Info Catalog eintr: Every eintr: Recursive Patterns eintr: Keep

eintr: Accumulate

 
 Recursive Pattern: _accumulate_
 ...............................
 
 Another recursive pattern is called the ‘accumulate’ pattern.  In the
 ‘accumulate’ recursive pattern, an action is performed on every element
 of a list and the result of that action is accumulated with the results
 of performing the action on the other elements.
 
    This is very like the ‘every’ pattern using ‘cons’, except that
 ‘cons’ is not used, but some other combiner.
 
    The pattern is:
 
    • If a list be empty, return zero or some other constant.
    • Else, act on the beginning of the list (the CAR of the list),
         − and combine that acted-on element, using ‘+’ or some other
           combining function, with
         − a recursive call by the function on the rest (the CDR) of the
           list.
 
    Here is an example:
 
      (defun add-elements (numbers-list)
        "Add the elements of NUMBERS-LIST together."
        (if (not numbers-list)
            0
          (+ (car numbers-list) (add-elements (cdr numbers-list)))))
 
      (add-elements '(1 2 3 4))
          ⇒ 10
 
    Making a List of Files Files List, for an example of the
 accumulate pattern.
 

Info Catalog eintr: Every eintr: Recursive Patterns eintr: Keep