C++: How can I switch between input and output from file?

Question

I'm trying to write up a program that will display the contents of a text file to the screen for a user. Specifically, the text file will be a list of names that the program will read and display each name to the user individually. The user will then either like the name and keep it or dislike the name and remove it.

My dilemma is: if the user elects to keep the name, the program will need to go from reading the file to "writing" (deleting the name) the file and then back to reading the file again! I found the following relevant code on http://www.tutorialspoint.com/cplusplus/cpp_files_streams.htm . It shows that one must use .close() to switch from reading to writing, but this seems funky to a newbie like me. Is there a better way to do it or is the code below just fine?

#include <fstream>
#include <iostream>
using namespace std;

int main ()
{

   char data[100];

   // open a file in write mode.
   ofstream outfile;
   outfile.open("afile.dat");

   cout << "Writing to the file" << endl;
   cout << "Enter your name: "; 
   cin.getline(data, 100);

   // write inputted data into the file.
   outfile << data << endl;

   cout << "Enter your age: "; 
   cin >> data;
   cin.ignore();

   // again write inputted data into the file.
   outfile << data << endl;

   // close the opened file.
   outfile.close();

This is where the file goes from write mode to read mode.

   // open a file in read mode.
   ifstream infile; 
   infile.open("afile.dat"); 

   cout << "Reading from the file" << endl; 
   infile >> data; 

   // write the data at the screen.
   cout << data << endl;

   // again read the data from the file and display it.
   infile >> data; 
   cout << data << endl; 

   // close the opened file.
   infile.close();

   return 0;
}

Also, I'm having a hard time finding how to read and modify individual characters in the file. I need to do this too, as the file needs to follow a specific pattern, with five names per line and one space between each name (newline at end of fifth name, obviously). Help with this would be appreciated.

Answer 1

Changing things mid-file is complicated .

What I would do is either create a temporary file, write the kept names to that file and replace the original file with this temporary file, (or just store the kept names in a vector and rewrite the file)

Answer 2

try

std::fstream ff("io.txt", std::fstream::in | std::fstream::out);

and fstream::read, fstream::write, fstream::seekg

Answer 3

If the file is small then load it into memory. Otherwise use fopen with "a+" or "r+" mode and fseek .