Java Rumblings

Monday, 2 August 2010

PowersOfTwo.java

/*************************************************************************
 *  Compilation:  javac PowersOfTwo.java
 *  Execution:    java PowersOfTwo N
 *  
 *  This program takes a command-line argument N and prnts a table of
 *  the powers of 2 that are less than or equal to 2^N.
 *
 *  % java PowersOfTwo 5
 *  0 1
 *  1 2
 *  2 4
 *  3 8
 *  4 16
 *  5 32
 *
 *  % java PowersOfTwo 6
 *  0 1
 *  1 2
 *  2 4
 *  3 8
 *  4 16
 *  5 32
 *  6 64
 *
 *  Remarks
 *  ------------
 *  Only works if 0 <= N < 31 since 2^31 overflows an int.
 *
 *************************************************************************/

public class PowersOfTwo {
    public static void main(String[] args) {

        // read in one command-line argument
        int N = Integer.parseInt(args[0]);

        int i = 0;                // count from 0 to N-1
        int powerOfTwo = 1;       // the ith power of two

        // repeat until i equals N
        while (i <= N) {
            System.out.println(i + " " + powerOfTwo);  // print out the power of two
            powerOfTwo = 2 * powerOfTwo;               // double to get the next one
            i = i + 1;
        }

    }
}

Harmonic.java


/*************************************************************************
 *  Compilation:  javac Harmonic.java
 *  Execution:    java Harmonic N
 *  
 *  Prints the Nth harmonic number: 1/1 + 1/2 + ... + 1/N.
 * 
 *  % java Harmonic 10
 *  2.9289682539682538
 *
 *  % java Harmonic 10000
 *  9.787606036044348
 *
 *************************************************************************/

public class Harmonic { 
    public static void main(String[] args) { 

        // command-line argument
        int N = Integer.parseInt(args[0]);

        // compute 1/1 + 1/2 + 1/3 + ... + 1/N
        double sum = 0.0;
        for (int i = 1; i <= N; i++) {
            sum += 1.0 / i;
        }

        // print out Nth harmonic number
        System.out.println(sum);
    }

}

Factors.java

/*************************************************************************
 *  Compilation:  javac Factors.java
 *  Execution:    java Factors N
 *  
 *  Computes the prime factorization of N using brute force.
 *
 *   % java Factors 81
 *   The prime factorization of 81 is: 3 3 3 3 
 *
 *   % java Factors 168
 *   The prime factorization of 168 is: 2 2 2 3 7
 *
 *   % java Factors 4444444444
 *   The prime factorization of 4444444444 is: 2 2 11 41 271 9091 
 *
 *   % java Factors 4444444444444463
 *   The prime factorization of 4444444444444463 is: 4444444444444463
 * 
 *   % java Factors 10000001400000049
 *   The prime factorization of 10000001400000049 is: 100000007 100000007 
 *
 *   % java Factors 1000000014000000049
 *   The prime factorization of 1000000014000000049 is: 1000000007 1000000007
 *
 *   % java Factors 9201111169755555649
 *   The prime factorization of 9201111169755555649 is: 3033333343 3033333343 
 *
 *   Can use these for timing tests - biggest 3, 6, 9, 12, 15, and 18 digit primes
 *   % java Factors 997
 *   % java Factors 999983
 *   % java Factors 999999937
 *   % java Factors 999999999989
 *   % java Factors 999999999999989
 *   % java Factors 999999999999999989
 *
 *   Remarks
 *   -------
 *   - Tests i <= N / i instead of i <= N for efficiency.
 *
 *   - The last two examples still take a few minutes.
 *
 *   - Tests i <= N / i instead of i * i <= N  to stave off overflow
 *     and enable us to factor 9201111169755555649; otherwise program
 *     goes into infinite loop. However, i * i <= N is almost twice
 *     as fast on some systems.
 *
 *************************************************************************/

public class Factors {

    public static void main(String[] args) { 

        // command-line argument
        long n = Long.parseLong(args[0]);

        System.out.print("The prime factorization of " + n + " is: ");

        // for each potential factor i
        for (long i = 2; i <= n / i; i++) {

            // if i is a factor of N, repeatedly divide it out
            while (n % i == 0) {
                System.out.print(i + " "); 
                n = n / i;
            }
        }

