How to implement priority queue in matlab - matlab

Question

Is there any library in matlab that provides the functionality of min priorityqueue
import java.util.PriorityQueue;
import java.util.*;
public class MyQueue {
Comparator<Double> c;
PriorityQueue<Double> PQ;
public MyQueue() {
c = new Comparator<Double>(){
public int compare(Double o1, Double o2){
if(o2 > o1) {
return -1;
} else if(o1 > o2) {
return 1;
} else {
return 0;
}
}
};
PQ = new PriorityQueue<Double>(1000,c);
}
public void addElement(double d) {
PQ.add(d);
}
public double removeElement() {
return(PQ.remove());
}
}
I have implemented this priorty queue in java. I can call it from matlab. However, I need to associate each cost with an index. I mean it's not only cost of the node that i need to store but also its index. How can I accomplish this. I need to pass the index from matlab

Answer 1

You can use Java's default PriorityQueue like so:
>> q=java.util.PriorityQueue;
>> q.add({value,index});
This is available since Java ≥ 1.5, which is pre-bundled in all MATLAB releases since 7.0.4 (R14).
Otherwise, you can use the one from the file exchange, which you'll have to compile.
There's also a Simulink block for it, but I doubt that's what you're after.

Answer 2

Below is a resizing-array implementation of a priority queue written entirely in matlab. You can attach/couple any kind of data/index you want along with the priority value. Also, you can switch/toggle the behaviour between a min and max priority queue through a boolean argument passed into the constructor when it is created.
classdef PriorityQueue < handle
properties (SetAccess = private)
numElements;
priorityList;
valueList;
flagMaxPriorityQueue;
end
methods (Access = public)
function obj = PriorityQueue( flagMaxPriorityQueue )
if ~exist( 'flagMaxPriorityQueue', 'var' )
flagMaxPriorityQueue = true;
else
if ~(isscalar(flagMaxPriorityQueue) && islogical(flagMaxPriorityQueue))
error( 'ERROR: invalid flagMaxPriorityQueue argument' );
end
end
obj.flagMaxPriorityQueue = flagMaxPriorityQueue;
obj.numElements = 0;
obj.priorityList = {};
obj.valueList = {};
end
function insert(obj, priority, value)
% increase the size of the array if full
if obj.numElements > 0 && obj.numElements + 1 > numel( obj.priorityList )
% double the size of the array and copy stuff
obj.priorityList = cat(1, obj.priorityList, cell(obj.numElements, 1));
obj.valueList = cat(1, obj.valueList, cell(obj.numElements, 1));
end
obj.numElements = obj.numElements + 1;
obj.priorityList{ obj.numElements } = priority;
obj.valueList{ obj.numElements } = value;
obj.swim(obj.numElements);
end
function [priority, value] = pop( obj )
if obj.isEmpty()
error( 'called pop() on an empty priority queue' );
end
priority = obj.priorityList{1};
value = obj.valueList{1};
obj.exch(1, obj.numElements);
obj.numElements = obj.numElements - 1;
obj.sink(1);
obj.priorityList{ obj.numElements + 1 } = [];
obj.valueList{ obj.numElements + 1 } = [];
% halve the size of the arrays if they get one-quarter full
if obj.numElements > 0 && obj.numElements == floor( numel( obj.priorityList ) / 4 )
obj.priorityList( 2 * obj.numElements + 1 : end ) = [];
obj.valueList( 2 * obj.numElements + 1 : end ) = [];
end
end
function [flagEmpty] = isEmpty( obj )
flagEmpty = (obj.numElements == 0);
end
function [qSize] = size( obj )
qSize = obj.numElements;
end
function [priority, value] = peek( obj )
if obj.isEmpty()
error( 'requested max() of an empty priority queue' );
end
priority = obj.priorityList{1};
value = obj.valueList{1};
end
end
methods (Access = private)
function swim(obj, elPos)
while elPos > 1 && obj.compare(floor(elPos / 2), elPos)
obj.exch(floor(elPos / 2), elPos);
elPos = floor(elPos / 2);
end
end
function sink(obj, elPos)
while 2 * elPos <= obj.numElements
j = 2 * elPos;
if j < obj.numElements && obj.compare(j, j+1)
j = j + 1;
end
if ~obj.compare(elPos, j)
break;
end
obj.exch(elPos, j);
elPos = j;
end
end
function [blnCmpResult] = compare(obj, e1, e2)
if obj.flagMaxPriorityQueue
blnCmpResult = (obj.priorityList{e1} < obj.priorityList{e2});
else
blnCmpResult = (obj.priorityList{e1} > obj.priorityList{e2});
end
end
function exch(obj, e1, e2 )
temp = obj.priorityList{e1};
obj.priorityList{e1} = obj.priorityList{e2};
obj.priorityList{e2} = temp;
temp = obj.valueList{e1};
obj.valueList{e1} = obj.valueList{e2};
obj.valueList{e2} = temp;
end
end
end % classdef