Download COMP 1001 : Introduction to Programming

Welcome to COMP 10110
Computer Programming 1
Lecturer: Fintan Costello
Where you are
• COMP 10110:
 ComputerProgramming
 (Tues@10 / Thurs@10)
• This class is for
 Computer Science Denominated Entry students
What will you learn on this course?
How to make a computer do what you want:
•Write computer programs to respond to different situations
•Write computer programs to manipulate data in various ways
•Write computer programs to solve problems.
You will also develop a useful attitude:
•Not to be afraid when things go wrong
•To try things on your computer and learn from mistakes
•To solve problems in a clear, unambiguous & precise way.
What is a computer program?
A general definition:
A computer program is a set of written instructions which,
when running (executing) on a computer, allow the computer
• Remember things (by placing them in ‘variables’)
• Produce new things by combining things in different ways
• Make decisions
• Work until an aim is achived
• Communicate with the user (or other programs)
These are all things we do when thinking. There is an
interesting relationship between computer programs & our
thought processes.
Why learn to program?
The technological reason:
The psychological reason:
To make useful & interesting things To gain understanding of ourselves
The scientific reason:
To gain understanding of our world
By learning to program, we gain the ability to control and understand the
most complex, sophisticated, and flexible tools humanity has ever
created, and ones which dominate many aspects of our lives.
By learning to program, we learn to think about our own thought
processes, to examine how we think in different situations, and to
transfer written versions of those thought processes into a computer.
By learning to program, we gain tools that contribute to scientific
understanding of our world (simulation of things around us, data analysis,
applying the computing viewpoint to things around us)
COMP10110 intro to programming: what to do
Attend programming lectures twice weekly
Attend practicals once weekly (1 * 2 hours)
Practicals start next week in Comp Sci building
Go through some of the textbook yourself (The
book is “Java Concepts”, by Cay Horstmann)
Marks based on practical work (40%) plus an
exam at christmas (60%).
Practical work must be submitted each week.
Practical work must be satisfactory to pass
COMP 10110
What is programming?
• A computer program is a set of written instructions which
explain how to do a certain task or solve a certain problem.
• These instructions are written in a ‘programming language’
which the computer can understand.
• When we give these instructions to a computer it can carry out
that task or solve that problem.
• A computer can carry out lots of tasks and solve lots of
problems, because it can run lots of different programs.
What’s so special about programming?
• When we give people instructions on how to solve a
problem, we can rely on their natural intelligence to
understand any steps we’ve fogotten to mention.
• With a computer, we must fully specify every single step:
we must give an absolutely complete explanation of how to
solve the problem.
• This need to competely specify our solutions to problems
is unique to programming.
• Programming, therefore, is
the study of problem-solving.
Programming languages.
• Just as there are lots of different natural languages
(French, English, Swahili…) there are lots of different
programming languages.
• We’re going to learn a language called Java (a widelyused and powerful general-purpose language).
• Our aim in studying Java is not simply to learn the
language; instead, we’re going to learn how to solve
problems, and will use this language to write and test
our solutions.
• But first: How can a computer understand a language
anyway?
How do computers understand instructions?
• A computer is made out of rock (silicon; that is, sand) and metal. How
can it understand instructions in a language?
• The basic idea is simple: instructions are represented by codes or
patterns of switches (patterns like ‘on-off-on-on-off-off-on-on’)
• A given code or pattern not only stands for a given instruction; when
certain switches deep inside the computer (in the ‘central processing
unit’: the CPU) are set in that pattern, they cause the action required
by that instruction to be carried out.
• In other words, the code representing the instruction (‘on-off-on-on-offoff-on-on’) also produces the action required by that instruction, when
the switches are set in that code.
• So, for example ‘on-off-on-on-off-off-on-on’ (or 10110011: we usually
represent these codes by 1s and 0s) could mean ‘start a new page on
the printer’ because when the switches in CPU are set in that code,
they cause a new page to be fed to the printer.
A very simple example: a toy car.
Codes:
on-on means ‘go forward’
on-off means ‘go left’
off-on means ‘turn right’
If this switch is on,
an electric signal
goes down these
wires, activating the
wheel motors
switches
off-off means ‘do nothing’
wires
off
0 off
0
Rather than using the words
‘on’ and ‘off’ to represent
switches turned on or off, we
use the numbers 1 and 0.
wheel motors
A very simple example: a toy car.
