Computer Science Fundamentals

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Computer Science Fundamentals

• A topics course in Computer Science

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Topics we hope to cover

• Data representation (especially binary and hex),
  arithmetic, base change algorithms
• Operating systems (including an introduction to
  Threads of execution)
• Boolean algebra and switching functions
• Combinational and flip-flop circuitry (an
  introduction to architecture)
• Computer communication (an introduction to
  sockets and/or datagrams)
• Theory of computation (an introduction to
  automata and computability)
• Assembly language programming
• Program translation
• Well also learn some java programming!

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Data representation

• Information in the computer is represented in
  base 2 (binary), although, for convenience, base
  16 is also sometimes used.
• In radix positional notation, a digit d, located
  in position p, in a number base b, represents
  dbp. So, the decimal value 8352 represents
  8(1000)3(100)5(10)21. Similarly, the
  binary value 1011101 represents the value
  16403211618140211.
• Note that the powers base 2 are 1,2,4,8,16,32,
  201, 212, etc. Make sure you are familiar with
  the two-powers.

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You will need to memorize the values 0..15 in
binary and hex
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Kinds of data

• There are many types of data stored in computer
  memory. Some memory cells hold instructions,
  some hold integers, some hold float values, some
  hold char data, other store composite data types.

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Char data is stored internally using the ASCII
(or UNICODE) collating sequence

• You can find a complete ASCII table in most
  computer texts.
• Exercise In C or Java, print your own ASCII
  table print the int value x 32255 in the left
  column of a table and print (char)(x) in the
  right column.
• You will need to know the collating sequence
  values for some characters like 0, 1, also
  a,b, and A, B,
• Exercise write these values in binary.

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Int data

• Integers may be stored internally in one of
  several formats
• BCD (binary coded decimal) often used in
  spreadsheet programs
• Unsigned int a positive binary value
• Signed int positive or negative value

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BCD

• BCD allocates 4 bits (a nibble) of binary storage
  for each decimal digit to be represented.
• Example 598 in decimal will require
  3digits4bits each 12 bits. In BCD it looks
  like this 010110011000
• Practice converting a few decimals to BCD

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Unsigned ints

• In unsigned (positive) representations, all bits
  are used to indicate the value.
• In 4 bits of binary, you can represent decimal
  values 0,,15. (see table a few slides back.)
  What values could be represented in 5 bits? In 8
  bits? In 16 bits?
• What is the general rule?
• Represent values 0, 48, 65, 97, and 127 in 8
  bits. Example 48 decimal is 00110000 in 8 binary
  bits.
• 8 bits is called a byte.

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Quick conversion between bases From base b to
decimal

• To convert from any base to decimal, use Horners
  algorithm for polynomial evaluation. A number in
  base b can be regarded as a polynomial p in the
  variable b. Let P(b) anbn a n-1 b
  n-1a1b1a0b0
• This would be written as the series of
  (coefficient) digits. For example, the base 6
  value 32501 represents 3(64)2(63)5(62)0(61)
  1(60)
• Using nested multiplication we can rewrite this
  without needing to compute exponentials of the
  base ((((36)2)65)60)61

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Base b to decimal via Horner

• If a value in a base b is entered as a String in
  a TextField named input, and the base is entered
  in a TextField called basefield, we could use the
  following Java code to do the conversion
• String value
• int b, answer
• answer0
• bInteger.parseInt(basefield.getText())
• valueinput.getText()
• for(int j0jltvalue.length()j)
• answer answerbase (int) (value.charAt(j)-48)
  

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Remarks on this code exercises

• Note that the length of a String in Java can be
  obtained using the class method length(). The
  String method charAt(p) returns the char in
  position of a String. The TextField method
  getText() returns the String entered into a
  TextField.
• For which bases would the java code fragment not
  provide the correct answer? Why?
• Fix this problem.
• Complete the JApplet. Write a JApplet which
  provides labels and textfields for a base, a
  value and an answer and performs the conversion
  from base b to base 10.

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Converting from decimal to other bases

• In determining what a decimal value would look
  like if represented in another base (b), we need
  to figure out what digit in base b goes in each
  position of the representation.
• Suppose n295 and b2, ie., we are trying to
  convert to binary. The 1s place will have a 1 in
  it since 295 is odd.
• 295 (10) ???..??1(2)
• But how many other bits are there, and what are
  each of their values? Since positions in the
  binary representation indicate powers of two,
  having figured out the ones place means we have
  only half of n (295/2 147) left to represent.

