(chapter 1) A Concise and Practical Introduction to Programming Algorithms in Java

1A Concise and Practical Introduction to Programming Algorithms in Java © 2009 Frank Nielsen
Frank NIELSEN
nielsen@lix.polytechnique.fr
A Concise and
Practical
Introduction to
Programming
Algorithms in Java
Chapter 1: Expressions, Variables, and Assignments

Contents
● Learn to program with/in Java
● Computing as a science
(some basic principles)
● Popular (computer) science
HCI BMI
MANET

Jobs &Jobs & Computer ScienceComputer Science
● Industry
● CS Industry
● Others (information systems)
● Administration
● Research & Development
Not feeling fluent with CS today, is like not being able to drive a car !

A Concise and Practical Introduction to Programming Algorithms in Java © 2009 Frank Nielsen
Digital worldDigital world
Benefits of the analog-to-digital paradigm shift?
• DissociateDissociate contentscontents fromfrom supportsupport : digitize/“binarize”
Contents become mere
binary 0/1 strings

• Universal player (machine) and dedicated devices
“Multiple 0/1 readers”

• Generic algorithms:
copying, compressing, transmitting, archiving, etc.
Raise the question: What is the (digital) information?
•Text
•Music
•Image
•Video
•Data

Digital world: Why 0/1 bits?Digital world: Why 0/1 bits?
Binary numeral systems:
Information, first needs of counting...
8
Unary numeral systems:
Logarithmic number of bits for countingLinear number of bits for counting vs
4 bits
for counting
0 to 15

Nature of computing?Nature of computing?
•Generic algorithms:
copying, transmitting... ...genetics...
DNA (double-helix structure of DNA)
1953, James Watson and Francis Crick (Nobel prize)
Genetics

Nature of computing?Nature of computing?
First envisioned by Erwin Schroedinger (What is life?, 1944)
Transmit crystals?
Nobel, Physics 1933
LIFE???

Digital world/computingDigital world/computing
Ubiquitous computing= computing everywhere
New features
Digital = Binary + Calculations
Example:
Computational
photography

Computer science is not programming PCs
Computers
=
computing machineries
Difference engine of Charles Babbage
(conceived in 1822 on paper, built much later on)
Computing is a principle of reality (and science)
Watson and Crick 1951 (DNA double helix heredity)
Computing is 21st
Century's Science of integration

INFORMATIQUE=INFORmation + autoMATIQUE
Information= Data sets, input (discrete binary sequences of 0/1)
Automatic= General recipe that works on any input
= ALGORITHM
Al-Khwarizmi
(790 - 840)
Al-Khwarizmi: Scholar of
scientifically flourishing Bagdad:
● Algorithmi (latinization) -> Algorithm
● Al jabr -> Algebra
Provide readers a generic pipelinepipeline solution
to solve a quadratic equation:
http://www.akiti.ca/Quad2Deg.html

2121stst
century computer sciencecentury computer science
● Computers (and computing) are omnipresent
-> Ubiquitous computing (Mark Weiser)
Computers are abundant and versatile:
1952-1999
Xerox parc chief scientist
● Computing impacted all Sciences:
Computational sciences
Eg., Biology -> Systems biology
(simulation-prediction-experience in wet lab)
● The Science of computing is Computer Science (CS):
Deep theoretical questions and important technologies
(eg., medical imaging such as DT-MRI, economy)
DW-MRI
(Many more devices than PCs)
Science of
Integration
(complex systems)

Flavor of my research in computer science
Visual computing:
● Computational geometry,
● Computer vision,
● Computer graphics,
● Machine learning
For example, tackling computational photography
Reinventing the photography: taking, sharing and experiencing photos...Reinventing the photography: taking, sharing and experiencing photos...
Smile shutterDigital camera
Analog camera
Everything has yetEverything has yet
to beto be
invented!!!invented!!!
Beyond 2D pixels
Beyond single flash
etc...

