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Determining the Element

Among other actions, the else-part of the if expression sets the value of kill-ring-yank-pointer to ARGth-kill-element when the kill ring has something in it and the value of do-not-move is nil.

The code looks like this:

     (nthcdr (mod (- n (length kill-ring-yank-pointer))
                  (length kill-ring))
             kill-ring)))

This needs some examination. Unless it is not supposed to move the pointer, the current-kill function changes where kill-ring-yank-pointer points. That is what the (setq kill-ring-yank-pointer ARGth-kill-element)) expression does. Also, clearly, ARGth-kill-element is being set to be equal to some cdr of the kill ring, using the nthcdr function that is described in an earlier section. (See copy-region-as-kill.) How does it do this?

As we have seen before (see nthcdr), the nthcdr function works by repeatedly taking the cdr of a list—it takes the cdr of the cdr of the cdr ...

The two following expressions produce the same result:

     (setq kill-ring-yank-pointer (cdr kill-ring))
     
     (setq kill-ring-yank-pointer (nthcdr 1 kill-ring))

However, the nthcdr expression is more complicated. It uses the mod function to determine which cdr to select.

(You will remember to look at inner functions first; indeed, we will have to go inside the mod.)

The mod function returns the value of its first argument modulo the second; that is to say, it returns the remainder after dividing the first argument by the second. The value returned has the same sign as the second argument.

Thus,

     (mod 12 4)
       ⇒ 0  ;; because there is no remainder
     (mod 13 4)
       ⇒ 1

In this case, the first argument is often smaller than the second. That is fine.

     (mod 0 4)
       ⇒ 0
     (mod 1 4)
       ⇒ 1

We can guess what the - function does. It is like + but subtracts instead of adds; the - function subtracts its second argument from its first. Also, we already know what the length function does (see length). It returns the length of a list.

And n is the name of the required argument to the current-kill function.

So when the first argument to nthcdr is zero, the nthcdr expression returns the whole list, as you can see by evaluating the following:

     ;; kill-ring-yank-pointer and kill-ring have a length of four
     ;; and (mod (- 0 4) 4) ⇒ 0
     (nthcdr (mod (- 0 4) 4)
             '("fourth line of text"
               "third line"
               "second piece of text"
               "first some text"))

When the first argument to the current-kill function is one, the nthcdr expression returns the list without its first element.

     (nthcdr (mod (- 1 4) 4)
             '("fourth line of text"
               "third line"
               "second piece of text"
               "first some text"))

Incidentally, both kill-ring and kill-ring-yank-pointer are global variables. That means that any expression in Emacs Lisp can access them. They are not like the local variables set by let or like the symbols in an argument list. Local variables can only be accessed within the let that defines them or the function that specifies them in an argument list (and within expressions called by them).