Next: Find the Length of a List:
cons must have a list to attach to.11 You
cannot start from absolutely nothing. If you are building a list, you
need to provide at least an empty list at the beginning. Here is a
cons expressions that build up a list of flowers. If
you are reading this in Info in GNU Emacs, you can evaluate each of
the expressions in the usual way; the value is printed in this text
after ‘⇒’, which you may read as “evaluates to”.
(cons 'buttercup ()) ⇒ (buttercup)
(cons 'daisy '(buttercup)) ⇒ (daisy buttercup)
(cons 'violet '(daisy buttercup)) ⇒ (violet daisy buttercup)
(cons 'rose '(violet daisy buttercup)) ⇒ (rose violet daisy buttercup)
In the first example, the empty list is shown as
() and a list
made up of
buttercup followed by the empty list is constructed.
As you can see, the empty list is not shown in the list that was
constructed. All that you see is
(buttercup). The empty list is
not counted as an element of a list because there is nothing in an empty
list. Generally speaking, an empty list is invisible.
The second example,
(cons 'daisy '(buttercup)) constructs a new,
two element list by putting
daisy in front of
and the third example constructs a three element list by putting
violet in front of
Actually, you can
cons an element to an atom to produce a dotted pair. Dotted
pairs are not discussed here; see Dotted
Pair Notation in The GNU Emacs Lisp Reference Manual.