COMS W1004 (CS I)
Intro to CS and Java Programming
Lecture 19: November 11, 2004

Lecture Outline

1. A prediction from the past
2. Review
3. Computer science as a discipline
4. Theory
5. Scanner class
6. Programming assignment #5
7. Reading assignment

1. Future home computer of 2004?

2. Review

• Representing data
• Floating-point numbers
• Images
• Sound

3. Computer science as a discipline

• Computer science is the study of
• computation
• computers
• complexity
• informatics
• Computer science is the study of computers and what they can do - the inherent powers and limitations of abstract computers, the design and characteristics of real computers, and the innumerable applications of computers to solving problems.
• The scientific method:
1. Formulate a hypothesis.
2. Design an experiment to test the hypothesis.
3. Conduct the experiment.
4. Analyze the results to see if they support the hypothesis.
• Computer science encompasses hardware, software and theory
• Subfields of computer science
• Algorithms and data structures
• Artificial intelligence
• Computational biology
• Computational science
• Computer engineering
• Computer vision
• Database systems
• Graphics
• Human-computer interaction
• Machine architecture
• Machine learning
• Networking
• Programming languages and compilers
• Operating systems
• Quantum computing
• Robotics
• Software engineering
• Security

4. Theory

• Turing machine: Reed, Fig. 10.4
• Is there a most powerful computer?
• Universal Turing machine
• We can construct a Turing machine that takes as input a specification of another Turing machine M and an input x, and simulates the behavior of M on x.
• Church-Turing thesis
• The set of Turing-computable functions is the same as the set of all computable functions.
• Colloquially, anything that can be computed can be computed by a Turing machine.
• Halting problem
• Given a program and an input to the program, answer "yes" if the program will eventually stop with this input; otherwise, answer "no."
• We can prove no such program exists.
• We say the halting problem is noncomputable (or undecidable).
• Nondeterminism
• A nondeterministic Turing machine can have moves in which it can make a choice of next move.
• Acceptance is defined if there exists a finite sequence of moves that leads to success.
• Nondeterministic Turing machines are no more powerful than deterministic Turing machines.
• P versus NP
• P is the set of problems that can be solved in polynomial time on a deterministic Turing machine.
• NP is the set of problems that can be solved in polynomial time on a nondeterministic Turing machine.
• Does P = NP? There is a one million prize for answering this question!
• NP-complete problems
• An NP-complete problem is a problem in NP that is as hard as any other problem in NP. The Traveling Salesman Problem is an example of an NP-complete problem.

5. Scanner class

• Java 1.5 has introduced the Scanner class for keyboard input and parsing
• Note: use
Scanner scan = new Scanner(System.in);
rather than
Scanner scan = Scanner.create(System.in);
• Here is a Java program that uses the Scanner class to read and print a line of text entered by the user:

//*********************************
// Echo.java
// Java program to read a line of text
//   and then print it
// Created by Al Aho on November 10, 2004
// Last modified on November 10, 2004
//*********************************

import java.util.Scanner;

public class Echo
{
   //--------------------------------------
   // Read a character string from the user
   // and then print it
   //--------------------------------------

   public static void main (String[] args)
   {
      String message;
      Scanner scan = new Scanner(System.in);
      System.out.println("Enter a line of text:");
      message = scan.nextLine();
      System.out.println("You entered: \"" + message + "\"");
   }
}

• Here is a Java program that asks for two integers from the keyboard, adds them, and then prints the sum.

//**************************************
// Addition.java
// Java program to add two integers
//   entered via keyboard
// Created by Al Aho on November 10, 2004
// Last modified on November 10, 2004
//**************************************

import java.util.Scanner;
public class KeyAddition
{
 //------------------------------------------
  // Adds two integers entered via keyboard
  //------------------------------------------

  public static void main(String[] args)
  {
    Scanner scan = new Scanner(System.in);
    int number1;
    int number2;
    int sum;
    String again;

    do {
      System.out.print("Enter first integer: ");
      number1 = scan.nextInt();
      System.out.print("Enter second integer: ");
      number2 = scan.nextInt();
      sum = number1 + number2;

      System.out.println("The sum is " + sum);
      System.out.print("Do Another? (y/n): ");
      again = scan.next();
    }
    while (again.equals("y"));

    System.exit(0);
  }
}






6. Programming assignment #5






7. Reading assignment

• Lewis and Loftus, Sections 2.6 and 4.1-4.4





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aho@cs.columbia.edu