18.1.1 Hash-Table Operations

The next figure lists some defined names that are applicable to hash tables. The following rules apply to hash tables.

• A hash table can only associate one value with a given key. If an attempt is made to add a second value for a given key, the second value will replace the first. Thus, adding a value to a hash table is a destructive operation; the hash table is modified.
• There are four kinds of hash tables: those whose keys are compared with eq, those whose keys are compared with eql, those whose keys are compared with equal, and those whose keys are compared with equalp.
• Hash tables are created by make-hash-table. gethash is used to look up a key and find the associated value. New entries are added to hash tables using setf with gethash. remhash is used to remove an entry. For example:

        (setq a (make-hash-table)) → #<HASH-TABLE EQL 0/120 32536573>
        (setf (gethash 'color a) 'brown) → BROWN
        (setf (gethash 'name a) 'fred) → FRED
        (gethash 'color a) → BROWN, true
        (gethash 'name a) → FRED, true
        (gethash 'pointy a) → NIL, false
  

  In this example, the symbols color and name are being used as keys, and the symbols brown and fred are being used as the associated values. The hash table has two items in it, one of which associates from color to brown, and the other of which associates from name to fred.

• A key or a value may be any object.
• The existence of an entry in the hash table can be determined from the secondary value returned by gethash.