Transcript Information Systems 1: IT 141

Tonga Institute of Higher Education
IT 141: Information Systems
Lecture 14:
Programming
Software Engineering
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Computer programming is also known as
software engineering.
It covers the entirety of developing
software, which includes systems
analysis and programming.
It can involve hundreds of people and
many months or years to finish a really
big software project
Microsoft Word has about 750,000 lines
of code. The US government only thinks
a program is big if it is over 1 million
lines of code
Problem Statement
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So how do we start engineering software?
It’s best to start with defining the things
we need to do and to know
A Problem Statement is a list of things
that
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Specifies all aspects of the problem
Clearly specifies all the information we know
Specifies when the problem is solved
With this information, we can figure out
the problems we want to solve and how
to solve them
Algorithms
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With this information about the problem,
we can create an algorithm to solve it.
An algorithm is a series of steps that can
be written down and coded into a
program.
If the instructions of an algorithm are
followed, then you will always get the
right answer (If the algorithm was made
correctly)
Usually, you must be pretty detailed with
your instructions. Computers are stupid.
How to write algorithms
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Structured English – This is using regular
language to write out your algorithm
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Pseudo-Code – Mixes programming code
and language to make something that
looks like code, but would never compile
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Multiply the length by the height to get the
area
Area = length x height
Flowchart – this uses pictures and
graphics to help you understand the
algorithm
Flowchart of Algorithm
Start
Input width
Input height
multiply
width
height
Output
area
Coding
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Programs are called sequential order
programs, because the commands
happen in order.
That is unless, you make them go
somewhere else
Control structures will control the
sequence of the instructions in the
programming.
Examples: if, else, functions, etc
They will change the flow of a program
Control Structures
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Sometimes we will draw pretty pictures to show how
a control structure works
Repetition Controls
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Repetition Controls are
ways to make something
happen many times.
These are called loops. In
programming you may
use "for" and "while" to
make this happen
They’ll have some
condition that either
makes them continue
doing the loop or stop the
loop.
Debugging And Documentation
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To make sure your
program works, you
have to do a lot of
testing.
The biggest problem is
finding errors or
"bugs" and removing
them
Sometimes testing
takes longer than
programming
People who write programs
with lots of problems look
like this
Errors
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There are three types of errors you may
get when you make a program
Syntax errors – when you don’t follow the
guidelines of the programming language.
Maybe you forgot semi-colons or spaces
Run-time errors – errors that happen
after you compile and start running a
program. Maybe you tried to divide by
zero.
Logic errors – when your algorithm and
procedures don’t do what they are
supposed to, but the program still runs
Documentation
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It’s a good idea to put comments within
your programs.
It makes it easier for people to read, for
you to remember what you did, and also
for other people to continue working with
your code
Once you have a program that runs, it’s
also a good idea to have documentation
that tells people how to actually use the
program.
Programming Languages
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Hundreds of programming languages have
been invented since computers started
Some were invented for specific purposes
(like business or math) and some are
general purpose
A “procedural” programming language has
a series of statements that execute one
after each other.
Programs start at the beginning and end at
the end. They follow a simple flow through
the code, it is easy to see what is
happening (one example, BASIC)
Programming Languages
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Object oriented languages – they try to
let programs use an object to manipulate
data and change things.
An object will be defined by a class.
Inside the class there will be methods
which allow people to do things to the
object.
There will also be attributes inside the
object which describe something about
the object. These are mostly variables
that define data inside the object
Object-Oriented
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So why do we use object oriented
programming?
It lets many people work on a program at
once because they can all work on small
parts themselves
It lets people use things that others did
without having to understand them
It lets things be reused a lot, instead of
redoing the same thing over and over
again
Declarative Languages
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Declarative languages let programmers
define a set of rules and statements
about some relationship or problem.
Then inside the compiler, it applies those
rules to get the answer.
The programmer does not have to
actually write out the answer, but instead
he just writes the rules.
Then the compiler does the hard work of
solving the problem.
One example is SQL
Markup and Scripting Languages
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We know that markup languages are not
really programming languages, because
they are simple tags inserted into regular
text to make it look different
Scripting languages are like most
programming languages, except they
cannot be run without an interpreter.
They are sometimes not as powerful as
programming languages that can be
compiled.
Low-Level Languages
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Low-level languages (also called Assembly
Languages) are used when the programmer
wants to work directly with hardware. People who
write compilers and who write operating systems
use this
Machine language - the language of 1s and 0s
that the processor uses to execute the program.
It cannot really be understood by humans. When
computers were first invented people had to use
1s and 0s in order to program
Assembly language is the direct translation of
machine language. It has simple commands that
will change memory or do simple arithmetic.
It can be understood by people, but not as easily
as high level languages (languages you need to
compile in order to change to a low-level
language)
Compilers
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Compilers turn your high-level programming
language into low level assembly language. Your
code is called “source code”
The low-level code that comes out of a compiler is
called “object code.” Sometimes it is ready to be
run right away, because it is an executable.
Sometimes it is turned into an “intermediate
language.”
One example of an intermediate language is with
Java. Java allows you to compile your program
into “byte-code.” This byte code can then be run
on any computer. Java’s goal is to eliminate the
problem that programs written on one system
cannot work on another.
Interpreters
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Interpreters will only compile one
instruction at a time. It will look at the
current instruction, turn it into machine
language and let it be executed. Then the
next instruction will be changed into
machine language.
It is a little slower than when a compiled
program runs because each line of code
needs to be changed into machine
language as it is running.
Event-Driven
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Some programs run based on
something that happens. This is the
case in graphical programming.
It might wait for a mouse-click,
something to move, or something
entered from the keyboard
These are called “events” and if
your program responds to these, it
is called an “event-driven” program
Components
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Because of object-oriented programming, you
can buy or download components (or objects)
that will do things for you.
You don’t even need to know how they work
You just plug them into your program and
use what they do.
For example, maybe your program needs to
make a graph, so you can get a graph
component.
This graph component might display a graph
for you with the data that you give it (data
independence!)
Programming Languages Overview
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Visual Basic and Basic – procedural
programs that can be compiled. Easy to
use. Visual Basic very easy to use to
make graphical programs.
C – procedural programming that gives
access to hardware, but is still high-level.
Smart people can use C to make their
programs very fast, but its hard to learn
C++ - like C, but with objects. Many
people can’t understand this idea of
objects, so they give up
Programming Languages Overview
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COBOL – language made in the 1960s for
large business mainframes. It is procedural,
easy to understand and has lasted for a long
time
Java – high level, object-oriented language
that is good for internet programs (called
applets) and can run on any computer
without being re-compiled
Pascal – high level, procedural language
made for students to learn with, but nobody
can make a real program out of it
SQL – the standard, declarative language for
using databases.
Programming Languages Overview
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Perl –It is a high-level scripting language that lets
you programming with objects or treat it like a
procedural program
PHP – Another high level scripting language that
is entered right into HTML.
.NET Languages (C#, VB.NET) – A new set of
languages made by Microsoft that extend older
languages by adding object oriented
programming and try standardize Windows
programming.
It doesn’t require types and lets you mix lots of
things together. It is also easy to make simple
programs. No compiling!
Summary
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Most of the information in this chapter
should be familiar with you, after having
been exposed to programming already
We went over some new things, like
different types of languages and different
things you can do with objects
We also looked at different programming
languages and what makes them unique