15.25.8 Value Accumulation

Clauses that accumulate values in lists and return them.

• ‘append’,
• ‘appending’,
• ‘collect’,
• ‘collecting’,
• ‘nconc’, and
• ‘nconcing’

Clauses that accumulate and return numerical values

• ‘count’,
• ‘counting’,¹⁸
• ‘maximize’,
• ‘maximizing’,
• ‘minimize’,
• ‘minimizing’,
• ‘sum’, and
• ‘summing’

Accumulation Variable

The preposition keyword

• ‘into’
  • can be used to name the variable used to hold partial accumulations. The variable is bound as if by the loop construct ‘with’ (see section 26.9).
  • If ‘into’ is used, the construct does not provide a default return value; however, the variable is available for use in any ‘finally’ clause.

Combining Accumulations

You can combine value-returning accumulation clauses in a loop if all the clauses accumulate the same type of data object. By default, the Loop Facility returns only one value; thus, the data objects collected by multiple accumulation clauses as return values must have compatible types.

• For example, since both the ‘collect’ and ‘append’ constructs accumulate objects into a list that is returned from a loop, you can combine them safely.

;;; Collect every name and the kids in one list by using
;;; COLLECT and APPEND.
(loop for name in '(fred sue alice joe june)
      for kids in '((bob ken) () () (kris sunshine) ())
      collect name
      append kids)

Multiple clauses that do not accumulate the same type of data object can coexist in a loop only if each clause accumulates its values into a different user-specified variable. Any number of values can be returned from a loop if you use the Common Lisp function values, as the next example shows:

;;; Count and collect names and ages.
(loop for name in '(fred sue alice joe june)
      as age in '(22 26 19 20 10)
      append (list name age) into name-and-age-list
      count name into name-count
      sum age into total-age
      finally
        (return (values (round total-age name-count)
                        name-and-age-list)))
; => 19 and (FRED 22 SUE 26 ALICE 19 JOE 20 JUNE 10)

• collect - collecting
• append - appending - nconc - nconcing
• count - counting
• maximize - maximizing - minimize - minimizing