Lecture 25 - Revision

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Objectives

• To give an overview of the work covered in this subject

1. Overview

This is a good time to review the aims and objectives of the subject:

Aims

• To introduce students to analysing, designing, constructing and documenting solutions to programming problems
• To familiarize students with commonly used operating systems and tools used in program development

Objectives

Students should be able to:

• Analyse a problem and construct a logical solution suitable for implementation as a computer program
• Represent a solution to a problem in pseudo code
• Use appropriate programming tools to develop solutions to programming problems
• Produce a correctly coded solution to a problem specification
• Apply the 3 basic programming constructs of sequence, selection and iteration
• Internally document a program with appropriate comments
• Develop external documentation for a programming problem
• Specify and execute test plans for programming problems
• Use standard Unix commands for: file management; printing, account management; compilation, ...
• Be aware of the nature and usage of Unix from a programmer's viewpoint

Topics

The main topics covered in this subject were:

• Introduction to Programming Environment
• Introduction to the Computer Networks at La Trobe Bendigo
• Problem solving and algorithm design
• Programming and design documentation
• Programming in Visual Basic
• The UNIX Operating System
• Programming in Java
• Programming Tools: ftp, winzip

2. Introduction to Programming Environment (lecture 1)

3. Networks at La Trobe Bendigo & Operating Systems (lectures 1 & 2)

Networks

• Computer networks allow resources to be shared.
• Networks have two types of computers: Servers and Workstations
• Servers are fast and store data centrally
• Workstations are PCs or UNIX workstations (or Macs...) that can do processing locally, but usually store data on the server
• The PC Network is a IBM compatible PC network. Servers on the network use Windows Sever 2003 as the Network Operating System. Workstations (PCs) use MS Windows XP Pro.

Operating Systems

• The two main categories of software are Application Software and System Software
• System Software includes BIOS and Operating Systems
• Operating Systems provide the basic functionality of a system
• The two main types of Operating Systems (OS) are command and graphical based
• UNIX is an example of a command based OS and Windows XP is an example of a graphically based OS
• Windows has three types of file systems: FAT, FAT32 and NTFS. NTFS is the most sophisticated
• Windows XP is a single user OS that is considerably more sophisticated than its predecessors
• Windows Server and Windows XP provide security for your account

4. Steps in Program Development (lecture 3)

• Programs are needed for problems that are: complex; repetitive; store data; involve communications etc
• Writing a program is not always the best solution to a problem. There are a number of alternatives, including: manual systems and using existing packages
• Building construction and software construction are similar.
  • Planning is essential.
  • Specifications are vital
  • A disciplined approach pays off
  • Finding problems early saves $$$
• The basic operations of a computer are: get data; output data; calculations; store data; retrieve data; decide between alternative instructions; and repeat instructions
• Steps in program development are: Understand the Problem; Specify a High Level Solution; Outline the Solution; Develop the Algorithm; Test the Algorithm; Write the Program; Test the Program; Debug the Program; and Maintain the Program
• It is essential that a problem is understood before attempting a solution
• Defining Diagrams give an overview of the problem, they broadly state what is to be used (inputs) and what is to be done (processing) to solve the problem and the results (outputs).

5. Pseudo Code (lectures 4 & 5)

• An algorithm is a series of steps to accomplish a task
• Pseudo Code is a commonly used method to express an algorithm
• Advantages of pseudo code:
  • Reduced complexity
  • Increased flexibility (language independent)
  • Ease of understanding
• A Data Dictionary is used to clarify the use of data in an algorithm
• A Desk Check is a manual check of an algorithm
• Some important statements used in pseudo code are: Assignment (=), Input and Display
• The arithmetic operators are: (), *, /, +, -
• Operator precedence  refers to the order that operators are evaluated in an expression

6. An Introduction to Visual Basic (lecture 6)

• Visual Basic in Access XP (2002) was used in this subject
• Visual Basic is often referred to as VB
• Comments help the reader understand the program
• Variables have a name and a value. Referring to the name allows the value to be obtained. Values can be stored in variables using = or InputBox. The value held in a variable can be changed while the program is running
• The main data types we will be using in VB are Long (for integers), Double (for floating point numbers) and String
• The  operators used in calculations are: (), *, /, +, -
• = is used to store a value in the variable on the left of the ='s
• InputBox is used to input a value
• Debug.Print is used to display value(s)
• We will be treating a module in Access/VB as a program
• Access/VB has an Integrated Development Environment (IDE) which includes an editor for writing programs and a compiler
• Syntax Errors are errors due to the incorrect use of a language

