IT

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IT som værktøj

• Bent Thomsen
• Institut for Datalogi
• Aalborg Universitet

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Introduction to Programming

• Bent Thomsen

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Why learn about programming?

• programming teaches you how to solve problems
• programming helps you be more precise
  (doesnt win you many friends though!)
• why did the computer scientist stay in the shower
  forever?
• the instructions on the shampoo said lather,
  rinse, repeat!
• programming gets you more out of your computer
• you may not be programming, but
• knowing a little bit about Computer Science
• and knowing a little bit about Programming
• will help you work with people who do

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Programs

• A program is a set of step-by-step instructions
  that directs the computer to do the tasks you
  want it to do and produce the results you want.

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You have already programmed!

• You wrote complex formulas in Excel
• D5EKSP(-LN(2)E4/C5)
• You used SQL to talk to databases
• SELECT FROM contacts WHERE age BETWEEN 18
  AND 35
• You programmed in MATLAB
• function r fz(x)
• global M p w1
• X cos(x), sin(x) -sin(x), cos(x)
• r1 M' - p' - Xw1'
• r r1'r1

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Programming

• Programming consists of two steps
• algorithmic design (the architects)
• coding (the construction workers)
• Programming requires
• a programming language to express your ideas
• a set of tools to design, edit, and debug your
  code
• either
• a compiler to translate your programs to machine
  code
• a machine to run the executable code
• or
• an interpreter to translate and execute your
  program

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Programming Languages

• A programming language is a set of rules that
  provides a way of telling a computer what
  operations to perform.

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Levels of Programming Languages

• Machine language
• Assembly Language
• High Level Languages
• Fourth Generation Languages (4GL)
• Fifth Generation Languages (5GL)

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Machine Languages

• different for each computer processor
• 0100
• 001101 100000 001101 110001
• 00101 10001 10000
• 01110
• 111001
• . . .

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Assembly Languages

• different for each computer processor
• main proc pay
• mov ax, dseg
• mov ax, 0b00h
• add ax, dx
• mov a1, b1
• mul b1, ax
• mov b1, 04h

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High-Level Languages

• Higher Level Languages
• Use traditional programming logic where the
  programming instructions tell the computer what
  to do and how to perform the required operations.
• 4GLs
• Use high-level English-like instructions to
  specify what to do, not how to do it .

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Types of high level Programming Languages

• Procedure-oriented languages
• Object-oriented languages
• Event-driven languages
• Declarative languages

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Procedure-Oriented Languages

• FORTRAN
• COBOL
• Pascal
• C
• Ada

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OOED Languages

• Object-oriented languages
• Smalltalk
• C
• Ada 95
• Java
• C
• Event-driven languages
• Visual Basic
• most Visual languages

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Declarative languages (5GL)

• Functional(?) Lisp, Scheme, SML
• Also called applicative
• Everything is a function
• Logic Prolog
• Based on mathematical logic
• Rule- or Constraint-based

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LanguageFamily Tree
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Lots more Languages

• There are many programming languages out there
• specification languages, e.g. Z, UML
• document languages, e.g. LaTeX, Postscript
• command languages, e.g. csh, MATLAB
• query languages, e.g. SQL
• Scripting languages, e.g. Perl, Python,
  JavaScript, VBScript, ASP, PHP,

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What determines a good language

• Formerly Run-time performance
• (Computers were more expensive than programmers)
• Now Life cycle (human) cost is more important
• Ease of designing, coding
• Debugging
• Maintenance
• Reusability
• FADS

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Why so many?

• Why does some people speak French?
• Most important the choice of paradigm, and
  therefore language, depends on how humans best
  think about the problem
• Other considerations
• efficiency
• compatibility with existing code
• availability of tools

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What can a program do?

• A program can only instruct a computer to
• Sequence
• Calculate
• Store data
• Compare and branch
• Iterate or Loop
• Write Output
• Read Input

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Sequence Control Structures

• Sequence control structures direct the order of
  program instructions.
• The fact that one instruction follows anotherin
  sequenceestablishes the control and order of
  operations.

