LexYacc program gives error including implicit declaration of 'yylex' - c++

Question

I am studying compilers and studying Lex and Yacc. I write a LexYacc code as my teacher shows:
here is exp.l:
/*%option outfile="scanner.cpp"*/
%{
/*#include "exp.tab.h"*/
#include "y.tab.h"
extern int yylval;
%}
%%
0|[1-9][0-9]* { yylval = atoi(yytext); return INTEGER; }
[+*()\n] { return yytext[0]; }
. { /* do nothing */ }
%%
and this is exp.y:
/*
%output "parser.cpp"
%skeleton "lalr1.cc"
*/
%{
#include <stdio.h>
%}
%token INTEGER
%left '+'
%left '*'
%%
input : /* empty string */
| input line
;
line : '\n'
| exp '\n' { printf ("\t%d\n", $1); }
| error '\n'
;
exp : INTEGER { $$ = $1; }
| exp '+' exp { $$ = $1 + $3; }
| exp '*' exp { $$ = $1 * $3; }
| '(' exp ')' { $$ = $2; } ;
%%
main () {
yyparse ();
}
yyerror (char *s) {
printf ("%s\n", s);
}
and I use linux command to run it:
flex exp.l
bison -d exp.y
gcc exp.tab.c lex.yy.c -o exp -lfl
and it shows this:
exp.tab.c: In function ‘yyparse’:
exp.tab.c:1217:16: warning: implicit declaration of function ‘yylex’ [-Wimplicit-function-declaration]
1217 | yychar = yylex ();
|
exp.tab.c:1374:7: warning: implicit declaration of function ‘yyerror’; did you mean ‘yyerrok’? [-Wimplicit-function-declaration]
1374 | yyerror (YY_("syntax error"));
|
| yyerrok
exp.y: At top level:
exp.y:28:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
28 | main () {
|
exp.y:33:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
33 | yyerror (char *s) {
|
exp.l:4:10: fatal error: y.tab.h: No such file or directory
4 | #include "y.tab.h"
|
compilation terminated.
The whole program is mainly a calculator to calculate addition and multiplication.
I don't know what happened and hope someone can help me.