How to write to a text file without deleting what's already in there

[mvvdsteen]

Hello all,

I'm quite new to c++. I made a small program that will help me analyse wind tunnel data. But now I want this program to write to a text file. This works just fine, except it discards whatever is in the textfile at that time.

I open the file, and then I go to the end, and write the stuff. I came up with the following code:

ofstream logfile;
logfile.open ("HWlog.txt");
logfile.seekp (0, ios::end);
logfile << " " << endl;
logfile << " Average speed: \t \t \t" << average_speed << "\t [m/s ]" << endl;
logfile << " Turbulence level: \t \t \t" << turb_level << " %" << endl;
logfile << " ----------------------------------------------------------- " << endl;
logfile.close();

I would also like to add time and date to these lines. I searched but I couldn't find a way of doing this. I sure there is some convenient predefined function to generate a string that contains time and date, but what is it?

Thanks in advance for your help!

Mac

[ragumaniraj]

Hello all,

I'm quite new to c++. I made a small program that will help me analyse wind tunnel data. But now I want this program to write to a text file. This works just fine, except it discards whatever is in the textfile at that time.

I open the file, and then I go to the end, and write the stuff. I came up with the following code:

ofstream logfile;
logfile.open ("HWlog.txt");
logfile.seekp (0, ios::end);
logfile << " " << endl;
logfile << " Average speed: \t \t \t" << average_speed << "\t [m/s ]" << endl;
logfile << " Turbulence level: \t \t \t" << turb_level << " %" << endl;
logfile << " ----------------------------------------------------------- " << endl;
logfile.close();

I would also like to add time and date to these lines. I searched but I couldn't find a way of doing this. I sure there is some convenient predefined function to generate a string that contains time and date, but what is it?

Thanks in advance for your help!

Mac

hi!
you must open the file in edit mode.
and then move the filepointer to the end of the file.
and then write the new contents whatever you want.

[mvvdsteen]

Thanks! It works magnificently.

Meanwhile, I solved the time and date issue (came across it sort of by accident). I'm affraid, however, that one more issue just popped up.

I'm using the code below to read in the data entries. However, for some dark reason the line in the data file is read twice. In this case it isn't a big deal because I have > 6000 points, so having the last point twice doesn't really change the outcome of the statistical analysis, but I'd still like to get it right. Should I first go over the file so see how many lines it has, and then make it a for loop (from 0 to amount of lines -1?) This will probably add unnecessary operations to my file, or not?

while(!file_to_read.eof()) {
float time,volt,speed;
file_to_read >> time >> volt >> speed;
// Store each value into the appropriate array
time_array.push_back(time);
volt_array.push_back(volt);
speed_array.push_back(speed);
sum_speed=sum_speed+speed;
}

[mvvdsteen]

By now I also solved the problem with the double last entry. I did this by implementing a while loop which only saves the values if the time entry is greater than the previous one. It's probably not the most efficient way but it works.

This leaves me with just one problem, which I really can't solve. That's the problem of the user defined file name. I.e., I'd like to be able to provide a name for the data file which must be read. The program accepts the string without complaining (and a char too), but it won't accept is as a file name.

So what I have now is:
ifstream file_to_read("HW.txt");

And what I'd like is something like
ifstream file_to_read(user_defined);
And be able to input the user_defined via the prompt.

Anybody any idea?

All help is greatly appreciated!

Mac

[toefraz]

Just split the the declaration in two:

ifstream file_to_open;
file_to_open.open(c_str);