CS 101
Chapter 1: Background
Aaron Bloomfield
Let’s begin
• Goal
– Teach you how to program effectively
• Skills and information to be acquired
–
–
–
–
Mental model of computer and network behavior
Problem solving
Object-oriented design
Java
What is a computer?
• Not a rhetorical question!
• “A device that computes…
especially a programmable electronic machine that
performs high-speed mathematical or logical
operations or that assembles, stores, correlates, or
otherwise processes information”
– From American Heritage® Dictionary of the English
Language, 4th Edition
So what is a computation?
• The act or process of computing
– Duh!
• Definition of computing:
– To determine by the use of a computer
– To determine by mathematics, especially by numerical
methods: computed the tax due
• My revised definition for computing:
– The act of taking a problem with specific inputs and
determining a specific answer (output)
Axiom
• By definition, a (properly functioning) computer will
always produce the same output given the same
input
• So how do we compute random numbers?
The first computers
• Scales – computed relative weight of two items
– Computed if the first item’s weight was less than, equal to, or
greater than the second item’s weight
• Abacus – performed mathematical computations
– Primarily thought of as Chinese, but also Japanese, Mayan,
Russian, and Roman versions
– Can do square roots and cube roots
Stonehenge
Computer Size
ENIAC then…
ENIAC today…

With computers (small) size does matter!
8
So what do we do with a computer now
that we have one?
• We have to tell a computer what to do
– Computers have no intelligence of their own
• We tell a computer what to do by writing a computer
program
– In this course, we’ll use Java
Algorithms
• What is an algorithm?
• “A step-by-step problem-solving procedure,
especially an established, recursive computational
procedure for solving a problem in a finite number of
steps”
– From American Heritage® Dictionary of the English
Language, 4th Edition
• We’ve seen lots of algorithms before…
Example algorithm: map directions
Example algorithm:
car radio removal
Example algorithm: Recipes
Bad algorithms
• Not all algorithms are “good”
• So then what makes an algorithm “bad”?
Bad algorithms: MapQuest directions
• Consider directions to get around grounds
Note that this is not an incorrect algorithm!
Just a very inefficient one
Bad algorithms: Shampoo directions
Bad algorithms: Shampoo directions
• Lather, rinse, repeat
• This algorithm repeats forever!
– Note that humans know to not to spend forever performing the
algorithm
– But computers do not!
• Remember, they have zero intelligence
• Hence the overplayed computer joke:
– How did the computer scientist die in the shower?
– He read the directions: lather, rinse, repeat
Bad algorithms: Inexact recipes
Our goal
• Is to write correct and efficient algorithms for a
computer to follow
– Remember that computers are dumb!
• We aren’t going to worry about the efficient part in
this course
• But what does “correct” mean?
“Correct” algorithms
• Consider an algorithm to display the color blue
• Is this blue?
Definitely
• Is this blue?
Also, yes: two correct results!
• What about this?
Maybe (could be green)
• And this one?
Definitely not
Note that this is not an incorrect algorithm!
Just a very inefficient one
So what does all this mean?
• Humans specify algorithms without a lot of precision
– Display the color “blue”
– Get me from “here” to “there”
– When there isn’t much precision, there are often multiple answers
• Computers need more precision
– Display the color 0x0000ff (royal blue):
• There is only one possible outcome
– Find the shortest route from “here” to “there”
• We need to be very specific when we specify things to a
computer
– Computers are dumb!
Onto the book chapter
• A lot of this terminology will be confusing at first
– We will be going over it in more detail throughout the
semester
Computer Organization
Memory
Output Devices
Input Devices
Central
Processing Unit
Computer Organization
• Computer advertisement specification
– Intel® Pentium 4 Processor
at 3.06GHz with 512K cache
– 512MB DDR SDRAM
– 200GB ATA-100 Hard Drive
(7200 RPM, 9.0 ms seek time)
– 17” LCD Monitor
– 64MB NVIDIA GeForce4
MX Graphics Card®
– 16x Max DVD-ROM Drive
– 48x/24x/48x CD-RW Drive
– 56K PCI Telephony Modem
– Windows XP Home Edition®
– 10/100 Fast Ethernet Network Card
512 million bytes of
3.06
Reads
memory
17”
Computer
Can
Can
Microprocessor
Stores
billion
on
send
read
DVDs
the
200
that
or
and
operations
operating
diagonal.
