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`nthcdr`

The `nthcdr`

function is associated with the `cdr`

function.
What it does is take the cdr of a list repeatedly.

If you take the cdr of the list ```
(pine fir
oak maple)
```

, you will be returned the list `(fir oak maple)`

. If you
repeat this on what was returned, you will be returned the list
`(oak maple)`

. (Of course, repeated cdring on the original
list will just give you the original cdr since the function does
not change the list. You need to evaluate the cdr of the
cdr and so on.) If you continue this, eventually you will be
returned an empty list, which in this case, instead of being shown as
`()`

is shown as `nil`

.

For review, here is a series of repeated cdrs, the text following
the ‘`⇒`’ shows what is returned.

(cdr '(pine fir oak maple)) ⇒(fir oak maple) (cdr '(fir oak maple)) ⇒ (oak maple) (cdr '(oak maple)) ⇒(maple) (cdr '(maple)) ⇒ nil (cdr 'nil) ⇒ nil (cdr ()) ⇒ nil

You can also do several cdrs without printing the values in between, like this:

(cdr (cdr '(pine fir oak maple))) ⇒ (oak maple)

In this example, the Lisp interpreter evaluates the innermost list first.
The innermost list is quoted, so it just passes the list as it is to the
innermost `cdr`

. This `cdr`

passes a list made up of the
second and subsequent elements of the list to the outermost `cdr`

,
which produces a list composed of the third and subsequent elements of
the original list. In this example, the `cdr`

function is repeated
and returns a list that consists of the original list without its
first two elements.

The `nthcdr`

function does the same as repeating the call to
`cdr`

. In the following example, the argument 2 is passed to the
function `nthcdr`

, along with the list, and the value returned is
the list without its first two items, which is exactly the same
as repeating `cdr`

twice on the list:

(nthcdr 2 '(pine fir oak maple)) ⇒ (oak maple)

Using the original four element list, we can see what happens when
various numeric arguments are passed to `nthcdr`

, including 0, 1,
and 5:

;; Leave the list as it was. (nthcdr 0 '(pine fir oak maple)) ⇒ (pine fir oak maple) ;; Return a copy without the first element. (nthcdr 1 '(pine fir oak maple)) ⇒ (fir oak maple) ;; Return a copy of the list without three elements. (nthcdr 3 '(pine fir oak maple)) ⇒ (maple) ;; Return a copy lacking all four elements. (nthcdr 4 '(pine fir oak maple)) ⇒ nil ;; Return a copy lacking all elements. (nthcdr 5 '(pine fir oak maple)) ⇒ nil