
Data Structure

This illustrates a data structure, a means of storing data within a program.

You may see other such structures in the Data Structures category.
 Illustration of FIFO behavior
Task

Implement a FIFO queue.

Elements are added at one side and popped from the other in the order of insertion.



Operations:

   push   (aka enqueue)    - add element
   pop     (aka dequeue)    - pop first element
   empty                             - return truth value when empty


Errors:

   handle the error of trying to pop from an empty queue (behavior depends on the language and platform)


See

   Queue/Usage   for the built-in FIFO or queue of your language or standard library.


See also

 Array
 Associative array: Creation, Iteration
 Collections
 Compound data type
 Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
 Linked list
 Queue: Definition, Usage
 Set
 Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
 Stack


USING: accessors kernel ;
IN: rosetta-code.queue-definition
 
TUPLE: queue head tail ;
TUPLE: node value next ;
 
: <queue> ( -- queue ) queue new ;
: <node> ( obj -- node ) node new swap >>value ;
 
: empty? ( queue -- ? ) head>> >boolean not ;
 
: enqueue ( obj queue -- )
    [ <node> ] dip 2dup dup empty?
    [ head<< ] [ tail>> next<< ] if tail<< ;
 
: dequeue ( queue -- obj )
    dup empty? [ "Cannot dequeue empty queue." throw ] when
    [ head>> value>> ] [ head>> next>> ] [ head<< ] tri ;