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Example convert 295 decimal to binary

• 295 is oddleast significant bit is a 1 295(10)
  ???..?1(2)
• 295/2 147. Since 147 is odd, 2s bit is also a
  1 295(10) ???..?11(2)
• 147/2 73. Since this is odd, next bit is also a
  1 295(10) ???..111(2)
• 73/236. Even, so next bit is a zero 295(10)
  ???..0111(2)
• 36/218, which is even. 295(10) ???00111(2)
• 18/29, odd. 295(10) ???100111(2)
• 9/24 even. ??0100111(2)
• 4/22 even. ?00100111(2)
• 2/21 0dd. 100100111(2)
• ½0 stop.

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Decimal to any other base

• Convert from decimal to any base using the
  remainder/division algorithm described
  previously. Each remainder operation generates
  one digit of the answer. Then divide the number
  by the base and continue.
• String answer
• int base Integer.parseInt(basefield.getText())
• int value Integer.parseInt(input.getText())
• while (value!0)
• answer valuebaseanswer
• value/base

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Remarks Exercises

• Note, we use the static Integer method parseInt()
  to convert a String to an int value. 
• For what input values would this Java code not
  work properly? Why?
• Can you fix this problem to convert decimal
  values into values in any other base?
• Write the JApplet which would provide labels,
  textfields and buttons enabling the user to enter
  a decimal int and a new base value and convert
  the decimal int to the new base.
• Practice doing these conversions by hand.

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Base conversions base b to base bn

• If you are converting between bases where one
  base is a power of the other, like binary to hex,
  or binary to octal, or base 3 to base 9 then the
  conversion may be done very easily. n digits of
  base b will be needed for each 1 digit of base bn
  .
• Consider converting A6E9 in base 16 to binary.
  Each hex digit will generate 4 binary bits. If
  you have not memorized your hex digits yet, this
  is a good time to start. In decimal A is 10, B
  is 11, C is 12, F is 15. So A 1010(2), etc.
• A6E9 (16) 1010011011101001 (2)
• Convert this number (right hand side above) to
  base 8 and base 4.
• Convert the base 9 value 783361 to base 3.

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An introduction to Java

• Java syntax is the same as C. In other words,
  an assignment statement, an if, a while, or a for
  statement in Java looks the same as in C.
• Java class methods are called, like in C, using
  the dot . operator
• ClassInstance.methodName(parameters)
• To get a String which a user has entered into a
  TextField named input we might use the code
• String value input.getText()//getText is a
  TextField method
• Java methods (functions) have return types,
  parameters, and their definitions are enclosed in
  a block , like in C programming. Prototypes
  are not used in Java.

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An introduction to Java

• Java provides components (classes) buttons,
  textfields, and labels, among many others. These
  three are called JTextField, JLabel, and JButton
  in Java 1.2 and higher. These classes must be
  instantiated with new. Example
• JButton bnew JButton(press me)
• In a small program, declarations are typically
  global (not inside a method) and data allocation
  occurs in the JApplet method init(). The init
  method is executed when a browser loads an
  applet.
• All java applets and applications must consist of
  an outer class
• //sample Java JApplet 
• import javax.swing. //contains Java2 classes 
• public class OuterClass extends JApplet
• //open block  
• //methods, declarations go in here
• //close outer block

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JApplet with JLabel, JTextField and JButton

• import javax.swing.  
• public class OuterClass extends JApplet
• //global applet declarations typically go here
• public JLabel label
• public JTextField text
• public JButton button
• public void init()//first method browser will
  execute
• Container cgetContentPane()//get the panel for
  this applet
• c.setLayout(new FlowLayout())//one sort of
  layout
• buttonnew JButton(press me)
• label new JLabel (enter value)
• textnew JTextField(20)
• c.add(label)//components appear in the order
  they were added to the panel
• c.add(text)
• c.add(button)
• //end init
• //end applet

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JApplet structure

• Note that imports appear the top of a java
  Applet. These are like includes in C.
• Applets may have many user defined methods.
• They may also override the Applet methods init(),
  paint(), start(), stop(), and many others.
• A Java applet or an application must consist of
  an outer class (with a block) and any data
  fields, and methods (with their blocks) defined
  inside the outer block.

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Applet deployment

• We must perform a number of steps in order to
  deploy an applet to our webspace so a visitor
  there could run it.
• Type the code into an editor or IDE
• Compile the .java file. This process will check
  syntax. If it is successful, a .class file will
  be created which has the same name as the
  original java source file. Ie. Prog.java when
  compiled with generate a file called prog.class.
  Create an appropriate html file to point a
  browser to your class file.
• Check things out on your desktop by running the
  applet in appletviewer.
• Use FTP to send appropriate html and class files
  to your webspace

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Using an editor

• Type the java source code into an editor or IDE
  (integrated development environment like
  JBuilder) and save it as a .java file.
• The filename must be the same as the outerclass
  name.
• We will use TextPad, a shareware program, as our
  editing environment, although you can also use
  the Borland C editor or Word. (If you use MS
  Word, make sure the file is saved as ASCII text
  format and has the extension .java)
• TextPad can be downloaded on your own computer.
  It is produced by Helios.com
• TextPad has some features of an IDE you can
  compile and run java applets and applications
  from TextPad.
• TextPad is available in the campus labs.

