One decision you must make when designing a program is how it will be used. If the person using your program must interact with it, the method of interaction can be important. For example, anytime the user must supply data, that data must be validated or it could cause errors. One way to minimize data errors is to change how the data is provided. In this example, if the data is one of several values that are known when the program is executed, the user can select a menu item instead of typing on the keyboard.
When you design programs to be interactive, you use a different structure than if you design them to be run in unattended batches. Interactive applications behave more like state machines than numerical algorithms, because they perform the actions you request when you request them. You may also find that once you can change what your program is doing while it runs, you will be more likely to experiment with it.
The QuickWin library lets you build simple Windows applications. Because QuickWin is a wrapper around a subset of the Windows API, there are limitations to what you can do, but it can fulfill the requirement of most users. If you need additional capabilities, you can call the Windows API directly rather than using QuickWin to build your program. (For more information, see Using QuickWin). You can also build a graphic user interface in either Microsoft Visual C++ or Visual Basic® that calls your Fortran code.