### 11.3 The Association Table

MIT/GNU Scheme provides a generalization of the property-list mechanism found in most other implementations of Lisp: a global two-dimensional association table. This table is indexed by two keys, called x-key and y-key in the following procedure descriptions. These keys and the datum associated with them can be arbitrary objects. `eq?` is used to discriminate keys.

Think of the association table as a matrix: a single datum can be accessed using both keys, a column using x-key only, and a row using y-key only.

procedure: 2d-put! x-key y-key datum

  Makes an entry in the association table that associates datum with x-key and y-key. Returns an unspecified result.

procedure: 2d-remove! x-key y-key

  If the association table has an entry for x-key and y-key, it is removed. Returns an unspecified result.

procedure: 2d-get x-key y-key

  Returns the datum associated with x-key and y-key. Returns `#f` if no such association exists.

procedure: 2d-get-alist-x x-key

  Returns an association list of all entries in the association table that are associated with x-key. The result is a list of `(y-key . datum)` pairs. Returns the empty list if no entries for x-key exist.

      (2d-put! 'foo 'bar 5)
      (2d-put! 'foo 'baz 6)
      (2d-get-alist-x 'foo)           ⇒  ((baz . 6) (bar . 5))

procedure: 2d-get-alist-y y-key

  Returns an association list of all entries in the association table that are associated with y-key. The result is a list of `(x-key . datum)` pairs. Returns the empty list if no entries for y-key exist.

      (2d-put! 'bar 'foo 5)
      (2d-put! 'baz 'foo 6)
      (2d-get-alist-y 'foo)           ⇒  ((baz . 6) (bar . 5))