How to Program

1
How to Program

• Dave Elliman

2
I am confusedWhat is a program?

• Prof Roland Backhouse
• A program changes the truth state of a set of
  predicates such that certain axioms on the pre
  and post state are true
• Dr Graham Hutton
• A program is a notation for evaluating
  mathematical functions using a computer

3
I am confusedWhat is a program?

• Dr Colin Higgins (Teaching LOG)
• A program is an expression in mathematical logic
  written using Horn clauses
• A Student
• A program is gobbledygook written in something
  called Java that is picky about every stupid
  semi-colon and which makes no sense at all to me.

4
I am confusedWhat is a program?

• Alan Turing
• A program is a sequence of instructions which may
  be executed on a Turing Machine
• Ada Lovelace
• We may say most aptly that the Analytical Engine
  weaves algebraical patterns just as the
  Jacquard-loom weaves flowers and leaves.

5
I am confusedWhat is a program?

• Dave Elliman (Lecturing CSA)
• A program is a sequence of machine instructions
  which include arithmetic and logical operations
  and which may move data between memory locations
  and which may test and change the machine state
  including the state of the next instruction to be
  executed
• Dave Elliman (Teaching C programming)
• A program is a list of instructions. It is a lot
  like a recipe, or the instructions that come with
  flat-pack furniture

6
Who is right?

• Everybody is
• Programs can be all these things
• Alan Turing is the most right
• From the Java point of view we need a different
  perspective
• Programming is about the design and interaction
  of objects
• But Objects are programs too

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Did that help you?Are you still confused like
Pat?
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Or Like Me

• Learning FORTRAN on day release from Rolls-Royce
  at Derby Tech College
• I I 1 ?!??

9
I understand the Syntax of Java

• Some student comments
• I understand the lectures but what is Java for?
• Where do I start?
• I have broken the computer!
• The computer wont run my program properly!

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You need a conceptual FRAMEWORK that works

• Programming is too hard
• Human beings can only do easy things
• Human beings make mistakes all the time
• You have to check what they do carefully
• Programming has to fit your skills

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A More Useful Definition of a Program

• A program does things that you can talk about in
  stories
• Start by writing short stories about what someone
  using your program will do
• These are called Use Cases and have a UML
  diagram

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A More Useful Definition of a Program

• A program is a model of the world
• Imagine it as a thing with knobs on
• The knobs are thing pressed in the user stories
  they will tell you what is needed
• Think what parts your model needs
• Make them simple
• Think how they work
• What do the knobs, buttons and lights do
• How could you test it was working right?
• Dont even think about code for a long time

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What do you come up with using this approach?

• A Box which does something
• An Interface on the Box which makes it work
• Then think what is needed inside the box to make
  it work
• A state (ingredients)
• Instructions (what to do next)

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Modelling the State

• My box needs to hold the height of the Duke of
  Edinburgh in metres
• What sort of variable is this? (TYPE)
• My box needs to hold the number of times button
  A has been pressed today
• My box needs to hold the name of a student
• My box needs to model whether the sun is shining
  or not?

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Types

• These are provided for you to model real things.
  You just choose the right type
• A Harder one My Box needs to hold the turnover
  of my company in pounds and pence. It is a BIG
  company.

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So what are Classes

• These are the boxes in your model
• The methods you use are the interface
• In Java you can have several interfaces for a
  class. This can be neat.

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What is Inheritance

• It gets tedious always building a box from
  scratch
• You can use something like what you need but
  simpler and add the bells and whistles
• Just like Blue Peter..
• I have this one I did earlier over here

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How do I decide on the Boxes to use in my Model

• Make it beautiful
• One Big Object with everything on it is ugly
• Lots of tiny little objects is a horrible rats
  nest
• Maximise Cohesion, minimise Coupling, hide
  complexity
• Use real things as your objects
• An Object will have some state to model inside
  it. Otherwise it is not an object

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What is the simplest useful story?

• Design your model to do just this simple thing.
• Anything harder will confuse you and take too
  long
• Think how you will know if it works
• Write a test for that in words
• Go on then code something now

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Most People think in Pictures

• That is one reason why mind-maps can be helpful
• Pictures are very useful for designing and
  documenting software
• UML has some useful diagrams
• Get ArgoUML its free!
• http//argouml.tigris.org/

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A program that needs to be written
Plagiarism is a nuisance to a Lecturer!
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How do we start?

• Step 1 Say in a sentence or two what the
  application will do
• Step 2 Give it a name

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I came up with

• This application will take coursework submitted
  by a students and check if it has been copied
  from web pages.
• I will call it Plagiarism Detector
• OK thats not very imaginitive.

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At this point many questions will taunt you

• Will it check if students have copied from one
  another?
• What file formats will it read?
• Will it run on PCs, Macs, Linux, Solaris?
• What programming language will I use? Etc..

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Too many questions paralyse progress

• I will do the simplest thing that is useful and I
  will make it work
• If a question has popped up to soon I will say
  undecided! and leave it for now
• Make everything as simple as possible. Small IS
  beautiful.

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Answers

• Will it check if students have copied from one
  another? NO. (not yet)
• What file formats will it read? .txt
• Will it run on PCs, Macs, Linux, Solaris?
  undecided
• What programming language will I use? Etc..
  undecided

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Make up user stories
The students assignments are placed in a
directory. The Plagiarism Detector Application is
started. The folder is selected in an open file
command. All selected files are checked for being
copied from web pages and a report generated for
the lecturer
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More questions

• What will the user-interface look like?
• How will it discover whether the work has been
  copied from the web?
• How will all the assignments arrive in this
  directory?

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Some Tentative Answers

• What will the user-interface look like? Very
  simple file open dialog box
• How will it discover whether the work has been
  copied from the web? Same as how you do it
  Dave?
• How will all the assignments arrive in this
  directory? Not my problem! (safileup?)

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Lets Design the Program Then

• First.. Can we may it any simpler?
• YES! Lets just do one file for now
• What was the output going to be?
• Yes or No!

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What are the Objects?

• Things in the real world
• Things that have a state and a behaviour
• Things that have an interface and complexity
  hidden inside

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How on earth can a program look up something on
the web?

• This is an algorithmic part
• Easy it can use Google
• It could web scrape (illegal)
• It could use the API
• It could use a web service (XML)

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Conclusion

• Can you see the process from idea to objects?
• That is most of what programming is about
• The GUI
• Clever (but not too clever) algorithms
• Testing