7. Selection (lectures 7 - 10)

• Only using sequence - performing instructions from top to bottom one after the other does not allow us to tackle real world problems
• Selection - conditionally executing instructions based on some conditions
• Problems that have alternative actions or optional actions suggest the presence of Selection.
• IF ... ELSE ... ENDIF is used in Pseudo Code to model selection
• If ... Else ... End If are used in Basic for selection
• Comparison operators are used in conditions to compare things. The operators are: =, <>, <, <=, >, >=
• Some problems require multi-way selection
• Nested-IFs are IF statements inside IF statements
• Nested IFs can be used for multi-way selection
• Compound conditions consist of simple conditions combined with NOT, AND, OR
• NOT negates a condition
• AND is True if both operands are True otherwise it is False
• OR is True if both or either operands are True, otherwise it is False
• Compound conditions may involve multiple variables
• Care needs to be taken to ensure that there aren't gaps in the logic. All possible cases in the problem and paths through the logic need to be examined
• In some cases it is not appropriate to use nested IFs
• Boolean variables hold one of the values: True or False

8. Miscellaneous (lecture 10)

• Debug.Print can print multiple literal values and/or variables.
• Semicolons and commas are used in Debug.Print to control where the values are printed
• Indentation and Whitespace are blanks (spaces and/or blank lines) that are used to make code easier to read
• The value of good variable names cannot be underestimated
• A Syntax error results from the incorrect use of the language - the code does not follow the rules of the language. Syntax errors are detected by the Complier.
• Logic errors occur when the program is running and are caused by mistakes in the logic - the code does not do what was intended and/or required
• Run-time errors occur when a statement attempts an illegal operation. These can can be difficult to detect. Sometimes run-time errors can be avoided.

9. Repetition (lectures 11 - 14)

• Repetition is also known as looping or iteration
• Repetition involves repeating instructions (statements) zero or more times
• There are 3 main types of loops:
  • fixed number of repetitions
  • variable number of repetitions, but known before entering the loop at run-time
  • unknown number of repetitions when entering the loop at run-time
• A FOR loop is a simple type of counting loop
• Signs of repetition in a problem are terms such as repeat, for, while or the use of plurals such as students or customers or repeated patterns in data

• While loops are more flexible than FOR loops, allowing the loop to be exited when the condition becomes false, rather than after a predefined number or repetitions.
• While loops are repeated while the condition is True
• Sentinel values are special input values that are used to indicate the end of input in a loop
• Infinite loops are loops that once entered, continue for ever
• Loops are often used for totalling, counting and averaging of numbers
• Validation is used to ensure that a value is correct
• Validation simplifies processing by (a) centralising the code to check a value and (b) allowing the programmer to assume that the value is correct elsewhere in the program
• IF, FOR and WHILE are control structures. They may be nested (place inside one another).
• Displaying tables of values often involves incrementing one value in a loop, from which the rest of the values on the same line in the table can be calculated

10. Subroutines (lectures 15 & 16)

• Sub-dividing a program into sub-programs allows for
  • Reduced complexity,
  • Permits work to be shared amongst developers
  • Allows for re-use of code.
• In Basic sub-programs are known as Subroutines or Functions
• When a subroutine is called the code specified for the subroutine is executed, then when the subroutine exits (finishes), control returns to the line after where it was called
• Subroutines may be called as many times as necessary
• In Pseudo code subroutine declarations begin with the subroutine name, followed by the subroutine code, and end with EXIT. To call (execute) a subroutine the name of the subroutine followed by brackets is used.
• In Basic subroutines begin with Sub, followed by the subroutine code, and end with End Sub. To call (execute) a subroutine the name of the subroutine is used.
• Local variables are only known in the subroutine they are declared in. Their value is lost when the subroutine exits
• Global variables can be used throughout a program. Their value is lost when the program ends
• Functions are a special type of subroutine that return a single value
• Basic has many built-in functions e.g. Format, Round.

11. Programming Revision

• Programmers build up a mental toolkit of problem types and solutions. This toolkit is gained through experience and study, and continues to grow throughout a programmers career.
• When solving new problems programmers draw on this toolkit to recognise problem types that they have encountered before and modify previous solutions to solve the problem at hand.
• The problem types studied in this subject are applicable to many programming problems.
• Programming Constructs
  • Sequence
    • Sequence involves executing a number of instructions one after the other in order from top to bottom.
  • Selection
    • Selection is used when different instructions are to be executed depending on the result of a condition. In this subject we have used the IF statement to implement selection.
  • Repetition
    • Repeating something zero or more times

Typical Uses

Sequence

Typical uses:

• User input of values
• Displaying literal values (e.g. 23 or "Hello World") or variables (e.g. the values stored in custName or custNumber)
• Initializing values (e.g. count = 0)
• Calculating, incrementing (adding 1), decrementing (subtracting one), totalling
• Calling a subroutine

Selection

Typical uses:

• Under a certain circumstance instructions are to be performed, otherwise nothing special is to be done (An IF, without an ELSE) e.g.
  • Calculate the average value if values have been input (count > 0)
  • Keep the smallest number input so far
  • Determine if a number is negative
• A set of instructions are to be performed if a condition is true, otherwise (the condition is false) another set of instructions are to be performed.
  • Characteristics: One of two different sets of instructions is to be performed depending on the result of a single condition.
  • Example: Display whether there is space left on a disk, if there isn't any space, display that, otherwise display how much space is left.
• A nested IF structure.
  • Characteristics: A dependency between two variables. The value of an unknown variable (dependent variable) can be determined solely based on the value of another variable of known value (independent variable)
  • Examples:
    • Range of values. e.g. Determine the cost per megajoule of gas (rate) for a customer.
    • Discrete values. e.g. Calculate the penalty applicable on car insurance depending on the risk associated with the insurance
• Compound condition.

