Intermediate Code Generator for If then else - Yacc Program - Compiler Lab

Program :

(lex Program : intif.l)

ALPHA [A-Za-z]
DIGIT [0-9]
%%
if                 return IF;
then                 return THEN;
else                 return ELSE;
{ALPHA}({ALPHA}|{DIGIT})*    return ID;
{DIGIT}+             {yylval=atoi(yytext); return NUM;}
[ \t]                 ;
\n                yyterminate();
.                 return yytext[0];
%%

(Yacc Program : intif.y)

%token ID NUM IF THEN ELSE
%right '='
%left '+' '-'
%left '*' '/'
%left UMINUS
%%

S : IF '(' E ')'{lab1();} THEN E ';'{lab2();} ELSE E ';'{lab3();}
  ;
E :V '='{push();} E{codegen_assign();}
  | E '+'{push();} E{codegen();}
  | E '-'{push();} E{codegen();}
  | E '*'{push();} E{codegen();}
  | E '/'{push();} E{codegen();}
  | '(' E ')'
  | '-'{push();} E{codegen_umin();} %prec UMINUS
  | V
  | NUM{push();}
  ;
V : ID {push();}
  ;
%%

#include "lex.yy.c"
#include<ctype.h>
char st[100][10];
int top=0;
char i_[2]="0";
char temp[2]="t";

int label[20];
int lnum=0;
int ltop=0;

main()
 {
 printf("Enter the expression : ");
 yyparse();
 }

push()
 {
  strcpy(st[++top],yytext);
 }

codegen()
 {
 strcpy(temp,"t");
 strcat(temp,i_);
  printf("%s = %s %s %s\n",temp,st[top-2],st[top-1],st[top]);
  top-=2;
 strcpy(st[top],temp);
 i_[0]++;
 }

codegen_umin()
 {
 strcpy(temp,"t");
 strcat(temp,i_);
 printf("%s = -%s\n",temp,st[top]);
 top--;
 strcpy(st[top],temp);
 i_[0]++;
 }

codegen_assign()
 {
 printf("%s = %s\n",st[top-2],st[top]);
 top-=2;
 }

lab1()
{
 lnum++;
 strcpy(temp,"t");
 strcat(temp,i_);
 printf("%s = not %s\n",temp,st[top]);
 printf("if %s goto L%d\n",temp,lnum);
 i_[0]++;
 label[++ltop]=lnum;
}

lab2()
{
int x;
lnum++;
x=label[ltop--];
printf("goto L%d\n",lnum);
printf("L%d: \n",x);
label[++ltop]=lnum;
}

lab3()
{
int y;
y=label[ltop--];
printf("L%d: \n",y);
}

Output:

nn@linuxmint ~ $ lex intif.l
nn@linuxmint ~ $ yacc intif.y
nn@linuxmint ~ $ gcc y.tab.c -ll -ly
nn@linuxmint ~ $ ./a.out
Enter the expression : if(k+8) then k=18;else c=s;
t0 = k + 8
t1 = not t0
if t1 goto L1
k = 18
goto L2
L1:
c = s
L2:
nn@linuxmint ~ $