The if
, when
, and unless
constructs
establish conditional control in a loop
. If the test
passes, the succeeding loop clause is executed. If the test does
not pass, the succeeding clause is skipped, and program control
moves to the clause that follows the loop keyword
else
. If the test does not pass and no else
clause is supplied, control is transferred to the clause or
construct following the entire conditional clause.
If conditional clauses are nested, each else
is paired
with the closest preceding conditional clause that has no
associated else
or end
.
In the if
and when
clauses, which are
synonymous, the test passes if the value of form is
true.
In the unless
clause,
the test passes if the value of form is false.
Clauses that follow the test expression can be grouped by using
the loop keyword and
to produce a conditional block consisting of
a compound clause.
The loop keyword it
can be used to refer to the result
of the test expression in a clause.
Use the loop keyword it
in place of the form in a
return
clause or an accumulation clause that is
inside a conditional execution clause.
If multiple clauses are connected with and
, the it
construct must be in the first clause in the block.
The optional loop keyword end
marks the end of the clause. If this
keyword is not supplied, the next loop keyword marks the end. The construct
end
can be used to distinguish the scoping of compound clauses.
;; 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)