Chapter 7

Chapter 7: Priority Queues - I

A priority queue is an ADT for storing a collection of prioritized elements that supports arbitrary element insertion but supports removal of elements in order of priority; that is, the element with first priority can be removed at any time.
Examples:

Standby passenger for a fully booked flight - priority here is measured in terms of the fare paid, frequent-flyer status, and check-in time.
In any operating system, each process is assumed a priority and the scheduler will always choose a process with higher priority over one of lower priority.

The priority is typically a numerical value - the smallest numerical value should have first (highest) priority.

7.1.1 Keys, Priorities, and Total Order Relations
We define a key to be an object that is assigned to an element as a specific attribute for that element and that can be used to identify, rank, or weight that element.

Comparing Keys with Total Orders

Keys in a priority queue can be arbitrary objects on which an order is defined
Two distinct items in a priority queue can have the same key
Mathematical concept of total order relation ≤ (a rule for comparing keys)

Reflexive property: x ≤ x
Antisymmetric property: x ≤ y ∧ y ≤ x ⇒ x = y
Transitive property: x ≤ y ∧ y ≤ z ⇒ x ≤ z

Linear ordering relationship among a set of keys (no contradictions)
Smallest key ≤ any key in our set

A priority queue stores a collection of items (cannot use positions!)
An item is a pair (k,e)
Main methods of the Priority Queue ADT

insertItem(k,e) inserts an item with key k and element e
removeMin() removes the item with the smallest key

Additional methods

minKey() returns, but does not remove, the smallest key of an item
minElement() returns, but does not remove, the element of an item with smallest key
size(), isEmpty()

Applications:

Standby flyer's

Example 7.1: Suppose a certain flight is fully booked an hour prior to departure. Because of possibility of cancellations, the airline maintains a priority queue of standby passengers hoping to get a seat. The priority of each passenger is determined by the fare paid, the frequent-flyer status, and the time when the passenger is inserted into the priority queue. When a passenger requested to fly standby, the associated passenger object is inserted into the priority queue with an insertItem operation. Shortly before the flight departure, if seats become available (for example, due to last-minute cancellations), the airline repeatedly removes, from the priority queue, a standby passenger with first priority, using a combination of minElement() and removeMin() operations, and lets this person board.

7.1.2 Sorting with a Priority Queue

We can use a priority queue to sort a set of comparable elements (total order in the set of elements)

Insert the elements one by one with a series of insertItem(e,e) operations
Remove the elements in sorted order with a series of removeMin() operations

The running time of this sorting method depends on the priority queue implementation

Algorithm PriorityQueueSort(S, P):
Input: A sequence S storing n elements, on which a total order relation is
    defined, and a priority queue P, that compares keys using the same total
    order relation
Output: The sequence S sorted by the total order relation
while !S.isEmpty() do
   e <- S.removeFirst()   {remove an element e from S}
   P.insertItem(e,e)         {the key is the element itself}
while !P.isEmpty() do
   e <- P.minElement()    {get a smallest element from P}
   P.removeMin()               {remove this element from P}
   S.insertLast(e)             {add the element at the end of S}

The running time of the algorithm is determined by the running times of operations insertItem(e,e), minElement(), and removeMin(), which do depend how P is implemented.
Example 7.2:

Operation	Output	Priority Queue
insertItem(5,A)	-	{(5,A)}
insertItem(9,C)	-	{(5,A),(9,C)}
insertItem(3,B)	-	{(3,B),(5,A),(9,C)}
insertItem(7,D)	-	{(3,B),(5,A),(7,D),(9,C)}
minElement()	B	{(3,B),(5,A),(7,D),(9,C)}
minKey()	3	{(3,B),(5,A),(7,D),(9,C)}
removeMin()	-	{(5,A),(7,D),(9,C)}
size()	3	{(5,A),(7,D),(9,C)}
minElement()	A	{(5,A),(7,D),(9,C)}
removeMin()	-	{(7,D),(9,C)}
removeMin()	-	{(9,C)}
removeMin()	-	{}
removeMin()	ERROR	{}
isEmpty()	true	{}

There are still two important issues that we have left undetermined to this point:

How do we keep track of the associations between elements and their keys?
How do we compare keys so as to determine a smallest key?

7.1.4 Compositions and Comparators

The Composition Pattern

The composition pattern defines a single object that is a composition of other objects.
A pair (k,e) is the simplest composition, for it composes two objects, k and e, into a single pair object.

html-7.2 (Item)

The Comparator Pattern

A comparator encapsulates the action of comparing two objects according to a given total order relation
When the priority queue needs to compare two keys, it uses its comparator
A generic priority queue uses a comparator as a template argument, to define the comparison function (<, =, >)
Example: Lexicographical less than operator:

bool operator<(const Point& p1, const Point& p2)
{ if (p1.getX() == p2.getX()) return p1.getY() < p2.getY();
  else return p1.getX() < p1.getX();
}

The comparator is external to the keys being compared. Thus, the same objects can be sorted in different ways by using different comparators.
Assume that comp is a comparison class object. comp(a,b) returns integer i, such that

i < 0 if a < b,
i = 0 if a = b, and
i > 0 if a > b.