16
billion
receive
can
write
times
for
be
faster
displaying
transferred
data
bytes
CDs.
up
Resolution
system
to
per
than
at
Can
of
56
two
second
data.
images
athousand
using
hold
at
basic
rates
up
double
You
650
to
awith
DVD
–
10
drive.
million
want
64
graphical
or
the
1,280
bits
million
100
high
normal
Can
per
bytes
by
million
hold
interface
RPM
second
bytes
1,024
of
rate
up
and
data
bytes
ofto
memory.
8 Reads
billion
low
per
seek
pixels
at
bytes
More
second
48
time.
memory
times
of data
supports
0.009
faster
A byteseconds
and
more
is 8writes
bits
colors
is
and
24 times
higher
average
faster
resolution
than
A bit
a basic
is a 0drive
or a 1
A bit of humor: Computer
Organization

Why I like adding humorous bits
into class…
30
Home network
Backbones
A bit of humor…
33
Network communication
•
Communication protocol
– Set of rules that govern how data is sent and received
•
TCP/IP
– Exchanging packets of information over the Internet
•
FTP
– Exchanging files between computes
•
SMTP
– Exchanging email over the Internet
•
POP
– Exchanging email between mail reader and the ISP
•
HTTP
– Exchanging files over the WWW
•
SSL
– How information is to be encrypted
First Programming Languages
• Ada Lovelace (1833)
– Programmed of Babbage’s analytical engine
• ENIAC (1945)
– Programmed by plugging wires
• Binary
– 100100010101001010101010
• Assembly
• FORTRAN (1954)
Java’s Direct Lineage
• C (1972) by Dennis Ritchie
• C++ (1985) by Bjarne Stroustrup
• Java (1991) by James Gosling and others at Sun
Microsystems
Computing units of measure
• A bit is either a 1 or a 0
– On or off, true or false, etc.
• A byte is 8 bits:
– 01001010
– As there are 8 bits per byte, each byte can hold 28=256 values
– 01001010 = 74
• All computing measurements are in terms of bytes
Computing units of measure
• Kilo (K) = 1,000 (thousand)
• Mega (M) = 1,000,000 (million)
• Giga (G) = 1,000,000,000 (billion)
• Tera (T) = 1,000,000,000,000 (trillion)
• Kilo = 210 = 1,024
= Kibi (Ki)
• Mega = (1024)2 = 1,048,576
= Mebi (Mi)
• Giga = (1024)3 = 1,073,741,824
= Gibi (Gi)
• Tera = (1024)4 = 1,099,511,627,776
= Tebi (Ti)
Computing units of measure
•
An unformatted text document (such as a Java program)
– 3 pages per kilobyte (1,000 bytes)
•
A formatted document (such as a Word file)
– About 5k per page with formatting
•
A digital camera picture
– About 1 Mb each (1,000,000 bytes)
•
An MP3 music file
– 5 Mb for a 5 minute song
•
A music file on a CD
– 50 Mb for a 5 minute song
– 10 times the size of an MP3!
•
A movie clip
– About 10 Mb per minute of (TV-sized) video
A marketing trick
• This hard drive has
250,059,350,016 bytes
– = 250.06 Gigabytes
– = 232.89 Gibibytes
• Guess which one they
use to advertise the
drive?
Software
• Program
– Sequence of instruction that tells a computer what to do
• Execution
– Performing the instruction sequence
• Programming language
– Language for writing instructions to a computer
• Major flavors
– Machine language or object code
– Assembly language
– High-level
Detailed
Program
knowledge
to which of
computer
the
machine
respond
is not
Java
Symbolic
is acan
high-level
language
For
program
to be
directly.
required.
Each machine
Uses
instruction
a
for
programming
coding
executed
itcode
must
be
is
a
vocabulary
binary
and
that
language
language
instructions
translated
structure
corresponds
closer to
to athe
problem
native being
instruction
solved
Translation
• Translator
– Accepts a program written in a source language and translates it
to a program in a target language
• Compiler
– Standard name for a translator whose source language is a highlevel language
• Interpreter
– A translator that both translates and executes a source program
Java translation
• Two-step process
• First step
– Translation from Java to bytecode
• Bytecodes are architecturally neutral object code
• Bytecodes are stored in a file with extension .class
• Second step
– An interpreter translates the bytecodes into machine instructions
and executes them
• Interpreter is known a Java Virtual Machine or JVM
Programming Task
• Display the forecast
I think there is a world market for maybe five computers.
