
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


def (queue seq)
  (tag queue (list seq lastcons.seq len.seq))
 
def (enq x q)
  do1 x
    let (l last len) rep.q
      rep.q.2 <- (len + 1)
      if no.l
        rep.q.1 <- (rep.q.0 <- list.x)
        rep.q.1 <- (cdr.last <- list.x)
 
def (deq q)
  let (l last len) rep.q
    ret ans car.l
      unless zero?.len
        rep.q.2 <- (len - 1)
      rep.q.0 <- cdr.l
 
def (len q) :case (isa queue q)
  rep.q.2