Apropos

gethashAccessor

    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.