Thomas Watson, IBM, 1943.
DisplayForecast.java
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
Anmethod
Three
A
application
statements
is a named
program
make
piece
is
uprequired
the
of code
action
to
that
have
of performs
method
a
main()action
some
public
static void
or implements
method named
a behavior
main().
Method main() is part of class DisplayForecast
Sample output
Java Documentation
• Familiarize yourself with the Java documentation
– It will save you lots of time!
• A link to it is on the website
– We will go over it in a future lab as well
Good Commenting
• Necessary so others can re-use your code
– And so the graders can understand it!
• A well commented program:
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
Bad commenting
//
//
//
//
Thomas J. Watson (February 17, 1874 - June 19, 1956) is
considered to be the founder of IBM. He was one of the
richest men of his time and called the world's greatest
salesman when he died.
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
While at NCR, he was convicted for illegal anticompetitive sales practices (e.g. he used to have
people sell deliberately faulty cash registers, either
second-hand NCR or from competitors; soon after the
second-hand NCR or competitors cash register failed,
an NCR salesperson would arrive to sell them a brand
new NCR cash register). He was sentenced, along with
John H. Patterson (the owner of NCR), to one year of
imprisonment. Their conviction was unpopular with the
public, due to the efforts of Patterson and Watson to
help those affected by the 1913 Dayton, Ohio floods,
but efforts to have them pardoned by President Woodrow
Wilson were unsuccessful. However, the Court of
Appeals overturned the conviction on appeal in 1915,
on the grounds that important defense evidence should
have been admitted.
//
//
//
//
//
//
//
//
//
//
//
Watson was born in Campbell, New York. His formal
education consisted of only a course in the Elmira
School of Commerce. His first job was at age 18 as
a bookkeeper in Clarence Risley's Market in Painted
Post, New York. Later he sold sewing machines and
musical instruments before joining the National Cash
Register Company as a salesman in Buffalo. He eventually
worked his way up to general sales manager. Bent on
inspiring the dispirited NCR sales force, Watson
introduced the motto, "THINK," which later became
a widely known symbol of IBM.
//
//
//
//
//
//
//
//
//
//
//
Although he is well known for his alleged 1943 statement: public class DisplayForecast {
"I think there is a world market for maybe five computers"
there is no evidence he ever made it. The author Kevin
// method main(): application entry point
Maney tried to find the origin of the quote. He has been
public static void main(String[] args) {
unable to locate any speeches or documents of Watson's
System.out.print("I think there is a world market for
that contain this, nor is it present in any contemporary
System.out.println(" maybe five computers.");
articles about IBM. The earliest known citation is from
System.out.println("
Thomas Watson, IBM, 1943.");
1986 on Usenet in the signature of a poster from Convex
}
Computer Corporation as "I think there is a world market }
for about five computers" --Remark attributed to Thomas
J. Watson (Chairman of the Board of International
More bad commenting
•
From the context-switching code of Unix V6 (file: slp.c)
/*
* If the new process paused because it was
* swapped out, set the stack level to the last call
* to savu(u_ssav). This means that the return
* which is executed immediately after the call to aretu
* actually returns from the last routine which did
* the savu.
*
* You are not expected to understand this.
*/
if(rp->p_flag&SSWAP) {
rp->p_flag =& ~SSWAP;
aretu(u.u_ssav);
}
•
Source: http://www.tuhs.org/Archive/PDP-11/Trees/V6/usr/sys/ken/slp.c
Java and the Internet
Your machine
Your friend's machine
I think ...