Codes:
1-1 means ‘go forward’
1-0 means ‘go left’
0-1 means ‘turn right’
0-0 means ‘do nothing’
1
1
1
1
0
0 0
1
1
1
1
1
1
Very rough structure of a computer
Memory: each
switch is on/off
(1 or 0)
Arithmetic unit:
circuits that add
together signals
from memory
Loads signals
from memory
into control unit
0001 0001
Control unit: different settings
of switches make different
things happen (select rows
from memory, cause arithmetic
computations, etc)
Very rough structure of a computer
Memory: each
switch is on/off
(1 or 0)
Arithmetic unit:
circuits that add
together signals
from memory
Loads signals
from memory
into control unit
0001 0001
Control unit: different settings
of switches make different
things happen (select rows
from memory, cause arithmetic
computations, etc)
Programming languages
• The commands controlling a computer are sequences of 1 and 0
(switches on and off). A given set of 1s and 0s will cause the
computer to carry out some specific action (because it will set the
switches to produce that action).
• It’s very hard to write instructions for a computer in 1s and 0s (in
“machine code”). Instead, we will write our instructions for our
computers in a programming language (which is a bit like english),
and use a special program to translate our programming language
instructions into machine code instructions.
• In the Java language, this translation process has two stages: we’ll
first use the Java “compiler” to translate our instructions into an
intermediate form called “bytecode” which all computers can
understand, and then use the Java “interpreter” to translate that
intermediate form into the machine code which our particular
computer uses.
Programming languages
Natural languages: French, English, Swahili…
Natural languages are forgiving: if you have wrong the grammar,
pronounciation, or speling, people can still understanding you be.
Programming languages: C, perl, Java...
Computers are not as smart as people: if you don’t have your
program’s grammar and spelling exactly right, the computer will
not understand and will give an error.
There are two different types of error. Can you guess what they are?
Errors can arise if the compiler cannot translate your instructions into
machine code (a ‘compiler error’), or if the computer cannot actually do
what you have asked it to do (a ‘run-time error’).
Your first java program
HelloTester.java (Book, p. 15)
public class HelloTester
{
public static void main(String[] args)
{
// displays a greeting at the console
System.out.println("Welcome to Java");
}
}
Reserved words
public class HelloTester
{
public static void main(String[] args)
{
// displays a greeting at the console
System.out.println("Welcome to Java");
}
}
The words in bold are reserved words: they mean something
special in the java programming language.
Reserved words are case-sensitive: Public is wrong.
What do these words mean?
public class HelloTester
{
public static void main(String[] args)
{
// displays a greeting at the console
System.out.println("Welcome to Java");
}
}
class is a word identifying a class (a set) of things in Java. In
this case, it’s the class of things that will print out a hello message.
public is a modifier which identifies something as publicly
accessible (other programs can “call” it, which means run it)
main identifies the main part of a program (the bit that does
most of the work). We’ll explain static and void later.
Blocks {......}
A block is a sequence of code between matching curly brackets
public class HelloTester
{
public static void main(String[] args)
{
System.out.println("Welcome to Java");
}
}
This block identifies everything that is in the public
class HelloTester
This block identifies everything that is in the main part
of the class.
Statements (commands)
A statement is an action.
A statement ends with a semi-colon
;
System.out.println("Welcome...");
println means ‘print this on a line of its own’ (you
can also use print, which doesn’t start a new line).
System.out means that println is an action carried
out by the output part of a java class which you can use
to interact with your computer’s System.
We’ll learn more about this later when we talk about methods
Comments in Java
Comments are simply notes for the programmer to remind
themseleves and other programmers what a program is
meant to do. Comments can contain any text you like
// this is a one line comment
// and here is another
/* this comment can
extend over several lines
.......
*/
Comments are not java commands. Always comment
your programs! It helps you remember what’s going on.
A more interesting program
Next time I’ll explain exactly the steps you go through to get
the HelloTester.java program to run.
I’ll also introduce a more complex program: one that uses
all the facilities of java to do some animation.
You can see that program running here:
http://inismor.ucd.ie/~fintanc/BouncingBallApplet.html
I’ll first explain how we get this program to run, and then use
it to give you a tour around the java programming language.
You can get all lecture slides etc for this course via
blackboard or at
http://inismor.ucd.ie/~fintanc/COMP101X0/