data streaming in c++ - c++

Question

I have a program that can generate a random integer every 1/10 second.
Here is the code:
int main()
{
ofstream myfile;
int max;
cout << "Max number: ";
cin >> max;
for (int i = 0; i < max; ++i)
{
myfile.open("test.txt",fstream::app);
myfile << random_int() << "\n";
myfile.close();
this_thread::sleep_for(chrono::milliseconds(100));
}
return 0;
}
int random_int()
{
return rand() % 10;
}
Now the question is, I need to write a program that calculate then output the average in the same rate. If the output of the number generator is:
1
2
3
4
5
The output of the average calculator should be
1
1
2
2
3
Every 1/10 second the program will output a number.
Note: The max number could be from 0 to couple millions. Calculating the average by adding all previous number during the time interval won't be ideal.
I am a sophomore student and a research assistant in a university. This is a simplified version of a problem that I encounter currently. Any suggestion will be greatly appreciated.
Update: Thanks for the help from Fei Xiang and oklar. Yes, remember the previous sum is the only way to make the calculation in time. However, since the random generator output file is changing constantly and the new output is appended to the old outputs, I am not sure how to get the most current data efficiently.

Answer 1

You don't need to calculate all the numbers that has been added each time, you only need to add the last one to a sum variable and divide by the amount of generated numbers.
Say you have:
1
2
3
4
5
Sum variable is 15. If you divide by the amount of numbers which is 5, you'll get the expected output of 3. Continuing, add the number 9 for instance to the sum variable and divide by the amount of generated numbers 6, you'll end up with an average of 4.
The i in your for loop can be used as a counter for the amount of generated numbers. Pseudo code:
sum += randomInt();
avg = sum/i;
EDIT:
I see that you are opening and closing the file each time in the for loop in your post. This can be done outside the loop, which will speed things up. If I understand you correctly, your mission is to generate a random number then calculate the average from the previous numbers and finally append it to the text file? If so, you're on point.
int i_random;
int avg;
int sum = 0;
myfile.open("avg.txt",fstream::app);
for (int i = 1; i < max + 1; ++i)
{
i_random = random_int();
sum += i_random;
avg = sum/i;
myfile << avg << "\n";
this_thread::sleep_for(chrono::milliseconds(100));
}
myfile.close();
See http://www.cplusplus.com/doc/tutorial/files/ for other operators. Check out cppreference for seek and tell if you want to skip to a position in the file.