Next: Symbols, Previous: Sequences Arrays Vectors, Up: Top
A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values. It differs from an alist in these ways:
Emacs Lisp (starting with Emacs 21) provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have no read syntax, and print in hash notation, like this:
(make-hash-table) => #<hash-table 'eql nil 0/65 0x83af980>
(The term “hash notation” refers to the initial ‘#’ character—see Printed Representation—and has nothing to do with the term “hash table.”)
Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Creating Symbols).