Interaction in programming is described as input/output (I/O). Of course, this communication flows in two directions, one in the form of output, which is information the program provides to outside parties, and the other in the form of input, which is information users provide to the program or information the program reads in from the outside world. It is easy to imagine a multitude of cases where such functionality could be useful. Imagine a program asking the user's name, for instance, or a program asking if it should terminate or ignore an error when something unexpected happens. These aspects are all covered in this chapter.
Interaction can happen not only between a program and a human end user, but can also involve other sources of information. Consider, for example, the fact that so far, whenever you closed and restarted a program, all its previous data values were lost. When writing a budget tracking application, you cannot expect your users to leave the program running indefinitely (what if the power goes out?) or expect them to re-enter all the information once they reopen the program. As such, you will also deal with ways of handling I/O between your program and data sources. This chapter covers the most basic of data sources, namely that of a file.