12.7) Exercise 18

Start by reminding yourself of how command line arguments work in C++.
Copy your Exercise 17 solution to a new file. Modify this program by first removing the code that populates the vector with data read from cin. In its place, add code that:

Checks that the argument count argc is equal to 2, terminating the program with the message "Error: filename required" if this is not the case
Opens a file for input, using argument argv[1] as the filename and terminating with the message "Error: cannot open file" if opening the file fails for any reason
Reads numbers from the file and appends them to the vector

Create a small file containing a list of numbers. Run the program on this file and check that it produces the correct output.