Most programs need to communicate with something outside themselves in order to be useful. The ``outside world'' can be a human being, or a data file, or a printer. The process of retrieving data from that outside world is called input, and the process of sending messages to the outside world is called output. These two concepts are often grouped together and abbreviated as I/O which stands for input/output.
Communicating with the outside world can be a very complicated task for a program. It can require a lot of programmer effort to get the output from a program to look just right or to handle all of the various problems that might arise. For example, a program that sends output to a printer might want to do something special when the printer runs out of paper.
We have discussed about everything in rbtjnts.f except for how the program ask for input. In a previous chapter we had discussed output A program that neither asks for input nor prints out its answers isn't very useful, so we'd better talk about these things now.