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(emacs-lisp-intro.info)Lists diagrammed

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Lists diagrammed
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   For example, the list `(rose violet buttercup)' has three elements,
`rose', `violet', and `buttercup'.  In the computer, the electronic
address of `rose' is recorded in a segment of computer memory along
with the address that gives the electronic address of where the atom
`violet' is located; and that address (the one that tells where
`violet' is located) is kept along with an address that tells where the
address for the atom `buttercup' is located.

   This sounds more complicated than it is and is easier seen in a
diagram:

         ___ ___      ___ ___      ___ ___
        |___|___|--> |___|___|--> |___|___|--> nil
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup




In the diagram, each box represents a word of computer memory that
holds a Lisp object, usually in the form of a memory address.  The
boxes, i.e. the addresses, are in pairs.  Each arrow points to what the
address is the address of, either an atom or another pair of addresses.
The first box is the electronic address of `rose' and the arrow points
to `rose'; the second box is the address of the next pair of boxes, the
first part of which is the address of `violet' and the second part of
which is the address of the next pair.  The very last box points to the
symbol `nil', which marks the end of the list.

   When a variable is set to a list with a function such as `setq', it
stores the address of the first box in the variable.  Thus, evaluation
of the expression

     (setq bouquet '(rose violet buttercup))

creates a situation like this:

     bouquet
          |
          |     ___ ___      ___ ___      ___ ___
           --> |___|___|--> |___|___|--> |___|___|--> nil
                 |            |            |
                 |            |            |
                  --> rose     --> violet   --> buttercup




In this example, the symbol `bouquet' holds the address of the first
pair of boxes.

   This same list can be illustrated in a different sort of box notation
like this:

     bouquet
      |
      |    --------------       ---------------       ----------------
      |   | car   | cdr  |     | car    | cdr  |     | car     | cdr  |
       -->| rose  |   o------->| violet |   o------->| butter- |  nil |
          |       |      |     |        |      |     | cup     |      |
           --------------       ---------------       ----------------




   (Symbols consist of more than pairs of addresses, but the structure
of a symbol is made up of addresses.  Indeed, the symbol `bouquet'
consists of a group of address-boxes, one of which is the address of
the printed word `bouquet', a second of which is the address of a
function definition attached to the symbol, if any, a third of which is
the address of the first pair of address-boxes for the list `(rose
violet buttercup)', and so on.  Here we are showing that the symbol's
third address-box points to the first pair of address-boxes for the
list.)

   If a symbol is set to the CDR of a list, the list itself is not
changed; the symbol simply has an address further down the list.  (In
the jargon, CAR and CDR are `non-destructive'.)  Thus, evaluation of
the following expression

     (setq flowers (cdr bouquet))

produces this:


     bouquet        flowers
       |              |
       |     ___ ___  |     ___ ___      ___ ___
        --> |   |   |  --> |   |   |    |   |   |
            |___|___|----> |___|___|--> |___|___|--> nil
              |              |            |
              |              |            |
               --> rose       --> violet   --> buttercup





The value of `flowers' is `(violet buttercup)', which is to say, the
symbol `flowers' holds the address of the pair of address-boxes, the
first of which holds the address of `violet', and the second of which
holds the address of `buttercup'.

   A pair of address-boxes is called a "cons cell" or "dotted pair".
Note: List Type, and *Note Dotted Pair Notation:
(elisp)Dotted Pair Notation, for more information about cons cells and
dotted pairs.

   The function `cons' adds a new pair of addresses to the front of a
series of addresses like that shown above.  For example, evaluating the
expression

     (setq bouquet (cons 'lily bouquet))

produces:


     bouquet                       flowers
       |                             |
       |     ___ ___        ___ ___  |     ___ ___       ___ ___
        --> |   |   |      |   |   |  --> |   |   |     |   |   |
            |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil
              |              |              |             |
              |              |              |             |
               --> lily      --> rose       --> violet    --> buttercup





However, this does not change the value of the symbol `flowers', as you
can see by evaluating the following,

     (eq (cdr (cdr bouquet)) flowers)

which returns `t' for true.

   Until it is reset, `flowers' still has the value `(violet
buttercup)'; that is, it has the address of the cons cell whose first
address is of `violet'.  Also, this does not alter any of the
pre-existing cons cells; they are all still there.

   Thus, in Lisp, to get the CDR of a list, you just get the address of
the next cons cell in the series; to get the CAR of a list, you get the
address of the first element of the list; to `cons' a new element on a
list, you add a new cons cell to the front of the list.  That is all
there is to it!  The underlying structure of Lisp is brilliantly simple!

   And what does the last address in a series of cons cells refer to?
It is the address of the empty list, of `nil'.

   In summary, when a Lisp variable is set to a value, it is provided
with the address of the list to which the variable refers.