gethash (Accessor)

Syntax:

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

Arguments and Values:

key—an object.

hash-table—a hash table.

default—an object. The default is nil.

value—an object.

present-p—a generalized boolean.

Description:

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.

Examples:

 (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

See Also:

remhash

Notes:

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.