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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))