(setf (gethash key hash-table &optional default) new-value)

hash-table - a hash table.

default - an object. The default is nil.

value - an object.

present-p - a generalized boolean.

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