IT – som værktøj

Bent Thomsen
Institut for Datalogi
Aalborg Universitet

Oktober 2002 Bent Thomsen - FIT 1-1 1

Introduction to Programming

Bent Thomsen
Why learn about programming?
• programming teaches you how to solve problems
• programming helps you be more precise
(doesn’t 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
 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.
 You have already programmed!
• You wrote complex formulas in Excel
=$D5*EKSP(-LN(2)*E$4/$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' - X*w1';
r = r1'*r1;
 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
 Programming Languages
• A programming language is a set of rules
that provides a way of telling a computer
what operations to perform.
 Levels of Programming
Languages
• Machine language
• Assembly Language
• High Level Languages
• Fourth Generation Languages (4GL)
• Fifth Generation Languages (5GL)
 Machine Languages
• different for each computer processor
0100
001101 100000 001101 110001
00101 10001 10000
01110
111001
. . .
 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
 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 .
Types of high level Programming
Languages

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

• FORTRAN
• COBOL
• Pascal
• C
• Ada
 OOED Languages

• Object-oriented languages
– Smalltalk
– C++
– Ada 95
– Java
– C#
• Event-driven languages
– Visual Basic
– most Visual languages
 Declarative languages (5GL)
• Functional(?): Lisp, Scheme, SML
– Also called applicative
– Everything is a function
• Logic: Prolog
– Based on mathematical logic
– Rule- or Constraint-based
Language
Family
Tree
 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, …
 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
 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
 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
 Sequence Control Structures
• Sequence control structures direct the order
of program instructions.
• The fact that one instruction follows another
—in sequence—establishes the control and
order of operations.
 Calculate

• A program can
instruct a computer Add 1 to
to perform Counter
mathematical
operations.
 Store

• A program will often

instruct a computer to Place 1
store intermediate in
results.
Counter
 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
 IF-THEN

Entry

Test false true

condition p

Exit
True
statement a
 IF-THEN-ELSE
Entry

Test
condition p false true

“false” “true”
statement a Exit statement a
 Iterate
• A program loop is a
form of iteration. A
computer can be
instructed to repeat
instructions under
certain conditions.

No
 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
 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.
 DO WHILE Loop

Entry

Exit
Test No
condition p
Yes
Loop
statement a
 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.
 DO UNTIL Loop

Entry

Loop
statement a
Exit
Test No Yes
condition p
 Programs are Solutions
to Problems
• Programmers arrive at these solutions by
using one or more of these devices:
• Logic flowcharts
• Pseudocode
• Structured Programming
• UML
• Object Oriented Programming
 Logic Flowcharts
• These represent the
flow of logic in a
program and help
programmers “see”
program design.
Common Flowchart Symbols
Common Flowchart Symbols

Terminator. Shows the starting and ending points of the program. A terminator
has flowlines in only one direction, either in (a stop node) or out (a start node).

Data Input or Output. Allows the user to inputdata and results to be displayed.

Processing. Indicates an operation performed by the computer, such as a variable

assignment or mathematical operation.

Decision. The diamond indicates a decision structure. A diamond always has two
flowlines out. One flowlineout is labeled the “yes” branch and the other is labeled the
“no” branch.

Predefined Process. One statement denotes a group of previously defined statements.

For instance, “Calculate m!” indicates that the program executes the necessary commands
to compute m factorial.

Connector. Connectors avoid crossing flowlines, making the flowchart easier to read.
Connectors indicate where flowlines are connected. Connectors come in pairs, one with
a flowline in and the other with a flowline out.

Off-page connector. Even fairly small programs can have flowcharts that extend several
pages. The off-page connector indicates the continuation of the flowchart on another
page. Just like connectors, off-page connectors come in pairs.

Flowline. Flowlines connect the flowchart symbols and show the sequence of operations
during the program execution.
 Flowchart for a
Cash Register Program
Start

sum=0

Input price

sum=sum+price

Yes More
items?

No
vat=sum x 0.25
total=sum+vat

Output sum, vat,

and total

Stop
 Psuedocode
• This device is not visual but is considered a
“first draft” of the actual program.
• Pseudocode is written in the programmer’s
native language and concentrates on the logic
in a program—not the syntax of a
programming language.
 Pseudocode for a
Cash Register Program
sum=0
While More items do
Input price
sum=sum+price
End While
vat=sum x 0.25
total=sum+vat
Output sum, vat, total
 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.
 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)
 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”)
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

 Programming and Debugging
• Write code
– Syntax
• Rules of the language
– Logic
• Order of execution of various parts of the program
 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
 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!
 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.