CSIS 10B
Assignment 13 Heaps and Priority Queues
Due: next Wed
In class Demos (0 points)
We develop a simple (non-generic) StrHeap class:
1) basic mapping of tree heirarchy into a vector
2) size, empty and clear methods
3) adding to a heap – percolateUp
Assignment (10 points) Do either Basic or Advanced
Basic -- do all these exercises in lab13basic folder inside mod13 download
1) Implement the following algorithm in the StrHeap class, which performs the percolateDown
operation on a heap, where data = the structure (vector or array) containing the elements in the
heap, r = the index of a "root" (which could be THE root or any subtree root in the heap),
and c= the index of a child (either left or right). Note that this is a general algorithm. Use the
appropriate access methods for the vector and the appropriate comparison method for String.
public void percolateDown(int r)
pre: r = a valid index in heap, the root index of a subtree
post: move the value at r into its proper min heap position within a path descending to a leaf
node.
While r < size do following
Set c = left(r)
// start by examining the left child
If c < size
if right(r) < size and data[c+1] < data[c] // if the right child is smaller
Increment c by 1
// we'll compare right instead of left
if data[c] < data[r]
// the child is smaller than its parent
i. Swap data[r] and data[c]
// exchange the parent with its child
ii. set r = c
// get ready to repeat this operation
else
// items are in order so done
break;
else
// we are at the end of heap so done
break;
End while
If you follow the steps carefully, this operation will be O(logN). When swapping make sure you
do not use the add method since that will give O(N log N) behavior due to the shifting of data
needed. Note: after this method works, change it from PUBLIC to PRIVATE—it is only a helper.
2) Code and test the above algorithm. Then, write the remove method for our heap:
public String remove()
get the first item (index 0) of the heap and store in variable minVal
remove the last item in the heap and store in variable lastVal
if there are more than 1 items remaining in the heap,
set the root of the heap to lastVal
start percolateDown at index 0 ( the root location)
finally, return the minVal item
If you follow the steps carefully, this algorithm will be O(logN) as well.
3) Having completed the remove operation, we can now use our StrHeap to sort a vector of String.
Simply make a loop that adds each item in the vector to the heap. Then, make another loop that
retrieves all of the items from the heap (in increasing order) and places them in a new sorted
vector. (There are more efficient ways to implement heap sort on an array or Vector "in place"
but this illustrates the main idea and is still O(N log N), which is generally the best one can do in
sorting.)
4) Complete the project shell given in OsSim.java using the following instructions:
Note1: you will need to define a compareTo method for the Task class that compares priorities
of two tasks. Note2: since javas PriorityQueue uses a min-heap, a task with priority 0 will be the
“highest” priority.
5. Add your answers to the following problems to the bottom of your HeapDemoApp file:
Advanced
For a more open-ended priority queue simulation you may do Lab 13.7 on p242 of the text.