Extend the program so that it can deal with single digit numbers of any value. A single digit number is one that consists only of thousands, hundreds, tens, or units. Thus **LXX** (70) and **CD** (400) are single digit numbers, but **XIV** (14) and **MC** (1100) are not. Use the same approach as for units digits, but with 4 different arrays, one each for the thousands, hundreds, tens, and units digits. Try looking for thousands digits first, then for hundreds, and so on. When you find a match in one of the arrays, print the corresponding value and stop.
Modify the program so that it reads and converts all input numbers until end of file (eof) on standard input. You will probably be able to do this by simply adding an appropriate reading loop around the code that reads a single line.
You are checking the THIRD command-line argument, not the second. Arrays are zero-based in C/C++.
Are you sure you want to require at least that many arguments, 3 or more? my_app 12345 makes more sense if you want command-line input.
Getting input via std::cin makes more sense for getting repeated input via a loop for testing purposes. Easily rewritten for file input after done testing with console input. No need to change the loop.