Computer science is (also) for creative minds
Not only the hardcore mathematical problems to solve,
but also innovation by unleashing the power of
digital calculus for soft problems:
Human Computer Interactions (HCI), design
Example: computational photography project (2004)
Non-photorealistic camera (NPR)
NPR Camera

Algorithms and their performances
(resource/complexities)
There is usually not a single recipe for solving the task:
Eg., compute 5422x2319
(human decimal, machine binary, indian base 60, many tricks, etc.)
How to evaluate and compare different algorithms?
Clean framework for assessing the use of ressources:
● time,
● memory,
● #communications,
● etc.
Judge the generic algorithms not for a given instance.
Therefore, analyze:
● Worst-case complexity
● Average-time complexity
● Modern challenges (inplace, i/o bottlenecks & streaming, etc.)
● Etc.
Donald Knuth

Programming algorithms in Java
● Conceived by Bill Joy (SUN co-founder) and James Gosling
●
Started in 1990, for the ''next wave in computing''
●
On-time for the Internet and WEB (applets are Java applications, Javascript, etc.)
Cross-platform= runs on various operating systems (Windows, UNIX, Leopard, etc.)
●
Typed language (a=b, with a and b from different types will generate a compiler error)
●
Object oriented (OO, ease the conception and modularity of applications)
● Rich set of Applications Programming Interface (API)
● Free Software Development Kit on many platforms (SDK)
● Verbose for catching bugs and debugging applications.

Why programming languages?
Machines are “stupid”: they obey you 100%
-> Need to fully and precisely specify your intentions
(no room for ambiguity, the bug is yours!!!)
... Machines only “understand” 0/1 binary sequences
(eg., instruction codes of microprocessors)
Machine = Processing + Peripherals (I/O)
... controlled by an Operating System (OS)
But Human masters “natural language”
... and we need to unleash ease of programming
ASSEMBLER, FORTRAN, ALGOL, BASIC, .......JAVA
Key principle of CS: Bootstrapping!
use existing languages to create more powerful languages:
Python, Ruby, etc.

My first (java) program
Programmers and CScientists cherrish...
... their “Hello World” programs
class FirstProgram{
public static void main (String[ ] args){
System.out.println("Hello INF311 !");
}
}
First programs often looks magic!
Special function main: entry of the program

● Type this program into a text editor (nedit, notepad)
Save this “text” as FirstProgram.java
● Compile the program FirstProgram.java
prompt% javac FirstProgram.java
● Execute the compiled program
prompt% java FirstProgram
prompt% Hello INF311 !

FirstProgram.java
FirstProgram.class
1) EDIT and SAVE
3) EXECUTE
2) COMPILE
(Java Byte code in .class)
java FirstProgram (Java Virtual machine: JVM)
... low-level language
instructions for processors
High-level language
concepts/abstraction

My first algorithm in Java:
A solver for quadratic equations
In Java
http://www.java.com/fr/
Input: a, b, c of the quadratic equations
Solution: the at most two real roots
Install the SDK
(you do not have
to do this in room machines)

Programming: Solver for quadratic equations
class QuadraticEquationSolver
{
public static void main(String[] arg)
{
double a,b,c;
a=Math.sqrt(3.0);
b=2.0;
c=-3.0;
double delta=b*b-4.0*a*c;
double root1, root2;
root1= (-b-Math.sqrt(delta))/(2.0*a);
root2= (-b+Math.sqrt(delta))/(2.0*a);
System.out.println(root1);
System.out.println("Let us check the roots:");
System.out.println(a*root1*root1+b*root1+c);
}
}
QuadraticEquationSolver.java

{
{
double a,b,c;
a=Math.sqrt(3.0);
b=2.0;
c=-3.0;
}
}
Programming simple formula
Variable
(declare)
Assignments
Declare+Assign

{
{
double a,b,c;
a=Math.sqrt(3.0);
b=2.0;
c=-3.0;
}
}
Programming simple formula
Arithmetic expressions
Display

Programming: Solver for quadratic equations
Use any text editor to program
(nedit in UNIX, notepad under windows)
Magic code for printing onto the
console
Indentation is up to you
-> helps read programs

CompilingCompiling andand executingexecuting a Java programa Java program
prompt>javac filename.java
prompt>java filename
If no compile error happens, it produces a file filename.class
Then excute the compiled code.
To store output to a file:
prompt>java filename > result.txt
Redirect console to filename result.txt

Fundamentals of Java: Variables
● A variable is uniquely named (not a reserved keyword)
● A variable stores a value in a memory slot
● The value of a variable is accessed by its name
● The value of a variable can be modified
A=32
B=16
C=A
Left hand side (reference) and right hand side (value) of = means different things
Memory bank
A
B
C
32
16
32
valuereference

