Writing from one file to another.

[bostock11]

I'm trying to take data from one file ("predata"), add the data together, and write those sums into another file ("postdata"). I can't seem to figure out how to use the while loop in the code. All help is appreciated.

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main()
{
	ifstream inputFile;
	ofstream outputFile;
	outputFile.open("predata.txt");
	
	outputFile<< 2 << "\t" << 9 << "\n";
	outputFile<< 14 << "\t" << 8 << "\n";
	outputFile<< 4 << "\t" << 21 << "\n";
	outputFile<< 9 << "\t" << 12 << "\n";
	
	
	
	inputFile.open("predata.txt");
	int value1 = 0;
	int value2 = 0;
	int value3 = 0;
	int value4 = 0;
	int value5 = 0;
	int value6 = 0;
	int value7 = 0;
	int value8 = 0;
	while(!outputFile.eof())
	{
	
		
		ifstream outputFile;
		inputFile.open("postdata.txt");
		inputFile>> value1;
		inputFile>> value2;
		cout<< value1 << "\t"<< value2 << "\t" << value1+value2 << "\n";
	
		inputFile>> value3;
		inputFile>> value4;
		cout<< value3 << "\t" << value4 << "\t" << value3+value4 << "\n";
	
		inputFile>> value5;
		inputFile>> value6;
		cout<< value5 << "\t" << value6 << "\t" << value5+value6 << "\n";
	
		inputFile>> value7;
		inputFile>> value8;
		cout<< value7 << "\t" << value8 << "\t" << value7+value8 << "\n";
		inputFile.close();
	}
	
	outputFile.close();
	
	return 0;

}

[OxBADC0DE]

You could simply open the same file for input and output, like this

#include //every necessary headers

int main()
{
std::fstream ioFile( "in_out.txt", std::ios::out );
//start writing to it manually like this below
ioFile << 9 << "\t" << 12 << "\n";
ioFile.close(); //although we can read w/o closing, I personally close it first.

int first_v {}, second_v{};
std::ostringstream ss{};

ioFile.open( "in_out.txt", std::ios::in );
if( ioFile ){ // if we successfully open it
    while( ioFile >> first_v >> second_v ) {
        ss << first_v << " " << second_v << " " << ( first_v + second_v ) << "\n";
    }
}
std::cout << ss.str() << std::endl;

std::ofstream out_file{ "out.txt" };
out_file << ss.str();

return 0;
}