~& vs ~% in FORMAT)?


Where possible, it is desirable to write functions that produce output
as building blocks. In contrast with other languages, which either
conservatively force a newline at various times or require the program
to keep track of whether it needs to force a newline, the Lisp I/O
system keeps track of whether the most recently printed character was
a newline or not. The function FRESH-LINE outputs a newline only if
the stream is not already at the beginning of a line.  TERPRI forces a
newline irrespective of the current state of the stream. These
correspond to the ~& and ~% FORMAT directives, respectively. (If the
Lisp I/O system can't determine whether it's physically at the
beginning of a line, it assumes that a newline is needed, just in case.)

Thus, if you want formatted output to be on a line of its own, start
it with ~& and end it with ~%. (Some people will use a ~& also at the
end, but this isn't necessary, since we know a priori that we're not
at the beginning of a line. The only exception is when ~& follows a
~A, to prevent a double newline when the argument to the ~A is a
formatted string with a newline at the end.) For example, the
following routine prints the elements of a list, N elements per line,
and then prints the total number of elements on a new line:

   (defun print-list (list &optional (elements-per-line 10))
     (fresh-line)
     (loop for i upfrom 1
           for element in list do
       (format t "~A ~:[~;~%~]" element (zerop (mod i elements-per-line))))
     (format t "~&~D~%" (length list)))