Common LISP Hints: Printing

## 8. Printing

Some functions can cause output. The simplest one is `print`, which prints its argument and then returns it.

> `

>     > (print 3)
>     3
>     3

> `

The first 3 above was printed, the second was returned.

If you want more complicated output, you will need to use `format`. Here's an example:

> `

>     > (format t "An atom: ~S~%and a list: ~S~%and an integer: ~D~%"
>               nil (list 5) 6)
>     An atom: NIL
>     and a list: (5)
>     and an integer: 6

> `

The first argument to format is either `t`, `nil`, or a stream. `T` specifies output to the terminal. `Nil` means not to print anything but to return a string containing the output instead. Streams are general places for output to go: they can specify a file, or the terminal, or another program. This handout will not describe streams in any further detail.

The second argument is a formatting template, which is a string optionally containing formatting directives.

All remaining arguments may be referred to by the formatting directives. LISP will replace the directives with some appropriate characters based on the arguments to which they refer and then print the resulting string.

Format always returns `nil` unless its first argument is `nil`, in which case it prints nothing and returns the string instead of printing it.

There are three different directives in the above example: `~S`, `~D`, and `~%`. The first one accepts any LISP object and is replaced by a printed representation of that object (the same representation which is produced by print). The second one accepts only integers. The third one doesn't refer to an argument; it is always replaced by a carriage return.

Another useful directive is `~~`, which is replaced by a single ~.

Refer to a LISP manual for (many, many) additional formatting directives.