data structures ah computing. description and exemplification of the following variable types/data...
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Data Structures
AH Computing
Description and exemplification of the following variable types/data structures:
2-D arrays, records, queues, stacks.
Data Constructs
Stack Queue Record
Stacks
Useful where the situation calls for the most recent item to accessed first
ExamplesStore code during compilation of HLLStore immediate results of
calculationsDuring interrupts were the status of
the program and contents of registers are stored
Stacks
Represented in the computer’s memory as a 1D array
Bottom fixed and a variable stack pointer stores the current top location
The stack pointer is a special register that is updated each time the contents of the stack changes
Stacks
Data elements can only be added or deleted from the top of the stack, akin to a real pile of plates or coins
Two operations
Push: an item is added to the top of the stack
Pop: the top item is taken from the stack
Push A
Stack
AB
Push B
Push C
CTop of Stack
Stack
ABCTop of Stack
Pop
Top of Stack
Top of Stack
Pop
Pop
LIFO
12
55
8
27
16
•Consider the integers 16, 27, 8, 55 and 12.
•Pushing them into a stack in the order given would produce…
Top of Stack
Bottom of Stack
LIFO
35
12
55
8
27
16
•If the number 35 is to be added to the list then it is pushed onto the top of the stack, the situation now looks like:
Top of Stack
Bottom of Stack
LIFO
35
12
55
8
27
16
•The last number in is always the first number out.
•A stack is therefore called a LIFO structure
•Last In First Out
Top of Stack
Bottom of Stack
Exercise 1
17
39
6
17
39
6
88
17
39
6
Consider the following stack sequence…
39
6
Stack Pointer
Explain the stack operations in terms of PUSH, POP and pointer changes
Stack Underflow
39
6
•If a further 2 POP operations took place, then the top of stack would become less than the bottom of the stack.
•The stack is now empty
•Attempting any further stack operations (before a PUSH operation took place) would result in an error known as a Stack Underflow
Stack Overflow
39
6
•Stack size usually limited
•If the maximum size is exceeded then a Stack Overflow will occur
Exercise 2
Given the output stream A,B,C,D,E,F Write down the sequence of operations (Push for stack and Pop for unstack) which would produce the sequence C,B,D,E,F,A
Implementation of a Stack
If Stack_Pointer> Maximum Then
Output “Stack Overflow”
Else
Stack_Pointer=Stack_Pointer + 1
Stack(Stack_Pointer)=Data item
EndIf
Push a new item on a stack
Implementation of a Stack
If Stack_Pointer< Minimum Then
Output “Stack Underflow”
Else
Data item =Stack(Stack_Pointer)
Stack_Pointer=Stack_Pointer - 1
EndIf
Pop an item off a stack
Stacks
Used To store code during the compilation
of a HLL To store the immediate results of
calculations Upon interrupts, the status of the
program and the contents of the registers are stored on top of a stack
Extension
Page 123 Reverse Polish Notation
The Queue
The Queue
Also a 1D array (linear list) Similar in structure to a stack Data items can be inserted and
deleted at different ends FIFO (First In First Out)
Queue Example
12
55
8
27
16
Head of queue
End of queue
If the number 35 is to be added then it joins the end of the queue (Pushed).
The queue now becomes…
Queue Example
12
55
8
27
16
35
Head of queue
End of queue
If a data item has to be removed from the queue then it is popped from the head of the queue.
In this case if 12 is popped then the situation becomes..
Queue Example
55
8
27
16
35
Head of queue
End of queue
An important aspect to realise here is that the data itself does not move but merely the pointers to the head and end of the queue.
Implementation of a Queue
If Rear=Maximum ThenRear=1
ElseRear=Rear+1
EndIfIf Rear = Start -1 Or (Rear =maximum and Start=1) Then
Output “Queue Full”Else
queue(Rear) = DataEndIf
Adding an item to the queue
Implementation of a Queue
If Rear=Start-1 or (Rear=Maximum and Start=1) ThenOutput “Queue Empty”
ElseData=queue(Start)
EndIfIf Start=Maximum Then
Start=1Else
Start=Start+1EndIf
Removing an item from the queue
Queues
These are used when multiple printing jobs have to be
processed During scheduling of tasks in a
multitasking environment
Exercise
Review questions 5 and 6 on page 128
Records
Definition of a Record
A data type created/customisable by the programmer
consisting of a set of fields/multiple data items
which can be of different data types
Records
Arrays can only hold data items with the same type
Records can contain different types of data
This makes them more complex to manipulate
Associated with databases
Record Example
Field Type
Surname String
FirstName String
Gender Character
Address String
PostCode String
Customer Number String
A record can be considered as a two dimensional array with different data types.
Record Example
Surname FirstName Sex Address 1 PostCode ‘phone
Tod Andy M 35 Brookside Dr
TY7 8UK
225 3625
Boyd Mary F 27 The Grange
OB7 RF1
335 2901
Bell Charles M 2 Larch Rd HT5 WA3
213 1157
Records
In the computer’s memory, records are stored as
Record 1
* Record 2 * Record 3
* Record 4
* Record 5
* EOF
Each record is terminated by a CR, LF (indicated by *) and EOF control codes.
Records in Visual Studio 2005
Structure Books Dim Title As String Dim Author As String Dim ISBN As String Dim Price As Decimal End Structure Dim BookInfo As Books
Dim BookInfo(100) As Books ‘100 records
} Fields
Record Datatype
Declare variable to hold 1 record
Assigning values
BookInfo.Title = InputBox("Enter the title of the book")
BookInfo.Author = InputBox("Enter the author's name")
BookInfo.ISBN = InputBox("Enter the ISBN")
BookInfo.Price = InputBox("Enter the price")
Displaying record values
ListBox1.Items.Add("Title-" & BookInfo.Title)
ListBox1.Items.Add("Author-" & BookInfo.Author)
ListBox1.Items.Add("ISBN-" & BookInfo.ISBN)
ListBox1.Items.Add("Price- £" & BookInfo.Price)
FilePut(1, BookInfo, Pointer) 'Write data to file
FileGet(1, BookInfo) ' get book details from file and display
Structure Competitor
dim Title as string
dim Rider as string
dim Round1score as single
dim Round2Score as single
dim Age as integer
End Structure
Dim CompetitorType(8) as Competitor
Comparison of Arrays v. Records
Arrays (1D & 2D)– all data must be of same type (Integer, string,..)
Records – fields of different types Arrays – simple to implement Records – more complex to
implement
Task 1
Task 2
Page 147 Program 4
2007 example
Charlie has a list of all the scores and competitors from a snowboard big air competition. Charlie has entered the results in a program which allows him to sort the list in order of “country”, “round one score” or “round two score” by clicking on the column heading. (Adapted)
The program uses a record data structure to store each competitor’s details.
(a) Define a suitable record structure to store each competitor’s details. (3)
(b) Describe a variable based on the record structure that could store the set of eight competitors. (3)