Repetition

Typical uses:

• The loop is always executed the same number of times e.g.
  • Displaying a table of values
• The loop is executed a variable number of times depending on a variable value that is known before the loop is executed e.g.
  • Displaying a repeated series of characters
  • Displaying a table of values
  • Inputting a series of numbers (and doing something with them)
• The loop is executed a variable number of times that is only known when the user indicates they have finished. e.g. looping until a sentinel value is input by the user e.g.
  • Inputting a series of numbers until a user types a sentinel value
  • Inputting numbers and performing some calculations on them
• Totalling, counting  and averaging e.g.
  • Totaling sales
  • Calculating average rainfall
  • Counting words in a document
• Validation e.g.
  • Validating Yes/No response by a user
  • Validating the school year 0 (prep) through to year 12
  • Validating a month number
  • Validating a menu item

12. Unix (lectures 17 - 24)

Introduction

• redgum: a Server is mainly for student use
• ironbark: Server used for staff accounts and IT Department web pages
• workstations: IBM Compatibles running Linux in the UNIX lab
• Unix
  • is Multi-User
  • has individual user accounts:
  • is command driven
  • has a Hierarchical File System

Unix File System

• Unix uses a Hierarchical File System
• There is a special root directory where it all starts (/)
• Files are referred to by their pathnames
  • A file can be referred to by its full or absolute pathname. Absolute files names start with a /
  • Files can be referred to by relative pathnames. Relative file names must NOT start with a /
  • Unix supports only one directory tree
  • Files from one computer are mounted (by the system administrator) onto the file system of another computer using NFS (the Network File System)

Unix Commands

Working with Files

• ls
• cat, less, more
• cp
• mv
• rm
• lpr
• chmod

Working with Directories

• pwd
• cd
• mkdir
• rmdir

Editors

• gedit
• pico
• ee
• vi

Special Directories

• .
• ..
• ~

Wildcards

• *
• ?

Redirection of Input/Output

• >
• >>
• <
• |

Java Programming

• javac
• java

Users and Processes

• who
• w
• ps

Miscellaneous

• man
• du
• df

Other Topics

• Using gedit
• Aliases
• .profile

13. Java  (lectures 21)

General

• Java is an object oriented language
• Java source code file are text files
• Compilation produces a java byte code file which can be executed using the Java Virtual Machine
• Java may be run on different types of computers using the Java Virtual Machine (JVM)
• Java programs are compiled using the javac command and executed using the java command

The Language

• Java is case sensitive
• {} are used to group statements
• // are used for comments to end of line
• Simple statements are terminated with a ;
• Data types are: int, double and String
• User input is done using JOptionPane.showInputDialog. This inputs a string. If the values input represent numbers they must be converted to numbers using Integer.parseInt or Double.parseDouble as appropriate
• Display is done using System.out.println
• Arithmetic operators: (), *, /, %, +, -
• / results in integer division if the values on both sides of the operator are integers, otherwise floating point division is performed
• & is used for concatenating strings
• if else statements are used for selection

14. Tools (lecture 22)

File Transfer

• Used to transfer files between different computers
• ftp - stands for File Transfer Protocol
• Requires a host computer running special software to act as a ftp server
• Users on a computer running client ftp software can connect to the host computer if they have an account or by anonymous ftp (if available)
• Files are transferred as ASCII or binary. Text file are normally transferred as ASCII, others as binary
• PCs, Unix and Macs have different end of line characters in text files requiring conversion when transferring between these different types of computers

Software

• ftp
  • A command line program for transferring files
  • Available on a variety of computers
• ws_ftp
  • Uses a Graphical User Interface
  • Available on PCs
  • Easier to use than command line ftp programs

File Compression

• Compression programs are used to reduce the space taken up by files

Software

• winzip
  • Graphical user interface. Available on PCs
• zip/unzip
  • Command line programs on Unix

Further Reading

• Lectures and Tutorials 1 to 24
• Guides: Basic, Data Dictionary, Defining Diagram, Desk Check, Java, Pseudo Code, Unix
• Robertson, L. A. (2006) Simple Program Design: A Step-by-Step Approach. 5th ed. Melbourne: Thomson.
• Taylor, D. (2005) Sams Teach Yourself UNIX in 24 Hours. 4th ed. Indianapolis, Indiana: Sams

Written by Tim Whitfort.