We use special comparator object, external to the keys, to supply the comparison rules.
The approach that we will use is to overload "()" operator (to create so called function object).

Example: Compare two points in the plane lexicographically.

class LexCompare {
public:
   int operator()(Point a, Point b) {
   if (a.x < b.x) return –1
   else if (a.x > b.x) return +1

   else if (a.y < b.y) return –1
   else if (a.y > b.y) return +1
   else return 0;
   }
};

html-7.3 (Compare)

Using Comparator Objects

To use the comparator, define an object of this type, and invoke it using its “()” operator:
Example of usage:

Point p(2.3, 4.5);
Point q(1.7, 7.3);
LexCompare lexCompare;
if (lexCompare(p, q) < 0) cout << “p less than q”;
else if (lexCompare(p, q) == 0) cout << “p equals q”;
else if (lexCompare(p, q) > 0) cout << “p greater than q”;

html-7.4 (Generic)

7.2 Implementing a Priority Queue with a Sequence ADT

7.2.1 Implementation with an Unsorted Sequence

Add the new pair object p = (k,e) at the end of sequence S, by executing function insertLast(p) on S - takes O(1) time, independent of whether the sequence is implemented using an array or a linked list.
To perform operations minElement(), minKey() or removeMin() on P, we must inspect all the elements of sequence S to find an element p = (k,e) of S with minimal k - takes O(n) time.

7.2.2 Implementation with a Sorted Sequence

We can implement functions minElement()or minKey() on P simply by accessing the first element of the sequence with the first() function of S. We can implement removeMin() function of P as S.remove(S.first()). O(1)
The function insertItem(k,e) of P requires that we scan through the sequence S to find the appropriate position to insert the element and key. O(n)

html-7.5 (SSPQ1)
html-7.6 (SSPQ2)

[priority.cpp]

Comparing the Two Implementations

Implementation with an unsorted sequence Store the items of the priority queue in a list-based sequence, in arbitrary order	Implementation with a sorted sequence Store the items of the priority queue in a sequence, sorted by key
Performance: insertItem takes O(1) time since we can insert the item at the beginning or end of the sequence removeMin, minKey and minElement take O(n) time since we have to traverse the entire sequence to find the smallest key	Performance: insertItem takes O(n) time since we have to find the place where to insert the item removeMin, minKey and minElement take O(1) time since the smallest key is at the beginning of the sequence

7.2.3 Selection-Sort and Insertion-Sort

Selection-Sort

Selection-sort is the variation of PriorityQueueSort where the priority queue is implemented with an unsorted sequence
Running time of Selection-sort:

Inserting the elements into the priority queue with n insertItem operations takes O(n) time
Removing the elements in sorted order from the priority queue with n removeMin operations takes time proportional to 1 + 2 + …+ n

Selection-sort runs in O(n²) time

	Sequence S	Priority Queue P (unsorted sequence)
Input	(7,4,8,2,5,3,9)	()
Phase 1 O(n)	(4,8,2,5,3,9) (8,2,5,3,9) (2,5,3,9) (5,3,9) (3,9) (9) ()	(7) (7,4) (7,4,8) (7,4,8,2) (7,4,8,2,5) (7,4,8,2,5,3) (7,4,8,2,5,3,9)
Phase 2 O(n^₂)	(2) (2,3) (2,3,4) (2,3,4,5) (2,3,4,5,7) (2,3,4,5,7,8) (2,3,4,5,7,8,9)	(7,4,8,5,3,9) (7,4,8,5,9) (7,8,5,9) (7,8,9) (8,9) (9) ()

Insertion-Sort

Insertion-sort is the variation of PriorityQueueSort where the priority queue is implemented with a sorted sequence
Running time of Insertion-sort:

Inserting the elements into the priority queue with n insertItem operations takes time proportional to
1 + 2 + …+ n
Removing the elements in sorted order from the priority queue with a series of n removeMin operations takes O(n) time

Insertion-sort runs in O(n²) time

	Sequence S	Priority Queue P (sorted sequence)
Input	(7,4,8,2,5,3,9)	()
Phase 1 O(n^₂)	(4,8,2,5,3,9) (8,2,5,3,9) (2,5,3,9) (5,3,9) (3,9) (9) ()	(7) (4,7) (4,7,8) (2,4,7,8) (2,4,5,7,8) (2,3,4,5,7,8) (2,3,4,5,7,8,9)
Phase 2 O(n)	(2) (2,3) (2,3,4) (2,3,4,5) (2,3,4,5,7) (2,3,4,5,7,8) (2,3,4,5,7,8,9)	(3,4,5,7,8,9) (4,5,7,8,9) (5,7,8,9) (7,8,9) (8,9) (9) ()