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elisp: Hash Tables

 
 7 Hash Tables
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 A hash table is a very fast kind of lookup table, somewhat like an alist
 (Association Lists) in that it maps keys to corresponding
 values.  It differs from an alist in these ways:
 
    • Lookup in a hash table is extremely fast for large tables—in fact,
      the time required is essentially _independent_ of how many elements
      are stored in the table.  For smaller tables (a few tens of
      elements) alists may still be faster because hash tables have a
      more-or-less constant overhead.
 
    • The correspondences in a hash table are in no particular order.
 
    • There is no way to share structure between two hash tables, the way
      two alists can share a common tail.
 
    Emacs Lisp provides a general-purpose hash table data type, along
 with a series of functions for operating on them.  Hash tables have a
 special printed representation, which consists of ‘#s’ followed by a
 list specifying the hash table properties and contents.  Creating
 Hash.  (Hash notation, the initial ‘#’ character used in the printed
 representations of objects with no read representation, has nothing to
 do with hash tables.  Printed Representation.)
 
    Obarrays are also a kind of hash table, but they are a different type
 of object and are used only for recording interned symbols (
 Creating Symbols).
 

Menu

 
· Creating Hash       Functions to create hash tables.
· Hash Access         Reading and writing the hash table contents.
· Defining Hash       Defining new comparison methods.
· Other Hash          Miscellaneous.
 

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