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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 |
| | | | | |
-------------- ----------------
|
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Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.