elisp: Association List Type

 
 2.3.6.3 Association List Type
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 An “association list” or “alist” is a specially-constructed list whose
 elements are cons cells.  In each element, the CAR is considered a
 “key”, and the CDR is considered an “associated value”.  (In some cases,
 the associated value is stored in the CAR of the CDR.)  Association
 lists are often used as stacks, since it is easy to add or remove
 associations at the front of the list.
 
    For example,
 
      (setq alist-of-colors
            '((rose . red) (lily . white) (buttercup . yellow)))
 
 sets the variable ‘alist-of-colors’ to an alist of three elements.  In
 the first element, ‘rose’ is the key and ‘red’ is the value.
 
    SeeAssociation Lists, for a further explanation of alists and
 for functions that work on alists.  SeeHash Tables, for another
 kind of lookup table, which is much faster for handling a large number
 of keys.