Randomness

You know what's cool? Having the computer randomly choose a number. This is the basis of pretty much every computer game ever.

To pick a random number, you first need to import java.util.Random;.

Then, you must create a random-number generator object, like so:

	Random r = new Random();

Once that's finished, you can have the computer pick a random integer like this:

	int x = 1 + r.nextInt(10);

That'll pick a random number from 1 to 10 (inclusive) and store it into the variable x. Enough of the explaining; let's look at some code!

Files Needed

import java.util.Random;

public class Randomness
{
	public static void main ( String[] args )
	{
		Random r = new Random();

		int x = 1 + r.nextInt(10);

		System.out.println( "My random number is " + x );

		System.out.println( "Here are some numbers from 1 to 5!" );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.print( 1 + r.nextInt(5) + " " );
		System.out.println();

		System.out.println( "Here are some numbers from 1 to 100!" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.print( 1 + r.nextInt(100) + "\t" );
		System.out.println();

		int num1 = 1 + r.nextInt(10);
		int num2 = 1 + r.nextInt(10);

		if ( num1 == num2 )
		{
			System.out.println( "The random numbers were the same! Weird." );
		}
		if ( num1 != num2 )
		{
			System.out.println( "The random numbers were different! Not too surprising, actually." );
		}
	}
}

What You Should See

Your random numbers will probably be different than these. Actually, that's kind of the point.

My random number is 8
Here are some numbers from 1 to 5!
1 1 5 4 2 2
Here are some numbers from 1 to 100!
25      25      39      34      93      13
The random numbers were different! Not too surprising, actually.

What You Should Do on Your Own

Assignments turned in without these things will receive no credit.

  1. Delete the 1 + in front of all six lines that pick numbers 1-5, so that they look like this: System.out.print( r.nextInt(5) + " " ); Run the program a few times, and see if you can figure out what range the new random numbers are in.
  2. Change the 1 + in front of all six lines that pick numbers 1-5, so that they look like this: System.out.print( 3 + r.nextInt(5) + " " ); Run the program a few times. Is it picking random numbers from 3 to 5? If not, what range are they?
  3. Change the line where you create the random number generator so that it looks like this: Random r = new Random(12353); This number is called a seed. Run the program a few times. What do you notice? What happened to the random numbers?
  4. Change to random seed to something else and observe the behavior. What happens to the random numbers?
  5. (Delete the random seed before turning in the assignment.)


©2013 Graham Mitchell

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