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case
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one clause and then executing all consequents of that clause. However,
case differs in the mechanism of clause selection.
The lrrst thing case does is to evaluate test·object, to produce an object
called the key object. Then case considers each of the clauses in turn. If
key is eql to any item in the clause, case evaluates the consequents of that
clause as an implicit progn.
If no clause is satisfied, case returns nil.
case returns the value of the last consequent of the clause evaluated, or nil
if there are no consequents to that clause.
The keys in the clauses are not evaluated; they must be literal key values.
It is an error for the same key to appear in more than one clause. TJ:1e order
of the clauses does not affect the behavior of the case construct.
Instead of a test, one can write one of the symbols t and otherwise. A
clause with such a symbol always succeeds and must be the last clause;
this is an exception to the order-independence of clauses.
If there is only one key for a clause, that key can be written in place of a
list of that key, provided that no ambiguity results. Such a "singleton key"
can not be nil (which is confusable with 0, a list of no keys), t, otherwise,
or a cons.
Examples:
(let «num 69»
(case num
«1 2) "math ... ack H )
«3 4) "great now we can count"») => NIL
(1 et «num 3»
(case num
«1 2) "one two")
«3 4 5 6) (princ "numbers") (princ " three") (fresh-line) )
(t "not today"») => numbers three
T
(let «object-one 'candy»
(case object-one
(apple (setq class 'health) "weekdays")
(candy (setq class 'junk) "weekends")
(otherwise (setq class 'unknown) "all week long"») => "weekends"
class => JUNK
For a table of related items: See the section "Conditional Functions" in
Symbolics Common Lisp: Language Concepts.