Lecture 8

Announcements

CS Application Deadlines: http://www.cs.virginia.edu/acad/declaring.html
1st year SEAS: via the normal Dean's office declaration form
2+ year SEAS: as CS Office in Rice hall, now through February 17
CLAS BA: at CS Office in Rice hall, February 24 through March 17
CLAS transfer to SEAS: via the normal Dean's office declaration form
SEAS Minor: http://www.cs.virginia.edu/~horton/cs-minor-apply by March 1
CLAS Minor: not offered

HW1 - what it will tell you when

Talking Points

Pick up where we left off

Strings and excape sequences: "I said \"hi\" to them" the \" is a quote character instead of the end of a string. "The word is \""+word+"\""

APIs on the Collab Wiki

Scanners: all you need is one (per input source)

Integer division

Casting – in the same way that -x means give me a negative version of x, (int)x means give me an integer version of x. In the same way that -"hi" doesn't make sense, (int)"hi" also doesn't make sense.

We almost always put parentheses around the thing being cast

public class CastingDivision {
    public static void main(String[] args) {
        System.out.println( (int)(3.14) );
        System.out.println( (double)(3) );
        System.out.println( (char)(68) );
        char c = 'w';
        c = (char)(c - 3);
        System.out.println(c);
        System.out.println( 9 / 5 );
        System.out.println( 9.0 / 5 );
        System.out.println( 9 / 5.0 );
        System.out.println( 9.0 / 5.0 );
        System.out.println( (double)(9 / 5) );
        System.out.println( (double)(9) / 5 );
    }
}

Constants: public static final

   public class Confusing {
public static void main(String[] args) {
        Scanner cheese = new Scanner(System.in);
        System.out.print("Enter number: ");
        
        int n = cheese.nextInt();
    double s = 0.00471 * n;
        int g = (int)(s);
        
        System.out.println("ans = " + g); }}

public class MP3Sizer {

    // accepted value for gigabytes per song
    public static final double GIGABYTES_PER_SONG = 0.00471;

    // main(): program starting point
    public static void main( String[] args ) {

        // set up input scanner
        Scanner stdin = new Scanner( System.in );

        // get number of songs
        System.out.print( "Enter number of songs: " );
        int nbrSongs = stdin.nextInt();

        // determine expected number of gigabytes to store songs
        double gigaBytes = GIGABYTES_PER_SONG * nbrSongs;

        // drop the decimal
        int storage = (int) ( gigaBytes );

        // display result
        System.out.println( "Amount of storage necessary for " + nbrSongs
                                +  " songs is " + storage + " GB" );
    }
}

DecimalFormat

e notation: 13e-4 means 13 × 10⁻⁴; read e as times ten to the

The Math library: Math.pow(4.0651,5), Math.sqrt(2), Math.PI, etc

DecimalFormat twoPlaces = new DecimalFormat("0.00");
DecimalFormat oneOrTwoPlaces = new DecimalFormat("0.0#");
double n1 = 1.0 / 99;
double n2 = Math.pow(1111, 0.59999);
String s1a = twoPlaces.format(n1);
String s1b = oneOrTwoPlaces.format(n1);
String s2a = twoPlaces.format(n2);
String s2b = oneOrTwoPlaces.format(n2);
System.out.println(n1 + " " + s1a + " " + s1b);
System.out.println(n2 + " " + s2a + " " + s2b);

Finite precision: ±2 billion for int, about 16 decimal places for double

System.out.println(1000 * 1000 * 1000);
System.out.println(1000 * 1000 * 1000 * 1000);
System.out.println(1.23456789 + 1 - 1);
System.out.println(1.23456789 + 1e10 - 1e10);
System.out.println(1.23456789 + 1e20 - 1e20);
System.out.println(0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1);