Computer Science 15-100 (Sections T & U), Spring 2008
Class Notes:  Ch 2:  Data and Expressions (1 of 2)

Logistics

  1. Schedule
    1. Quiz 1 today!
    2. Hw2 due Friday
    3. Quiz 2 on Tuesday
  2. Hw1 returned today.
  3. Reading:
    1. L&L Chapter 2:  Data and Expressions
      Sections 2.1 - 2.2 and 2.6 today
      Sections 2.3 - 2.5, 2.7, 2.9 for Thursday

Topic Outline:

  • Data and Expressions
    1. Teasing apart HelloWorld.java

      public class HelloWorld {
            public static void main(String[] args) {
                System.out.println("Hello World!");
            }
      }
       
    2. print vs println 
      class MyCode {
  public static void main(String[] args) {
    System.out.print("This is");
    System.out.print("a test");
    System.out.println("that will run, albeit");
    System.out.println();
    System.out.println("with some problems...");
  }
}
    3. String concatenation 
      class MyCode {
  public static void main(String[] args) {
    System.out.println("a" + "b");
    System.out.println("a" +  1 );
    System.out.println("a" +  1 + "2");
    System.out.println("a" +  1 +  2 );
    System.out.println( 1  +  2 + "a");
  }
}
    4. Printing numbers (not Strings) 
      class MyCode {
  public static void main(String[] args) {
    System.out.println(1);
    System.out.println(1 + 2);
  }
}
    5. Escape Sequences (or, how do we print a double-quote?) 
      class MyCode {
  public static void main(String[] args) {
    System.out.println("This is a double-quote: \"");
  }
}
    6. More Escape Sequences (tabs, newlines, and backslashes) 
      class MyCode {
  public static void main(String[] args) {
    System.out.println("These items are tab-delimited, 3-per-line");
    System.out.println("abc\tdef\tg\nhi\tj\\\tk\n---");
  }
}
    7. "int" variables

      a)  Declare a variable and use it

      class MyCode {
  public static void main(String[] args) {
    int x = 3;
    System.out.println(x);
  }
}

      b)  Same, but with a nicer UI

      class MyCode {
  public static void main(String[] args) {
    int x = 3;
    System.out.println("x = " + x);
  }
}

      c)  Use a variable without declaring it

      class MyCode {
  public static void main(String[] args) {
    System.out.println("x = " + x);  // ERROR!  No such variable as x
  }
}

      d)  Declaring and assigning on different lines

      class MyCode {
  public static void main(String[] args) {
    int x;  // declare the variable
    x = 5;  // initialize it (assign it its first value)
    System.out.println("x = " + x);
  }
}

      e)  Assigning and re-assigning

      class MyCode {
  public static void main(String[] args) {
    int x;
    x = 1;
    System.out.println("x = " + x); // prints "x = 1"
    x = 2;
    System.out.println("x = " + x); // prints "x = 2"
  }
}

      f)  Using before assigning a value

      class MyCode {
  public static void main(String[] args) {
    int x;  // declared, but not assigned a value
    System.out.println("x = " + x);  // ERROR!  x is not initialized
  }
}

      g)  Using two variables

      class MyCode {
  public static void main(String[] args) {
    int x = 1;
    int y = 2;
    System.out.println("x = " + x);
    System.out.println("y = " + y);
  }
}

      h)  Declaring two variables on one line

      class MyCode {
  public static void main(String[] args) {
    int x = 1, y = 2;
    System.out.println("x = " + x);
    System.out.println("y = " + y);
  }
}

      i)  Using two variables in an expression

      class MyCode {
  public static void main(String[] args) {
    int x = 1, y = 2;
    System.out.println("x = " + x);
    System.out.println("y = " + y);
    System.out.println("x + y = " + (x + y));
  }
}

      j)  Same, but with a nicer UI

      class MyCode {
  public static void main(String[] args) {
    int x = 1, y = 2;
    System.out.println("x = " + x);
    System.out.println("y = " + y);
    System.out.println("x + y = "
                       + x + " + " + y
                       + " = " + (x + y));
  }
}
    8. I/O (Reading/Writing from/to Consoles, Strings, and Files)

      a)  Reading an "int" variable from the console

      import java.util.Scanner;
public class ReadFromConsole {
  public static void main(String[] args) {
    // declare local variables
    Scanner scanner = new Scanner(System.in);
    int x;
    
    // get the input (with a suitable prompt)
    System.out.print("Enter an integer: ");
    x = scanner.nextInt();
    
    // display result
    System.out.println("You entered: " + x);
  }
}

      b)  Reading an "int" variable from a String

      import java.util.Scanner;
public class ReadFromString {
  public static void main(String[] args) {
    // declare local variables
    Scanner scanner = new Scanner("5"); // simulated input in a string!
    int x;
    
    // get the input (with a suitable prompt)
    System.out.print("Enter an integer: ");
    x = scanner.nextInt();
    
    // display result
    System.out.println("You entered: " + x);
  }
}

      c)  Reading an "int" variable from a file

      import java.util.Scanner;
import java.io.File;
public class ReadFromFile  {
  public static void main(String[] args) throws Exception {
    // declare local variables
    Scanner scanner = new Scanner(new File("SampleFile.txt"));
    int x;    

    // get the input (with a suitable prompt)
    System.out.print("Enter an integer: ");
    x = scanner.nextInt();

    // display result
    System.out.println("You entered: " + x);
  }
}

      d)  Writing to a file

      import java.io.File;
import java.io.PrintStream;
public class WritingToAFile {
  public static void main(String[] args) throws Exception {
    PrintStream out = new PrintStream(new File("SomeFile.txt"));    
    out.println("This will output to the file 'SomeFile.txt'");
  }
}
    9. More about "int" Variables

      a)  Overflow

      class MyCode {
  public static void main(String[] args) {
    int x = 1234567890;  // about 1.2 billion
    int y = 1355779246;  // about 1.3 billion
    System.out.println("x = " + x);
    System.out.println("y = " + y);
    System.out.println("x + y = "
                       + x + " + " + y
                       + " = " + (x + y));
    // Prints:  x + y = 1234567890 + 1355779246 = -1704620160
  }
}

      b)  Integer division

      class MyCode {
  public static void main(String[] args) {
     System.out.println("20/3 = " + (20/3));
     System.out.println(" 6/3 = " + ( 6/3));
     System.out.println(" 5/3 = " + ( 5/3));
     System.out.println(" 2/3 = " + ( 2/3));
  }
}

      c)  The modulus or remainder operator (%)

      class MyCode {
  public static void main(String[] args) {
     System.out.println("20%3 = " + (20%3));
     System.out.println(" 6%3 = " + ( 6%3));
     System.out.println(" 5%3 = " + ( 5%3));
     System.out.println(" 2%3 = " + ( 2%3));
     System.out.println(" 0%3 = " + ( 0%3));
  }
}

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