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(librep.info)Association Lists

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Association Lists
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   An "association list" (or "alist") is a list mapping keys to to.
Each element of the alist is a cons cell, the car of which is the
"key", the cdr the value that it associates to. For example an alist
could look like,

     ((fred . 20)
      (bill . 30))

this alist has two keys, `fred' and `bill' which both associate to an
integer (20 and 30 respectively).

   It is possible to make the associated values lists, this looks like,

     ((fred 20 male)
      (bill 30 male)
      (sue  25 female))

in this alist the symbol `fred' is associated with the list `(20 male)'.

   There are a number of functions which let you interrogate an alist
with a given key for its association.

 - Function: assoc key alist
     This function scans the association list ALIST for the first
     element whose car is `equal' to KEY, this element is then
     returned. If no match of KEY is found false is returned.

          (assoc 'two '((one . 1) (two . 2) (three . 3)))
              => (two . 2)

 - Function: assq key alist
     Similar to the function `assoc' except that the function `eq' is
     used to compare elements instead of `equal'.

     It is not usually wise to use `assq' when the keys of the alist
     may not be symbols--`eq' won't think two objects are equivalent
     unless they are the _same_ object!

          (assq "foo" '(("bar" . 1) ("foo" . 2)))
              => ()
          (assoc "foo" '(("bar" . 1) ("foo" . 2)))
              => ("foo" . 2)

 - Function: rassoc association alist
     This function searches through ALIST until it finds an element
     whose cdr is `equal' to ASSOCIATION, that element is then returned.
     false will be returned if no elements match.

          (rassoc 2 '((one . 1) (two . 2) (three . 3)))
              => (two . 2)

 - Function: rassq association alist
     This function is equivalent to `rassoc' except that it uses `eq'
     to make comparisons.