Lecture 13: Implementing a Queue with an array or ArrayList

Announcements:

Exams are graded; average score = 80.4 – good result!

H4 is graded – your grade is in your handin directory

P3 is posted, due Mar 15 – read & bring questions on Thursday

H5 will be posted on Thursday, due on March 15

first try:

complexity of enqueue & dequeue:

front	enqueue(ob)	time	dequeue()	time
first item	add(ob)	O(1)	remove(0)	O(N)
last item	Add(0,ob)	O(N)	Remove(size()-1)	O(1)

second try:

· keep frontIndex, rearIndex & let front of queue move

· enqueue: //basic code

rearIndex++;

items[rearIndex] = ob;

numItems++;

· dequeue: //basic code

numItems - -;

frontIndex++

return items[frontIndex-1];

both operations are O(1) - that's good!

need to take care of when rearIndex = items.length-1

· could just expand array, but that wastes a lot of space

· instead: let rear of queue wrap around & reuse items[0]…

wrap around & reuse items[0]

o circular list within the array

o still might need to expand the array, but use all of it

You try:

write the code for enqueue, assuming array still has space

then add call to expandCircularArray if out of space

public void enqueue(Object ob) {

if (numItems == items.length)

expandCircularArray;

if (rearIndex == items.length -1)

rearIndex = 0;

else

rearIndex++;

items[rearIndex] = ob;

numItems++;

}

You try:

write steps for expandCircularArray() in English

if you have extra time, write the code

Solution:

Steps:

1. Allocate new array, tmp, twice the size

2. copy values in items[frontIndex] .. items[rearIndex] into tmp[0] … tmp[numItems-1]

3. set items = tmp

4. set frontIndex = 0, rearIndex = numItems-1;

for (k = 0; k < numItems; k++) {

items[k] = items[frontIndex];

frontIndex ++;

if (frontIndex = items.length)

frontIndex = 0;

}

Compare: Queue implemented via an array vs LinkedList

space:

o linked list stores pointer + data

o but the array can be ½ empty

time:

o linked list all O(1)

o array also all O(1)

ease of implementation:

o linked list is easier if have LinkedList class

o otherwise, both implementations are similar