Replacing the data in a specific row in the txt file - c++

Question

I'm building a TCP Server application. Two different types of requests come from the client. In the first request type, the client requests a certain number of line information starting from a certain line. For example, the client throws a request like I want to read 3 lines starting at line 5.It's okay so far, I've completed this part, but now let's come to the part where I have difficulty.In the second type of request, the client wants to change the characters in a certain number of lines starting from a certain line.For example change 4 rows starting at row 6.txt file consists of 1s and 0s.txt file is as follows.
1
0
1
0
1
The txt file should be as follows when two line change requests are received, starting from the fourth line.
1
0
1
1
0
I don't use index numbers to identify line numbers, instead I first read the entire file with a loop and push the read data into a vector at each iteration. Thus, when I want to read a particular line, I call the desired index of that vector.
The parts of the code that interest us are shown below.
std::fstream myfile("fc3-4.txt", std::ios_base::in);
int reg;
std::vector<int> registers_vec;
while (myfile >> reg)
{
registers_vec.push_back(reg);
}
As a result of my research, I found to delete a specific data, but this does not work for me because for example, if I write 1 to the value to be deleted, it deletes all the 1s in the txt file.How can I replace a certain number of rows starting from a certain row as I mentioned above.Thanks in advance

Answer 1

First read the complete file into memory. Then close the file.
Make the modifications in memory.
Then, after modifications have been done, write the complete vector to the file.
E.g.:
#include <iostream>
#include <fstream>
#include <vector>
#include <iterator>
int main() {
// Open the file and check, if it could be opened
if (std::ifstream dataStream{ "fc3-4.txt" }; dataStream) {
// Read all vallues in vector data
std::vector data(std::istream_iterator<int>(dataStream), {});
// And close the file
dataStream.close();
// Do any modification. Example flip bits for line 6 to 8
for (size_t line{ 6 }; data.size() > 8 and line < 8; ++line)
data[line] = 1 - data[line];
// Write all file to file
{
if (std::ofstream dataStream{ "fc3-4.txt" }; dataStream) {
// Write all data to file
std::copy(data.begin(), data.end(), std::ostream_iterator<int>(dataStream, "\n"));
}
else std::cerr << "\nError, Cannot open file for writing\n";
}
}
else std::cerr << "\nError, Cannot open file for reading\n";
}