% in rotate-yank-pointer
When the kill-ring-yank-pointer points to the
beginning of the kill ring, and the argument passed to
rotate-yank-pointer is 1, the % expression returns 1:
(- length (length kill-ring-yank-pointer))
=> 0
therefore,
(+ arg (- length (length kill-ring-yank-pointer)))
=> 1
and consequently:
(% (+ arg (- length (length kill-ring-yank-pointer)))
length)
=> 1
regardless of the value of length.
As a result of this, the setq kill-ring-yank-pointer expression
simplifies to:
(setq kill-ring-yank-pointer (nthcdr 1 kill-ring))
What it does is now easy to understand. Instead of pointing as it did
to the first element of the kill ring, the
kill-ring-yank-pointer is set to point to the second element.
Clearly, if the argument passed to rotate-yank-pointer is two, then
the kill-ring-yank-pointer is set to (nthcdr 2 kill-ring);
and so on for different values of the argument.
Similarly, if the kill-ring-yank-pointer starts out pointing to
the second element of the kill ring, it length is shorter than the
length of the kill ring by 1, so the computation of the remainder is
based on the expression (% (+ arg 1) length). This means that
the kill-ring-yank-pointer is moved from the second element of
the kill ring to the third element if the argument passed to
rotate-yank-pointer is 1.
Go to the first, previous, next, last section, table of contents.