Info Catalog eintr: Build a list eintr: cons

eintr: length

 
 7.2.1 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
 
      Lisp error: (wrong-number-of-arguments 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 ‘length’ is the name of the
 function.
 

Info Catalog eintr: Build a list eintr: cons