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(cl)Accumulation Clauses

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Accumulation Clauses
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These clauses cause the loop to accumulate information about the
specified Lisp FORM.  The accumulated result is returned from the loop
unless overridden, say, by a `return' clause.

`collect FORM'
     This clause collects the values of FORM into a list.  Several
     examples of `collect' appear elsewhere in this manual.

     The word `collecting' is a synonym for `collect', and likewise for
     the other accumulation clauses.

`append FORM'
     This clause collects lists of values into a result list using
     `append'.

`nconc FORM'
     This clause collects lists of values into a result list by
     destructively modifying the lists rather than copying them.

`concat FORM'
     This clause concatenates the values of the specified FORM into a
     string.  (It and the following clause are extensions to standard
     Common Lisp.)

`vconcat FORM'
     This clause concatenates the values of the specified FORM into a
     vector.

`count FORM'
     This clause counts the number of times the specified FORM
     evaluates to a non-`nil' value.

`sum FORM'
     This clause accumulates the sum of the values of the specified
     FORM, which must evaluate to a number.

`maximize FORM'
     This clause accumulates the maximum value of the specified FORM,
     which must evaluate to a number.  The return value is undefined if
     `maximize' is executed zero times.

`minimize FORM'
     This clause accumulates the minimum value of the specified FORM.

   Accumulation clauses can be followed by `into VAR' to cause the data
to be collected into variable VAR (which is automatically `let'-bound
during the loop) rather than an unnamed temporary variable.  Also,
`into' accumulations do not automatically imply a return value.  The
loop must use some explicit mechanism, such as `finally return', to
return the accumulated result.

   It is legal for several accumulation clauses of the same type to
accumulate into the same place.  From Steele:

     (loop for name in '(fred sue alice joe june)
           for kids in '((bob ken) () () (kris sunshine) ())
           collect name
           append kids)
          => (fred bob ken sue alice joe kris sunshine june)