Two aspects of programs are the following.

• What the customer sees - customer facing - functional requirements.
• What the programmer sees - programmer facing - nonfunctional requirements.

In this course, the customer-facing parts of the program are the input and output statements.

• Customers can be very particular about the output. Every character and space must be what they want.
• The same goes for input prompts and input echo - text for this course, but the graphical user interface in general.