A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list (rose violet buttercup)
:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup
In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.
In this example, the first box, which holds the CAR of the first
cons cell, refers to or holds rose
(a symbol). The second
box, holding the CDR of the first cons cell, refers to the next
pair of boxes, the second cons cell. The CAR of the second cons
cell is violet
, and its CDR is the third cons cell. The
CDR of the third (and last) cons cell is nil
.
Here is another diagram of the same list, (rose violet
buttercup)
, sketched in a different manner:
--------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil
. In other words, nil
is both a symbol
and a list.
Here is the list (A ())
, or equivalently (A nil)
,
depicted with boxes and arrows:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil
Here is a more complex illustration, showing the three-element list,
((pine needles) oak maple)
, the first element of which is a
two-element list:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles
The same list represented in the second box notation looks like this:
-------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ----------------