Fundamentals of Java: Expressions
● Formed by variables, operators (+,-,/, x, etc.) and constants (1, Math.PI, etc.)
● Expressions are evaluated and return a result (eventually stored in a variable)
● Operators follow priority rules: 5x3+2 ?
...avoid overuse of parenthesis 5x3+2 = (5x3)+2
Few examples of expressions in Java:
// Expressions
5+3*x/y
“Hello “+”INF311!”
// Assignment (expressions) terminate with a ;
x=cx + r*Math.cos(theta);
y=cy+ r*Math.sin(theta);

Fundamentals of Java: Affectation (sign =)
Var = Expression ;
● Var is the name of a variable
● Expression is a well-formed expression
Assignment left hand side=right hand side is decomposed as :
● The Expression is evaluated yielding value v
● The reference (memory slot) of Var is determined
● Value v is stored in the memory slot of Var
lhs = rhs
Reference Value
lhs rhs
Memory bank
Atomic
instruction

Basic typesBasic types
Type = Domain of values of variables
All variables must be typed in JavaAll variables must be typed in Java
Basic types (=basic data structures):
Integers:
byte 8 bits
short 16 bits
int 32 bits [-2**31,2**31-1]
long 64 bits [-2**63,2**63-1]
Reals:
float (single precision, 32 bits)
double (double precision, 64 bits)
char 16 bits (Unicode, world languages)
boolean true or false

Why do we type variables?Why do we type variables?
To ensure homogeneous operations
2 +3 =5
3 +4 =7
5 +2 =???
3 +4 =7

Basic types:Basic types: castingcasting expressionsexpressions
Euclidean (integer) division versus usual (real) division
int p=2;
int q=3;
int quotient=p/q;
int reminder=p%q; // modulo
double div=p/q;
double realdiv=(double)p/(double)q;
System.out.print(quotient);
System.out.print(“ “);
System.out.println(reminder);
System.out.println(div);
System.out.println(realdiv);
Cast (coercion)

Typing:
Safeguards for basic bugs in programs
Allows one to perform static analysis of programs
CastingCasting expressionsexpressions
Implicit casting for assignment
x=Expression;
Should be of the same type. Casting: Var=(TypeOfVar)Expression;
double x=2; // implicit casting
double x=(double)2;// explicit
double x=2.0; // same type

Implicit castingImplicit casting
Implicit casting rules
char c='X';
int code=c;
System.out.println(code);
Answers 88 (ASCII code of X)

Fundamentals of Java: Types
● Everything is typed (variables, constants, etc.)
● Require to declare types of variables
● The result of an expression has a type
● Variable and expression should have the same type for assignment
(d=5.0f)
(different types)
ERROR
ERROR
Compiler warns you of implicit casting
(possible loss of precision!)

Recap of simple (formula) programs
Declare variables of basic types: Type var;
double x;
int n,m; //separate with a comma variables
char c;
Assignment: var=Expression;
x=2.71;
n=2008;
c='X';
Arithmetic expression: Expression1 Operator Expression2
m=300%23;
delta=b*b-4*a*c;
Declare+Assign at once (shortcut):
int year2secs=365*24*60*60;

Incrementing/Decrementing
x=x+1;
x=x+step;
// Instructions equivalent to
x+=1;
x+=step;
// Decrement now
x-=3;
i=2;
i++; // equivalent to i=i+1;
++i; // similar, equivalent to i=i+1;
Incrementing is useful for loops

Pre- and post-incremention
i=5;
j=i++; // post-incrementation
ii=5;
jj=++ii; // pre-incrementation
compare...
Var++ returns the value of var and then increment it
++Var first increment var and then return its value
Thus j=5 but jj=6

Chopping Programs (Language)
Syntax of programs
Word
Sentence
Paragraph
Chapter
Book
Library
Reserved keywords
Variables
Instruction I;
Block (of instructions) {I;}
Function
Program
Library (API)

Commenting programs
● Adopt conventions
Eg., class ClassName .... stored in file ClassName.java
● Name variables explicitly (so that you can remember them easily)
● Comment programs (single line // or multiple lines /* */)
// Written for INF311
class CommentProgram
{
/* This is a simple Java program that
illustrates how to comment source code */
// Entry of the program
public static void main(String[ ] args)
{// it does nothing
}
}

A basic skeleton program in Java
Name of your program: Prog.java
Magic formula 2
Magic formula 1
> javac Prog.java
(builds a Prog.class file)
> java Prog
(execute the program)
2008
// Basic skeleton program for INF311
class Prog
{
{
int x=2008;
System.out.println(x);
}
}

Integrated Development Environment (IDE)
An IDE allows one to create, edit, compile and debug seamlessly
applications at the tip of mouse clicks.
(Eg., Jcreator, www.jcreator.com/ )
Eclipse

