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#include <iostream>
#include <fstream>
using namespace std;
int main(){
int a=0,b=0;
string s;
ifstream file(input.txt);
if (file.is_open){
file >> a;
getline(file, s);
   int n=0;
   n = atoi(s.c_str());
}
else cout << " error";
 
 
ofstream file1(output.txt);
 
file.close();
return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:7:15: error: ‘input’ was not declared in this scope
 ifstream file(input.txt);
               ^~~~~
prog.cpp:8:17: error: cannot resolve overloaded function ‘is_open’ based on conversion to type ‘bool’
 if (file.is_open){
                 ^
prog.cpp:17:16: error: ‘output’ was not declared in this scope
 ofstream file1(output.txt);
                ^~~~~~

Standard output is empty