1. Computer Science 2
Queue

2. Learning Objectives Improve reading a program that uses linked lists.
Be able to model a line using a linked list.

3. program DryRunPointers; type ptrtype = ^rectype; rectype = record
score:integer;
next:ptrtype;
end;
var
front, back:ptrtype;
count:integer;
procedure a(var front, back:ptrtype;count:integer);
temp:ptrtype;
begin
new(temp);
temp^.score:=count;
temp^.next:=nil;
if front = nil then
front:=temp;
back:= temp;
end
else
back^.next:= temp;
back:=temp;
procedure b(front:ptrtype);
begin
while front<> nil do
writeln(front^.score);
front:=front^.next;
end;
procedure c(var front, back:ptrtype);
var
temp:ptrtype;
if front=nil then
writeln('The list is empty')
else
temp:=front;
dispose(temp);
begin {Of the main body};
front:=nil;
back:=nil;
for count:= 1 to 4 do
a(front,back,3*count);
b(front);
for count := 1 to 5 do
c(front,back);
end.

4. Queue
Definition: A data structure where you the first one on the list is the first one off of the list.

5. Quick Check What does a stack look like after the following?
Push: 10, 20, 5, 16
Pop: Two Values
Push: 8
Pop: 2 Values
Push: 47, 18
What does a Queue look like after the following?
Enqueue (Add): 5, 12, 27
Dequeue (Remove): 2 values
Enqueue: 51, 12, 92, 26
Dequeue: 2 values 12 92 26

6. Uses: Modeling a line of people. Modeling a bank, fast food place, …
Setting up a printer queue. This is where several computers print using one printer. A computer is set up as the print server and processes who gets to print and when.
Modeling traffic pattern at a signal.

7. Example: Initializing the queue: Setting the front and back to nil.
Adding to a queue: Enqueue
Empty Queue
Non-empty Queue
Picture
Code
Showing all.
Taking off of a queue: Dequeue

8. From Idea to Code Initialize the Queue Add to an empty queue
Front := nil;
Back:= nil;
Add to an empty queue
If front = nil then
Begin
New(temp);
Readln(temp^.name);
Temp^.next:= back;
Back:= temp;
Front:= temp;
End;

9. Idea to Code Add to non-empty queue If front <> nil then begin
New(temp);
Temp^.next:= nil;
Back^.next:= temp;
Back:= temp;
end

10. Idea to Code: Show all Temp:= front; While temp<> nil do Begin
Writeln(Temp^.name);
Temp:= temp^.next;
End;

11. Remove One: Dequeue Temp := front; If front<> nil then Begin
Front:= front^.next;
Writeln(temp^.name , ‘ is being removed.’);
Dispose(temp);
end;

12. Program options:
Write a program that will model a line in a bank. Using a menu and a while loop.
Add person to the line. Get their name and put them in the line.
Show all of the people in the line.
Serve a person. Say the name of the person being served.
Close the bank. (When this is chosen, everyone left in the line is dismissed personally. Ex. Sorry we are closed, please come back tomorrow, John, Jan , Jane, Jal)
Push:
Time how long each person waits in the line.
Add a provision for a person cutting in line.
Create a screen saver that uses a queue help modeling the motion. Have the lines around the screen saver where the lines are added to one end of the series and the line in the back of the series is removed. Note: You need to incorporate the pointers into this option.