CLHS: Section 18.1.1

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## 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.

    clrhash           hash-table-p     remhash
    gethash           make-hash-table  sxhash
    hash-table-count  maphash

Figure 18-1. Hash-table defined names

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- HASH-TABLE-SIZE:INTENDED-ENTRIES
- HASH-TABLE-TESTS:ADD-EQUALP

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