DisplayForecast.java
Java
Compiler
JVM
DisplayForecast.class
DisplayForecast.class
Modem
Modem
Internet
Engineering software
• Complexity of software grows as attempts are made to
make it easier to use
– Rise of wizards
Software engineering
• Goal
– Production of software that is effective and reliable,
understandable, cost effective, adaptable, and reusable
• Design
Makes
Work
Because
Cost
tocorrectly
sense
software
develop
of thedue
and
long
and
sotonot
that
lifetime
maintain
thefail
new
great
many
features
should
costspeople
involved
not
andexceed
will
capabilities
to
behave
involved
expected
flexible
can be
benefit
components
added
that– can
be used in other software
Creation
– Debugging
– Maintenance
– Enhancement
• Two-thirds of the cost is typically beyond creation
Principles of software engineering
• Abstraction
• Encapsulation
• Modularity
• Hierarchy
Ranking
Separate
Construct
Determine
or ordering
components
a the
system
relevant
of from
objects
into
properties
external and
and
andpackages
internal
features
components
while ignoring
aspects
nonessential
details
A bit of humor:
1989 Computer
Advertisement
Guess the price!
55
Object-oriented design
• Purpose
– Promote thinking about software in a way that models
the way we think and interact with the physical word
• Including specialization
• Object
– Properties or attributes
– Behaviors
Programming
• Class
– Term for a type of software object
• Object
– An instance of a class
with specific properties
and attributes
Programming
• Problem solving through the use of a computer
system
• Maxim
– You cannot make a computer do something if you do not
know how to do it yourself
Problem Solving
• Why do you care?
– We are all assigned tasks to do
• At work
• At home
• At school
• Why not do them
– Right
– Efficiently
Problem Solving
• Why care about computerbased problem
solving (i.e.,
programming)?
–
–
–
–
–
Neat
Frontier of science
Profitable
Necessary
Quality of life
Problem Solving
Remember
• Solutions
Accept
goal
is not
a clever
solution
a correct
solution
– Often
The process
require
isboth
iterative
concrete
and but
abstract
thinking
• In
Teamwork
solving the problem increased understanding might require
restarting
Problem Solving Process
• What is it?
–
–
–
–
Analysis
Design
Implementation
Testing
Describe the components and associated processes for solving
the problem
Determine
the inputs,
and other
components
the
Develop
solutions
for outputs,
the components
and
use those of
components
Test the components
individually
and collectively
problem
to produce
Straightforward
an overalland
solution
flexible
Description
should
beflexible
sufficiently specific to allow you to solve
Method – process
Straightforward
and
the problem
Object – component and associated methods
Problem Solving Process
Determine
problem features
Describe objects
and methods
Produce the
classes and code
Examine for
correctness
Analysis
Rethink as
appropriate
Design
Implementation
Testing
Tips
• Find out as much as you can
• Reuse what has been done before
• Expect future reuse
• Break complex problems into subproblems
Tips
• Find out as much as you can
• Reuse what has been done before
• Expect future reuse
• Break complex problems into subproblems
Consider
Research
can is
require
and generate questions
Find out what
knownsignificant
about thetime
problem
Sketching
apresenter
solution
andbecause
then repeatedly
its components
The
worthwhile
the resultrefine
is a better
Talk effort
to theis
until
the entire process is specified
understanding
Determine what attempts have succeeded and what attempts
True
of the problem makes it easier to solve
have understanding
failed
Tips
• Find out as much as you can
• Reuse what has been done before
• Expect future reuse
• Break complex problems into subproblems
Your
Be
open
timetoisindirect
valuable
use of existing materials
Correctness is probably even more valuable
Use existing infrastructure that is known to work
Tips
• Find out as much as you can
• Reuse what has been done before
• Expect future reuse
• Break complex problems into subproblems
Make as few assumptions as necessary
Maximizes the likelihood that your effort can be used in future
situations
Tips
• Find out as much as you can
• Reuse what has been done before
• Expect future reuse
• Break complex problems into subproblems
Divide-and-conquer
Solve subproblems and combine into an overall solution
Tips
• Read
– Problem solving texts
• George Polya, How to Solve It; A New
Aspect of Mathematical Method,
Princeton Press, 1988
• Wayne Wickelgren, How to Solve Mathematical Problems, Dover
Publications, 1995
• Paul Zeitz, The Art and Craft of Problem Solving, John Wiley, 1999
– Sociological examination of different problem solving styles
• Sherry Turkle and Seymour Papert, Epistemological Pluralism: Styles and
Voices Within the Computer Culture, Signs: A Journal of Women in Culture
and Society, 1990
Today’s demotivators
70