ABSTRACT
Functions that need to communicate interactively with users must present some information to the user and then interpret the response from the user. For writing to the user, the function cat provides for precise control over the output, while print provides neatly formatted displays automatically. Word processing commands can also be used, through the unix interface. The functions cat and print are the basic tools for writing to users. The print function produces an attractive display of an arbitrary S object; it is the function used to automatically print the value of the S expression a person types. Another function for generating user output is format. It takes a single argument and converts it to a character vector, with property that each of elements has the same number of characters. This is often used for getting output labels to line up.
