1. Computer Science 2
Dynamics

2. Learning Objectives
Be able to read a program that uses dynamic data structures
Be able to fix a program that uses dynamic data structures
Be able to write a program that uses dynamic data structures.
Be able to add to and remove from a stack.

3. What can you use when you have…
A large group of similar information?
You want to use the information more than once.
You don’t know how many items there will be
You want fast access (too fast for files)

4. Or
What can you use to link different pieces of information together? Like relationships, passageways from one room to another, …
Why can’t we use the ^ for exponents.
How can you model a line of people, items, components,… where you make a variable when you add someone to the line, and free up space when a person is served?

5. Pointers/Dynamics
Lets you dynamically (while the program is running) create variables.
Lets you link/connect variables in non-linear ways.
Lets you dynamically erase variables when you are finished with them.

6. Pointer (Definition)
A dynamic data structure that allows the program to create, link and dispose memory locations.
Dynamic: Creating and disposing while the program is running.
Data Structure: A place to save information. (We’ll be using records)

7. Declaring the data structure
Type
pointertype = ^ NameRecType;
NameRecType = record
name:string;
next:pointertype; {This is how the data is
connected}
end;
Var
top, temp:pointertype;
These work a lot like file pointers.

8. Commands New(PointerVariable); Dispose(PointerVariable);
Allocates memory and points PointerVariable to it.
Dispose(PointerVariable);
Disposes the information where PointerVariable is pointing.
There must be something to dispose in order to dispose it. Otherwise Crash!

9. Other pointer stuff nil ^
A pointer constant. It is used to initialize pointers and check if you are at the end of a list (Like EOF in files.) ^
Used to access the information being pointed to.

10. How do you link them together?
top
Linked lists
Stacks
Models hiring/firing
Lines (Queues)
Circularly linked lists
Binary trees
Sparse Arrays
nil A B C D

11. Stacks Draw a stack using Top and Temp pointers
How would you start a stack? (Initialize)
How would you add to the stack?

12. Starting and Adding Start Adding (Pushing) Top :=nil; Make the memory
Put stuff in it
Connect to the top
Update the top

13. To code Begin Top:=nil;{Start/Initialize the list}
New(temp);{Creates memory}
Writeln(‘Please enter name’);
Readln(temp^.name); {Put stuff in}
Temp^.next:=top; {Connect to top}
Top:=temp; {Update top}

14. Adding a bunch to the stack?
Begin
Top:=nil;{Start/Initialize the list}
Writeln(‘Please enter name -1 to quit’);
Readln(shuttle);
while shuttle<> ‘-1’ do
begin
New(temp);
Temp^.name:=shuttle;
Temp^.next:=top; {Connect to top}
Top:=temp; {Update top}
End;
Type
pointertype = ^ NameRecType;
NameRecType = record
name:string;
next:pointertype;
end;
Var
top, temp:pointertype;
shuttle:string;

15. Remove from a stack (Pop)
How can you do it and not lose the stack?
Point to the person to be fired.
Update the top
dispose

16. Pop Temp:=top; Top:=top^.next Dispose(temp);
What could go wrong with the above code?
How can you fix it?
If top<>nil then
begin
Temp:=top;
Top:=top^.next
Dispose(temp);
End;

17. Quick review Declarations Nil New() Dispose() Pop Push
Declare pointer type, then the record it will point to.
Nil
Like eof
New()
Allocates memory
Dispose()
Disposes the info where pointer is pointing
Pop
Remove from a stack
Push
Add to a stack

18. Stack Define a stack. Show a stack after the following actions
Push: 5, 8, 15, 31
Pop: 2 values
Push: 4, 26, 12
Pop: One value

19. Warm-up
Write the declarations (Type and Var) for a stack that will hold the following
Name, Age, Phone Number
Write the code for the following
Initialize the stack
Add one value to the stack
Remove one value from the stack.

20. Declarations Type Var pntrType = ^ rectype; Rectype = record End;
Name:string;
Age:integer;
Phone:string;
Next:pntrType;
End;
Var
Top, temp: pntrtype;

21. Begin Top:= nil Writeln(‘Please enter your name’); New(temp);
readln(temp^.name);
Temp^.next:= top;
Top:= temp;
//Remove
If top<> nil then
Temp:= top;
Top:= top^.next;
Dispose(temp);
End;

22. Quick review Declarations Nil New() Dispose() Pop Push
Declare pointer type, then the record it will point to.
Nil
Like eof
New()
Allocates memory
Dispose()
Disposes the info where pointer is pointing
Pop
Remove from a stack
Push
Add to a stack

23. What do you recall about…
Dynamics
Pointer
New
Dispose
Next ^
Stack
queue

24. Warm-up Show the stack after completing the following
Push: 10, 30, 15, 20
Pop two values
Push: 90, 80, 100
Pop one value

25. How do you show the list? Temp:=top; While temp<>nil do Begin
Writeln(temp^.name);
Temp:=temp^.next;
End;

26. How do you delete the list?
Dispose(top); {What’s wrong with this?}
Temp:=top;
While temp<>nil do
Begin
Top:=temp^.next;
Dispose(temp);
End;

27. Don’t do this Writeln(top);
Dispose a pointer that is pointing to nothing
Leave information lost in space… Keep something pointing to it, so you can find it.

28. Review
Dynamics/pointers: Let you create and dispose memory on the fly.
Uses the ^ to point
Stack: Last in First Out (LIFO)
Nil is like the end of file.

29. Dry Run procedure c(var top:ptrtype); var temp:ptrtype; begin
Pascal 2: Pointer Worksheet #2
program DryRunPointers;
type
ptrtype = ^rectype;
rectype = record
score:integer;
next:ptrtype;
end;
var
top:ptrtype;
procedure a(var top:ptrtype);
temp:ptrtype;
count:integer;
begin
for count:= 1 to 4 do
new(temp);
temp^.score:= 3*count;
Temp^.next:= top;
top:=temp;
procedure b(top:ptrtype);
while top<> nil do
writeln(top^.score);
top:=top^.next;
procedure c(var top:ptrtype);
var
temp:ptrtype;
begin
temp:=top;
top:=top^.next;
dispose(temp);
end;
procedure d(var top:ptrtype);
while temp^.next<> nil do
temp:=temp^.next;
temp^.next:=top;
begin {Of the main body};
top:=nil;
a(top);
b(top);
c(top);
d(top);
End.
Dry Run

30. FixStack1.pas on the class website.
program fixstaque;
type
recType = record
number:integer;
end;
pointertype = *rectype;
var
top:integer;
temp:pointertype;
begin
{Add a number into the stack}
new(temp);
new(temp2);
new(temp3);
temp^.1:= 5;
temp2:= 8;
temp 3 = 14;
for top:= 1 to 3 do
writeln(top);
top:= top+1;
end.
Fix the following
FixStack1.pas on the class website.
It is supposed to put three values into a stack and then show them from the stack.

31. Program options
Create, show and delete a stack of names and ages using the following main body (You write the procedures)
Top:=nil;
Create(top);{Adds an unknown # of people}
Show(top); {Shows the list}
Delete(top); {Deletes the entire list}
Like the first option, but add a loop and a menu for each of the options.
Like the first option, but show the information in reverse order.
Write a recursive show procedure.