Random Numbers in Java

Random Numbers in Java

by Lisa Donlan

​How do random numbers work?

​Math.random() returns a random number between 0.0-0.99.

​

(int)(Math.random( )*range) + min moves the random number into a range starting from a minimum number.

​

The range is the (max number - min number + 1).

​

​What do you think the output is?

​public class Test3

{

   public static void main(String[ ] args)

   {

     System.out.println(Math.random());

     System.out.println(Math.random());

   }

}

​One possible result is

​0.3842967819081746

0.30067392084109024

​System.out.println(Math.random());

System.out.println(Math.random());

Multiple Choice

What does 

  $$double\ rnd=Math.random(\ );$$  

return?

• returns a random number between 0.0-0.99

• returns a random number between 0.0-1.00

• returns a random number between 0 and 1

• a negative number

​How do you use random to choose numbers?

​Example: Randomly choose numbers on a die.

​

Multiple Choice

$$int\ rnd2=(int)(Math.random(\ )*10)+1;$$  

$$System.out.print\left(rnd1\right);$$  

What is the output?

• any integer between 1 and 10 exclusive

• any integer between 1 and 10 inclusive

• any integer between 0 and 10 inclusive

• any integer between 0 and 10 exclusive

Multiple Choice

How would you choose the numbers from 1 to 100?

• System.out.println((int) (Math.random( ) * 10)+10);

• System.out.println((int) (Math.random( ) * 10)+100);

• System.out.println((int) (Math.random( ) * 100)+100);

• System.out.println((int) (Math.random( ) * 100)+1);

​How do you get a random number you can use?

​// rnd1 is an integer in the range 0-9 (including 9).

​

int rnd1 = (int)(Math.random( ) * 10);

​

​Explain to your shoulder partner

​

1. Why do we have to use (int)?

2. What is this called (the concept is tested on the AP test EVERY year!!)?

3. ​Why do we have to multiply by 10?

Multiple Choice

Which of the following would be true about 40% of the time?

• Math.random() > 0.4

• Math.random( ) < 0.4

• Math.random() == 0.4

Multiple Choice

Which of the following would return a random number from 1 to 5 inclusive?

• ((int) (Math.random( ) * 5))

• ((int) (Math.random( ) * 6))

• ((int) (Math.random( ) * 5) + 1)

Multiple Choice

Which of the following would return a random number from 0 to 10 inclusive?

• ((int) (Math.random( ) * 11))

• ((int) (Math.random( ) * 10))

• ((int) (Math.random( ) * 10) + 1)

Multiple Choice

Which of the following would be true about 75% of the time?

• Math.random( ) < 0.25

• Math.random( ) == 0.25

• Math.random( ) > 0.25

Multiple Choice

Which of the following statements assigns a random integer between 25 and 60, inclusive, to rn?

This is a released AP MC question.

• int rn = (int) (Math.random( ) * 25) + 36;

• int rn = (int) (Math.random( ) * 36) + 25;

• int rn = (int) (Math.random( ) * 25) + 60;

• int rn = (int) (Math.random( ) * 60) + 25;

• int rn = (int) (Math.random( ) * 26) + 60;