Computer programs often interact with the user in some manner. For example:
Web browsers accept inputs from the keyboard and mouse and display its output on the monitor.
Editors for text, images, or audio can take data stored on the user's computer (input), alter it, and put the updated file back into storage (output).
It is important that whatever data is input into an application, it should be valid in terms of type, range and format. This checking is called validation.
This section will discuss the topic of validating data, as well as storing data permanently in files.