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Find the Length of a List: `length'
-----------------------------------

   You can find out how many elements there are in a list by using the
Lisp function `length', as in the following examples:

     (length '(buttercup))
          => 1
     
     (length '(daisy buttercup))
          => 2
     
     (length (cons 'violet '(daisy buttercup)))
          => 3

In the third example, the `cons' function is used to construct a three
element list which is then passed to the `length' function as its
argument.

   We can also use `length' to count the number of elements in an empty
list:

     (length ())
          => 0

As you would expect, the number of elements in an empty list is zero.

   An interesting experiment is to find out what happens if you try to
find the length of no list at all; that is, if you try to call `length'
without giving it an argument, not even an empty list:

     (length )

What you see, if you evaluate this, is the error message

     Wrong number of arguments: #<subr length>, 0

This means that the function receives the wrong number of arguments,
zero, when it expects some other number of arguments.  In this case,
one argument is expected, the argument being a list whose length the
function is measuring.  (Note that _one_ list is _one_ argument, even
if the list has many elements inside it.)

   The part of the error message that says `#<subr length>' is the name
of the function.  This is written with a special notation, `#<subr',
that indicates that the function `length' is one of the primitive
functions written in C rather than in Emacs Lisp.  (`subr' is an
abbreviation for `subroutine'.)  Note: What Is a Function?,
 for more about subroutines.