gethash

gethash Accessor

Syntax:

gethash key hash-table &optional default → value, present-p

(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.

Expanded Reference: gethash

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