ANSI Common Lisp 6 Iteration 6.1 The LOOP Facility 6.1.6 Conditional Execution Clauses
6.1.6.1 Examples of WHEN clause
;; Signal an exceptional condition.
(loop for item in '(1 2 3 a 4 5)
when (not (numberp item))
return (cerror "enter new value" "non-numeric value: ~s" item))
Error: non-numeric value: A
;; The previous example is equivalent to the following one.
(loop for item in '(1 2 3 a 4 5)
when (not (numberp item))
do (return
(cerror "Enter new value" "non-numeric value: ~s" item)))
Error: non-numeric value: A
;; This example parses a simple printed string representation from
;; BUFFER (which is itself a string) and returns the index of the
;; closing double-quote character.
(let ((buffer "\"a\" \"b\""))
(loop initially (unless (char= (char buffer 0) #\")
(loop-finish))
for i of-type fixnum from 1 below (length (the string buffer))
when (char= (char buffer i) #\")
return i))
2
;; The collected value is returned.
(loop for i from 1 to 10
when (> i 5)
collect i
finally (prin1 'got-here))
GOT-HERE
(6 7 8 9 10)
;; Return both the count of collected numbers and the numbers.
(loop for i from 1 to 10
when (> i 5)
collect i into number-list
and count i into number-count
finally (return (values number-count number-list)))
5, (6 7 8 9 10)
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