gethash key hash-table &optional default => value, present-p
(setf (gethash key hash-table &optional default) new-value)
Arguments and Values:
hash-table---a hash table.
default---an object. The default is nil.
present-p---a generalized boolean.
Value is the object in hash-table whose key is the same as key under the hash-table's equivalence test. If there is no such entry, value is the default.
Present-p is true if an entry is found; otherwise, it is false.
setf may be used with gethash to modify the value associated with a given key, or to add a new entry. When a gethash form is used as a setf place, any default which is supplied is evaluated according to normal left-to-right evaluation rules, but its value is ignored.
(setq table (make-hash-table)) => #<HASH-TABLE EQL 0/120 32206334> (gethash 1 table) => NIL, false (gethash 1 table 2) => 2, false (setf (gethash 1 table) "one") => "one" (setf (gethash 2 table "two") "two") => "two" (gethash 1 table) => "one", true (gethash 2 table) => "two", true (gethash nil table) => NIL, false (setf (gethash nil table) nil) => NIL (gethash nil table) => NIL, true (defvar *counters* (make-hash-table)) => *COUNTERS* (gethash 'foo *counters*) => NIL, false (gethash 'foo *counters* 0) => 0, false (defmacro how-many (obj) `(values (gethash ,obj *counters* 0))) => HOW-MANY (defun count-it (obj) (incf (how-many obj))) => COUNT-IT (dolist (x '(bar foo foo bar bar baz)) (count-it x)) (how-many 'foo) => 2 (how-many 'bar) => 3 (how-many 'quux) => 0
Side Effects: None.
Affected By: None.
Exceptional Situations: None.
The secondary value, present-p, can be used to distinguish the absence of an entry from the presence of an entry that has a value of default.