C++ | How Do I Write to the Top of the Next Column in a CSV File After Finishing an Iteration in a for-loop?

I am currently working on a school project that requires me to test algorithms multiple times and calculate average times. I would like to dump all of my data into a CSV file and make a graph of the average runtimes after everything is said and done. My problem is that I'm not entirely sure about the syntax of it all. Here is what I have so far:

//test number
            for (int a = 1; a <= 40; ++a)
            {
                //run function 41 times per test and take data
                for (int i = 0; i <= 200; i += 5)
                {
                    int vector_size;

                    if (i == 0)
                        vector_size = 10;
                    else
                        vector_size = 10 * i;

                    std::vector<int> medians_vector = make_vector(vector_size);
                    auto time_before_function = std::chrono::high_resolution_clock::now();
                    //function call
                    auto time_after_function = std::chrono::high_resolution_clock::now();


                    execution_time = std::chrono::duration_cast<std::chrono::microseconds>(time_after_function - time_before_function).count();
                    if (a == 1)
                    {
                        csv_files.open("vector.size-time.csv", std::ios::app);
                        csv_files << vector_size << ", " << execution_time << std::endl;
                        csv_files.close();
                        csv_files.clear();
                    }
                    else
                    {
                        csv_files.open("vector.size-time.csv", std::ios::app | std::ios::end);
                        csv_files << vector_size << ", " << execution_time << std::endl;
                        csv_files.close();
                        csv_files.clear();
                    }

                    total_time += execution_time;
                    ++iterations;

                }
            }
            
            
            
            average_time = total_time / iterations;
            std::cout << "Merge_select took a total time of ";
            print_time(total_time);
            print_average_time(average_time, iterations);

As you can see, I have most of the logic down. All I need is the loop or logic to get me onto the next column. Something has to be done here:

else
                    {
                        csv_files.open("vector.size-time.csv", std::ios::app | std::ios::end);
                        csv_files << vector_size << ", " << execution_time << std::endl;
                        csv_files.close();
                        csv_files.clear();
                    }

That's all I know. I was hoping there was an ios::start function or something like that, but there is not.

With all of this being said, how do I make a proper CSV table with all test times in their respective columns?

Thank you,

--Matt