        // if biggest factor occurs only once, n > 1
        if (n > 1) System.out.println(n);
        else       System.out.println();
    }
}

Converting an integer to binary string - Binary.java


/*************************************************************************
 *  Compilation:  javac Binary.java
 *  Execution:    java Binary n
 *  
 *  Prints out n in binary.
 * 
 *  % java Binary 5
 *  101
 *
 *  % java Binary 106
 *  1101010
 *
 *  % java Binary 0
 *  0
 * 
 *  % java Binary 16
 *  10000
 *
 *  Limitations
 *  -----------
 *  Does not handle negative integers.
 *
 *  Remarks
 *  -------
 *  could use Integer.toBinaryString(N) instead.
 *
 *************************************************************************/

public class Binary { 
    public static void main(String[] args) { 

        // read in the command-line argument
        int n = Integer.parseInt(args[0]);

        // set v to the largest power of two that is <= n
        int v = 1;
        while (v <= n/2) {
            v = v * 2;
        }
  
        // check for presence of powers of 2 in n, from largest to smallest
        while (v > 0) {

            // v is not present in n 
            if (n < v) {
                System.out.print(0);
            }

            // v is present in n, so remove v from n
            else {
                System.out.print(1);
                n = n - v;
            }

            // next smallest power of 2
            v = v / 2;
        }

        System.out.println();

    }

}

Though written a program, we can use inbuilt lib of java - See here.

Sunday, 1 August 2010

Input functions in java (UDFs)

Inputing strings
public int inputInt() throws IOException
{
    int k=0;
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String str;
    str = br.readLine();
    k = Integer.parseInt(str);
    return k;

}
For inputing integers
public int inputInt() throws IOException
{
    int k=0;
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String str;
    str = br.readLine();
    k = Integer.parseInt(str);
    return k;

}

Input or read in java from Standard IO

Lets first see what we mean by standard stream in java. Click here.

Using scanner

To read console input, you first construct a Scanner that is attached to the "standard input stream" System.in.
```
Scanner in = new Scanner(System.in);
```
Now you use the various methods of the Scanner class to read input. For example, the nextLine method reads a line of input.
```
System.out.print("What is your name? ");
String name = in.nextLine();
```
Here, we use the nextLine method because the input might contain spaces. To read a single word (delimited by whitespace), call
```
String firstName = in.next();
```
To read an integer, use the nextInt method.
```
System.out.print("How old are you? "); 
int age = in.nextInt();
```
Similarly, the nextdouble method reads the next floating-point number.

Finally, add the line
```
import java.util.*;
```

Using JOptionPane

String input = JOptionPane.showInputDialog(promptString)

eg.

String name = JOptionPane.showInputDialog("What is your name?");

Entering Integers

String name = JOptionPane.showInputDialog("What is your age?");
int age = Integer.parseInt(input);


Import import javax.swing.*;

Finally, whenever your program calls JOptionPane.showInputDialog, you need to end it with a call to System.exit(0). The reason is a bit technical. Showing a dialog box starts a new thread of control. When themain method exits, the new thread does not automatically terminate. Toend all threads, you call the System.exit method.

Using BufferedReader Its most commonly used constructor is shown here:

BufferedReader(Reader inputReader)
Here, inputReader is the stream that is linked to the instance of BufferedReader that is being created. Reader is an abstract class. One of its concrete subclasses is InputStreamReader, which converts bytes to characters. To obtain an InputStreamReader object that is linked to System.in, use the following constructor:
InputStreamReader(InputStream inputStream)

So basic step is :
```
InputStreamReader inp = new InputStreamReader(System.in)
BufferedReader br = new BufferedReader(inp);
```
Now to read character :
```
char c = br.read();
```
To read String:
```
String str = br.readLine();
```
- Reading Characters
  int read( ) throws IOException
  Each time that read( ) is called, it reads a character from the input stream and returns it
  as an integer value. It returns –1 when the end of the stream is encountered. As you can
  see, it can throw an IOException.
```
char c;
BufferedReader br = new
BufferedReader(new InputStreamReader(System.in));
c = (char) br.read();
System.out.println(c)
```
- Reading strings
  String readLine( ) throws IOException
```
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
String str;
str = br.readLine();
```