How can I find and replace a line of data in a text file c++

Question

I am trying to find and replace a line of data in a text file in c++. But I honestly have no idea where to start.

I was thinking of using replaceNumber.open("test.txt", ios::in | ios::out | ios_base::beg | ios::app);

To open the file at the beginning and append over it but this doesn't work.

Does anyone know of a way to achieve this task?

Thanks

Edit: My text file is only one line and it contains a number for example 504. The user then specifies a number to subtract then the result of that should replace the original number in the text file.

Answer 1

Yes, you can do this using std::fstream, here's an example implementation. You open the file, iterate over each line in the file, and replace any occurrences of your substring. After replacing the substring, store the line into a vector of strings, close the file, reopen it with std::ios::trunc, and write each line back to the empty file.

std::fstream file("test.txt", std::ios::in);

if(file.is_open()) {
    std::string replace = "bar";
    std::string replace_with = "foo";
    std::string line;
    std::vector<std::string> lines;
    
    while(std::getline(file, line)) {
        std::cout << line << std::endl;
        
        std::string::size_type pos = 0;
        
        while ((pos = line.find(replace, pos)) != std::string::npos){
            line.replace(pos, line.size(), replace_with);
            pos += replace_with.size();
        }
        
        lines.push_back(line);
    }

    file.close();
    file.open("test.txt", std::ios::out | std::ios::trunc);
    
    for(const auto& i : lines) {
        file << i << std::endl;
    }
}