help with the file input

Question

It is a project of the four fundamental operations of arithmetic
can anyone help me to add the file input and output part
if the the input file is "in.txt"
and the output file is "out.txt"
Thankyou

#include <iostream>
#include <stdio.h>
#include <cstdlib>
#include <stack>
using namespace std ;

int main(){
int N ; scanf("%d",&N) ; getchar() ;
while(N--){
stack<int> house ;
string in ; getline(cin,in) ;
int l=in.size(),ok=0,num=0,ten=1 ;
for(int i=l-1 ; i>=0 ; i--){
if(in[i]-'0'>=0 && in[i]-'9'<=0){
num+=ten*(in[i]-'0') ;
ten*=10 ; ok=1 ;
}
if(in[i] == ' ' && ok){
house.push(num) ;
num=0 ; ok=0 ; ten=1 ;
}

if(isalpha(in[i])){
int in1=house.top() ; house.pop() ;
int in2=house.top() ; house.pop() ;
if(in[i] == 'D'){
int in3=in1+in2 ; house.push(in3) ;
}
else if(in[i] == 'B'){
int in3=in1-in2 ; house.push(in3) ;
}
else if(in[i] == 'L'){
int in3=in1*in2 ; house.push(in3) ;
}
else if(in[i] == 'X'){
int in3=max(in1,in2) ; house.push(in3) ;
}
else if(in[i] == 'N'){
int in3=min(in1,in2) ; house.push(in3) ;
}
i-=2 ;
}
}
printf("%d\n",house.top()) ; house.pop() ;
}

return 0 ;
}

Answer 1

to read/write file in C++, include the <fstream> library and read this: http://www.cplusplus.com/doc/tutorial/files/