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I want a clojure data structure that:
(assoc q 0 1) would set the value of the front to 1)Is there something like that in Clojure (unfortunately PersistentQueue doesn't fulfill Nr.3), or should I built it on top of vector?
230
The key difference between a queue and an associative array is how individual elements are added or removed. You have to explicitly "push" to allocate elements to the front or back of a queue, and the elements are always indexed with a value from 0 at the front to the size of the queue minus 1 at the back.
Associative array SystemVerilog When the size of the collection is unknown or the data space is sparse, an associative array is a better option. Dynamic arrays are useful for contiguous collections of variables whose number changes dynamically.
The most frequently used general purpose implementation of an associative array is with a hash table: an array combined with a hash function that separates each key into a separate "bucket" of the array.
A queue is just a data structure, and a mailbox is an higher level concept that is built around a combination of queues and semaphores. If you have only one process reading and writing to the data structure, there is no need to use a mailbox.
Queue has a dynamic and fixed size. Array has a fixed size. Stack has a dynamic and fixed size. Queue can contain elements of different data type.
There isn't a data structure in standard Clojure that will meet these requirements efficiently.
There was some talk on the Clojure-Dev mailing list about using RRB trees for vectors, which would be a great data structure for this:
Not sure how far that has developed - but if you are interested in this kind of data structure then it is definitely worth taking a look at this.
89
If you do not require persistency of the data structure,
you could use java.util.LinkedList in your Clojure programs.
Example:
;;; Creation
user> (import 'java.util.LinkedList)
java.util.LinkedList
user> (def linked-list (LinkedList. [:a :b :c :d :e]))
#'user/linked-list
;;; Pop from the front
user> (.pop ^LinkedList linked-list)
:a
user> linked-list
#<LinkedList [:b, :c, :d, :e]>
;;; Push to the rear, but costly
user> (.addLast ^LinkedList linked-list :x)
nil
user> linked-list
#<LinkedList [:b, :c, :d, :e, :x]>
;;; Assoc (cf. (assoc linked-list 0 :y)
user> (.add ^LinkedList linked-list 0 :y)
nil
user> linked-list
#<LinkedList [:y, :b, :c, :d, :x]>
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