A Glimpse at Chapter 2:
Block of instructions

Euclid's Greatest Common Divisor (GCD)
Input: Two numbers a,b
Output: Find the greatest common divisors c of a and b
Euclid's original algorithm
History, proof, etc http://en.wikipedia.org/wiki/Euclidean_algorithm
For example, GCD of (30,105):
Mathematical proof:
GCD(30,105)
=GCD(30,75)
=GCD(30,45)
=GCD(30,15)
=GCD(15,15)
=GCD(15,0)
=> GCD(30,105)=15
30=2*5*3
105=7*5*3

Euclid's Greatest Common Divisor (GCD)
Input: Two numbers a,b
Output: Find the greatest common divisors c of a and b
Euclid's original algorithm
History, proof, etc http://en.wikipedia.org/wiki/Euclidean_algorithm
class GCD {
{
// Parse arguments into integer parameters
int a= Integer.parseInt(arg[0]);
int b= Integer.parseInt(arg[1]);
while (a!=b)
{
if (a>b) a=a-b;
else b=b-a;
}
// Display to console
System.out.println(a);
}
}

Euclid's greatest common divisor (GCD)
> javac gcd.java
(compile in a gcd.class)
> java gcd 234652 3456222 > gcd.txt
(execute and store result in gcd.txt)
arg[0] arg[1]

Geometric interpretation of Euclid's GCD
Visualize a (65) and b (25) on two axes
a=b=5, Stopping criterion + GCD

Programming is helpful for simulation
Simulation by Monte Carlo methods:
Eg., approaching PI=3.41592... using simulation
Draw a random point uniformly in a square:
Probability of falling inside a centered unit disk?
Monte-Carlo sampling extremely used
in graphics and financial economy !!!
How do we get (pseudo-)random numbers in Java?
Call function random() of class Math
Math.random();

Monte-Carlo estimation of PI in Java
Monte-Carlo simulation techniques proved useful in computational sciences
import java.util.*;
class MonteCarloPI
{
public static void main(String [] args)
{
int iter = 10000000; // # iterations
int hits = 0;
for (int i = 0; i < iter; i++)
{
double rX = 2*Math.random() - 1.0;
double rY = 2*Math.random() - 1.0;
double dist = rX*rX + rY*rY;
if (dist <= 1.0) // falls inside the disk
hits++;
}
double ratio = (double)hits/iter; // Ratio of areas
double area = ratio * 4.0;
System.out.println("Estimation of PI: " + area+ " versus
machine PI "+Math.PI);
}
}

Human versus Machine #transistors x2 every 18 months
http://www.kurzweilai.net/articles/art0134.html?printable=1
The Law of Accelerating Returns of Ray Kurzweil
● Machines are dull but extremely fast
● Designing software is difficult
(as difficult as building an Airbus)
● Artifical intelligence (AI) is a
key topic in Computer Science
Bug:
● Abnormality of the system
● Not by the faulty machine but by the programmer!
● Small bugs, big consequences!!!
(Ariane 501, Intel's Pentium division bug, etc.)
● Cost 100 billion$/ years (Dept. of Commerce)

Small bugs, big consequences: Numerical errors
If (a<b)
then
Block 1
else
Block 2
Predicate
Expressions
lhs=expression(rhs)
Branching
instruction
Finite precision, roundings of arithmetic operations may cause devastating effects
Wrong evaluation of a predicate
yields a different path of instructions: Bug!
Small numerical errors may not be so
capital here.

CAPTCHA versus SPAM (Human vs Machine)
Completely Automated Public Turing test to tell Computers and Humans Apart
Image-recognition CAPTCHAs:
Difficult task (OCR, segmentation, etc.)
To fight undesirable bulk spam, we need
to differentiate whether it is the action of
a human or an automated jam program.
(visual) CAPTCHA

Turing test...
Alan Turing, 1912-1954
(41 years old)
Pioneer of modern computer science
Proposed the “universal” Turing machine:
A ribbon, a head, a state and an action table
(automaton)
Turing test: proposal for a test of machine's capability to demonstrate
intelligence. Originally, for natural language conversation (and processing).
Initially, by text-only channel such a teletype machine
DNA, ribosome
Association for computing machinery (ACM)'s Turing Award (250000$)
[Nobel prize in computer science]

Versatility of Turing tests
The Continuator of F. Pachet (Sony CSL)
www.csl.sony.fr

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