Transcript Document

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
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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
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Procedure-oriented languages
Object-oriented languages
Event-driven languages
Declarative languages
Procedure-Oriented Languages
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FORTRAN
COBOL
Pascal
C
Ada
OOED Languages
• Object-oriented languages
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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
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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
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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:
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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
to perform
mathematical
operations.
Add 1 to
Counter
Store
• A program will often
instruct a computer to
store intermediate
results.
Place 1
in
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
condition p
Exit
false
true
True
statement a
IF-THEN-ELSE
Entry
Test
condition p
false
“false”
statement a
true
Exit
“true”
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
No
Test
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
Test
condition p
Exit
No
Yes
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?
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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.