;
// US : Statement
cond : PO expr PF { printf("Condition");} ;
//cond : PO ENTIER LT ENTIER PF { printf("ok"); if($1<$3){$$=1;}else{$$=0;}};
stmt : expr
| assgn
| IF expr stmt {$$ = code(IF,)}
;
// US : Statement List
stmtList : {data_t d; d.value = 0; push(d);}
| stmt
| stmtList DEL stmtList
| AO stmtList AF
;
// US : Statement List
block :
AO stmtList AF
;
// US : sym, nbr, var, opAlg, opCmc
sym : nbr | var | PREDEF | opAlg | opCmd | opCOMP
;
nbr : ENTIER | REEL
;
var : UNDEF | IVAR | FVAR
;
opAlg : ADD | SUB | MUL | DIV
;
opCmd : PR_TS | PR_TS2 | DBG_TS | DBG_TS2
;
opCOMP : LT | GT | EQ | NE
;
%%

Answer 1

Generally, for this you'll want an "embedded" action in the middle of the rule. Something like:
stmt: IF expr {
// generate code to branch to a label if the preceeding expression was false
} stmtList {
// generate the label that is branched to
}
You can communicate between these two fragments by setting $$ to something in the first action and then using that value (as $3) in the second action. The final action needs to set $$ for the final result of the rule.
For an IF/ELSE, you need something more complex with two labels (one at the end and one just after the ELSE) and two branches (conditional to the else label as with the simple if and unconditional to the end label just before the else label)