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Calculate

• A program can instruct a computer to perform
  mathematical operations.

Add 1 to Counter
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Store

• A program will often instruct a computer to store
  intermediate results.

Place 1 in Counter
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Compare and Branch

• A program can instruct a computer to compare two
  items and do something based on a match or
  mismatch which, in turn, redirect the sequence of
  programming instructions.
• There are two forms
• IF-THEN
• IF-THEN-ELSE

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IF-THEN
Test condition p
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IF-THEN-ELSE
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Iterate

• A program loop is a form of iteration. A computer
  can be instructed to repeat instructions under
  certain conditions.

No
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Iteration Control Structures

• Iteration control structures are looping
  mechanisms.
• Loops repeat an activity until stopped. The
  location of the stopping mechanism determines how
  the loop will work
• Leading decisions
• Trailing decisions

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Leading Decisions

• If the stop is at the beginning of the iteration,
  then the control is called a leading decision.
• The command DO WHILE performs the iteration and
  places the stop at the beginning.

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DO WHILE Loop
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Trailing Decisions

• If the stop is at the end of the iteration, the
  control mechanism is called a trailing decision.
• The command DO UNTIL performs the iteration and
  puts the stop at the end of the loop.

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DO UNTIL Loop
Loop statement a
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Programs are Solutionsto Problems

• Programmers arrive at these solutions by using
  one or more of these devices
• Logic flowcharts
• Pseudocode
• Structured Programming
• UML
• Object Oriented Programming

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Logic Flowcharts

• These represent the flow of logic in a program
  and help programmers see program design.

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Common Flowchart Symbols
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Flowchart for aCash Register Program
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Psuedocode

• This device is not visual but is considered a
  first draft of the actual program.
• Pseudocode is written in the programmers native
  language and concentrates on the logic in a
  programnot the syntax of a programming language.

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Pseudocode for aCash Register Program
sum0 While More items do Input price
sumsumprice End While vatsum x
0.25 totalsumvat Output sum, vat, total
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Structured Programming

• Structured program languages lend themselves to
  flowcharts and pseudocode.
• Structured programming languages work best where
  the instructions have been broken up into small,
  manageable parts.

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Object Oriented Programming

• Everything is an object
• A program is a bunch of objects telling each
  other what to do by sending messages
• Each object has its own memory made up of other
  objects
• Every object has a type
• All objects of a particular type can receive the
  same messages
• (Alan Kay)

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The object concept

• An object is an encapsulation of data and
  behaviour, modeled after real-world objects
• An object is an instance of an abstract data type
• An abstract data type is implemented via a class
• An object has
• identity (a unique reference)
• state (also called characteristics)
• behaviour
• Behaviour is implemented via methods
• Methods are often implemented using structured
  programming
• An objects methods and state are access via dot
  notation
• I.e document.write(Hello World)

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The Program Development Cycle
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation
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Programming and Debugging

• Write code
• Syntax
• Rules of the language
• Logic
• Order of execution of various parts of the program

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Programming and Debugging

• Programming Errors
• Syntax error
• Misuse of syntax
• e.g., typing fer instead of for
• Logic errors
• Unintended operation of program
• e.g., Infinite loop

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Programming and Debugging

• Debugging
• Tracing and resolving errors in a program
• Coined by Admiral Grace Hopper
• Moth short-circuited a relay
• bug in the system
• Removed it ? system debugged
• Not an exact science more a black art
• Human against evil machine!

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So really, why learn about programming?

• Programmers make lots of money.
• Programming really is fun.
• Programming is very intellectually rewarding.
• Programming makes you feel superior to other
  people.
• Programming gives you complete control over an
  innocent, vulnerable machine, which will do your
  evil bidding with a loyalty not even your pet dog
  can rival.