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Compile the .java file.

• This process will check syntax. If it is
  successful, a .class file will be created which
  has the same name as the original java source
  file. Ie. Prog.java when compiled with generate
  a file called Prog.class.Java is case sensitive.
• Compile in TextPad by selecting compile from the
  tools menu.
• Alternative. If youve downloaded JDK1.3.X onto
  your own computer, you have java, the java
  compiler as part of the kit. Open a DOS window
  (run command.com)and navigate to your jdk bin
  directory where you should have saved your .java
  file. At the prompt, type java Prog.java
• This will create a file called Prog.class.
• A class file is not an executable, it contains
  portable byte-code. It requires a java
  interpreter (named java) to be executed.

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Create an appropriate html file to point a
browser to your class file

• Sample html
• lthtmlgt
• ltapplet code Prog.class height300 width500gt
• lt/appletgt
• lt/htmlgt
• This must be typed into a file and saved with the
  .html extension. You could call this file
  Prog.html to make it easier to remember which
  applet it pointed to.

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A possible problem with the html and current
browser versions (IE Netscape)

• Depending on what browser you use, and its
  version, it may require a plugin to run java2.
  The html file will then need to be modified to
  tell the browser to go load the plug in. Campus
  labs have the plugin already downloaded.
• There should also be a java program called
  HTMLConverter you can use to convert the html
  file we created into a more complicated one that
  tells the browser it needs to load a plugin to
  run the applet.
• My class page has examples of this converted html
  file format you can cut and paste from the
  examples there to create a new html.
• See class notes about this topic.

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Check things out on your desktop by running the
applet in appletviewer

• If you can get into DOS (on your own computer)
  and have downloaded the jdk you can use the
  simple html file format I presented earlier to
  check whether your applet works now.
• Assuming you have an applet called Prog.java, a
  class file named Prog.class and an html file
  named Prog.html, navigate to the bin directory
  and type appletviewer Prog.html
• appletviewer is a program in the jdk which will
  run your applet as a browser would, so you can
  check if it works ok.
• Textpad is also configured to run appletviewer in
  the labs. If youve created an html file for
  your applet, select appletviewer from the textpad
  tool menu. You may need to have saved these
  files in particular directories, like the
  java/bin directory, for this to work properly in
  the labs. See class notes on this.

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Use FTP to send appropriate html and class files
to your webspace

• I have a screen shot of how to configure your
  WinFTP session to ship files to your webspace on
  our class pages.
• Make sure to send .class files as type binary.
  Later, youll use binary type for jpg and wav
  files also.
• HTML can be sent as ascii.
• Type your applet url into the browser to go visit
  your applet. It will be something like
• http//www.webserver2/students/smitxy87/Prog.html

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An applet with a listener

• Applets are event-driven programs. This means
  things happen when the user click buttons, moves
  the mouse, or enters values into fields.
• It is up to the programmer to insure the applet
  listens for button clicks (or whatever) and does
  what needs to be done if a button is clicked.
• This is done by means of listeners in Java.
  There are many (!) listeners. One of the
  simplest is the one which checks if values have
  been entered into textfields or if a button is
  clicked. It is called ActionListener.
• Listeners are types of interfaces. A java applet
  or application program may use many interfaces.

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Implementing the ActionListener

• import javax.swing.  
• public class OuterClass extends Japplet
  implements ActionListener
• //global applet declarations typically go here
• public JLabel label
• public JTextField text1, text2
• public JButton button
• public void init()//first method browser will
  execute
• Container cgetContentPane()//get the panel for
  this applet
• c.setLayout(new FlowLayout())//one sort of
  layout
• buttonnew JButton(press me)
• label new JLabel (enter value)
• text1new JTextField(20)
• Text2new JTextField(20)
• c.add(label)//components appear in the order
  they were added to the panel
• c.add(text1) c.add(text2)
• button.addActionListener(this)//button is
  listening now for user to press it
• c.add(button)
• //end init
• //see next slide!

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ActionListener interface requires the programmer
to define the method actionPerformed()

• The above applet is not complete. It is missing
  the outer close block, but even so, will not
  compile without the definition of the method
  actionPerformed which tells the aplet what to do
  if the button is pressed.
• public void actionPerformed(ActionEvent x)
• int z Integer.parseInt(text1.getText())//get
  int entered by user
• int p 2z// do elaborate computations
• text2.setText(p)
• //when button is clicked, 2input value is
  calculated and displayed
• //end actionPerformed
• //end outer class