I am just learning swift and tried to make a queue class with enqueue and dequeue.
- The node holds the data for each element
- The enqueue adds an element to the "queue"
- The dequeue deletes the first element in the queue and returns the data
- Head and Tail are references to node elements, which also tells if the queue is empty when they are
nil
(null)
Now I am wondering, have I done this correctly, both the theory and can the code be improved in Swift (used to code in Java)?
class Queue {
class Node {
// variables
var data: String
var prev: Node?
init(data: String){
self.data = data
}
}
// variables
var head: Node?
var tail: Node?
func enqueue(e: String){
if head == nil && tail == nil {
head = Node(data: e)
head?.prev = nil
tail = head
} else {
let temp = Node(data: e)
tail?.prev = temp
tail = temp
}
}
func dequeue() -> String{
let result = head?.data
if head === tail {
head = nil
tail = nil
} else {
head = head?.prev
}
if (result == nil){
return "empty"
}
return result!
}
}
Testing the program:
var q: Queue
q = Queue()
q.enqueue("One")
q.enqueue("Two")
q.enqueue("Three")
print(q.dequeue())
print(q.dequeue())
print(q.dequeue())
print(q.dequeue())
Output:
One Two Three empty
1 Answer 1
Your code is correct as far as I can see. However, there are some things which can be improved or simplified.
Let's start with the class Node
definition:
- Strictly speaking,
data
andprev
are properties. prev
is – as I understand it – a pointer to the node that was added after this one, so I would call itnext
instead.- The class should be private because it is not indented to be
used outside of
Queue
. data
should be a constant because it is not mutated after the creation of a node (attributions go to @Feldur who noticed that in a comment).- Minor note: There should be a space before curly braces.
Then we have:
private class Node {
// properties
let data: String
var next: Node?
init(data: String) {
self.data = data
}
}
Consequently, head
and tail
are private properties as well:
// properties
private var head: Node?
private var tail: Node?
The enqueue()
method can be simplified. head
and tail
are both
nil
or not, so there is no need to check both. Here it makes sense
to check the tail via optional binding, so that a new node can be
appended if the list was non-empty, or a single-node list created
otherwise:
func enqueue(e: String) {
let node = Node(data: e)
if let lastNode = tail {
lastNode.next = node
} else {
head = node
}
tail = node
}
The dequeue()
method returns the string "empty" if the queue is
empty. This is a "magic string" and makes it impossible to store the
string "empty" itself in the queue. Swift optionals were make
for this very purpose: The method should return an optional String?
which is nil
if the queue is empty.
The method can be simplified in a similar manner as enqueue()
.
Now we check the initial node via optional binding. Note that there
is no forced unwrapping (!
) anymore, which should generally be
avoided:
func dequeue() -> String? {
if let firstNode = head {
head = firstNode.next
if head == nil {
tail = nil
}
return firstNode.data
} else {
return nil
}
}
In your test code, the queue variable can be defined and initialized in a single statement:
var q = Queue()
Since dequeue()
now returns an optional, we can remove elements in
a loop like this:
q.enqueue("One")
q.enqueue("Two")
q.enqueue("Three")
while let s = q.dequeue() {
print(s)
}
Finally: There is nothing in the implementation which is particular to strings. You can make the class generic so that it can be used with other data types as well. Only minor modifications are necessary:
- Replace
Node
byNode<T>
(and move it outside ofQueue
, as Swift currently does not allow nested types in generic classes). - Replace
Queue
byQueue<T>
. - Replace
String
byT
.
Then it looks like this:
private class Node<T> {
let data: T
var next: Node?
init(data: T) {
self.data = data
}
}
class Queue<T> {
private var head: Node<T>?
private var tail: Node<T>?
func enqueue(e: T) {
let node = Node(data: e)
if let lastNode = tail {
lastNode.next = node
} else {
head = node
}
tail = node
}
func dequeue() -> T? {
if let firstNode = head {
head = firstNode.next
if head == nil {
tail = nil
}
return firstNode.data
} else {
return nil
}
}
}
Test code:
var q = Queue<Int>()
q.enqueue(1)
q.enqueue(2)
q.enqueue(3)
while let i = q.dequeue() {
print(i)
}
-
\$\begingroup\$ Great idea with generic classes, I am still learning Swift and this helped a lot! \$\endgroup\$Sigils– Sigils2016年07月07日 07:53:40 +00:00Commented Jul 7, 2016 at 7:53
-
\$\begingroup\$ 1. Why classes vs structure, which are generally preferred? \$\endgroup\$Feldur– Feldur2016年07月16日 16:25:26 +00:00Commented Jul 16, 2016 at 16:25
-
\$\begingroup\$ 2. Why not make the generic version conform to SewuenceType? \$\endgroup\$Feldur– Feldur2016年07月16日 16:26:30 +00:00Commented Jul 16, 2016 at 16:26
-
\$\begingroup\$ 3. Why are the properties of Node var's when they never need to be mutated? \$\endgroup\$Feldur– Feldur2016年07月16日 16:28:01 +00:00Commented Jul 16, 2016 at 16:28
-
1\$\begingroup\$ I appreciate the replies. They make good sense. \$\endgroup\$Feldur– Feldur2016年07月17日 07:04:05 +00:00Commented Jul 17, 2016 at 7:04