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How to generate random numbers 1 to 9 and 1 to 7 using function that generates 1 to 5

Another highly popular interview question – how do you generate random numbers 1 through 7 or 1 through 9 (or any other range) using a function that generates numbers 1-5. In this article, I will show you how to do that, with some further analysis.

To generate a random number 1- 5, I have used the following function. It is to note that Math.random() generates 0.0 through 1.0. I have used Math.ceil function to generate numbers through 1-5. Math.ceil(0.0) is 1 strangely. It’s probably because of the double representation such as 0.0 being 0.00000000001 etc.

	public static int randomFive() {
		return (int) (Math.ceil(Math.random() * 5));
	}


To generate random number in another range, I have used called this method twice to get a good sample and to include all the numbers in the range, then taking a modulus to make sure the numbers are not out of range. In the following program, I have then called these methods randomSeven() and randomNine() one thousand times each just to see how often different numbers in the given range get generated.

Here is the full source code:

package com.icodejava.blog;

import java.util.Arrays;

/**
 * @author Kushal Paudyal
 * www.icodejava.com
 * Created On -  Mar 8, 2014
 * Last Modified On - Mar 8, 2014
 * 
 * Generates random number 1-9 and 1-7 
 * using function that generates random number 1-5
 */
public class RandomGenerator {

	public static void main(String args[]) {

		generateRandomNineAndPrint();
		generateRandomSevenAndPrint();
	}

	/**
	 * This method calls randomNine() method 1000 times and updates the count of
	 * each number from 1-9 being generated.
	 * 
	 * randomNine() method generates numbers from 1-9 using randomFive() method.
	 */
	private static void generateRandomNineAndPrint() {
		int[] counts = new int[9];
		for (int i = 0; i < 1000; i++) {
			int value = randomNine();
			counts[value - 1]++;
		}

		System.out.println("Count of Generated Random Numbers 1- 9:" + Arrays.toString(counts));
	}

	/**
	 * This method calls randomSeven() method 1000 times and updates the count
	 * of each number from 1-7 being generated.
	 * 
	 * randomNine() method generates numbers from 1-7 using randomFive() method.
	 */
	private static void generateRandomSevenAndPrint() {
		int[] counts = new int[7];
		for (int i = 0; i < 1000; i++) {
			int value = randomSeven();
			counts[value - 1]++;
		}

		System.out.println("Count of Generated Random Numbers 1- 7:" + Arrays.toString(counts));
	}

	/**
	 * Uses randomFive() method that generates 1-5 to generate numbers from 1-9
	 * 
	 * @return
	 */
	public static int randomNine() {
		int random;

		while (true) {
			/**
			 * When you call randomFive() two times, there are a lot of possible
			 * combinations.25 to be exact. (1,1) (1,2) (1,3) ... (5,5)
			 * 
			 * We need to call it two times and use the formula below to make
			 * sure we generate numbers from 1-9. Otherwise, it will not
			 * generate some numbers.
			 */
			random = (randomFive() - 1) * 3 + (randomFive() + 1) * 2;
			return random % 9 + 1;
		}
	}

	/**
	 * Uses randomFive() method that generates 1-5 to generate numbers from 1-7
	 * 
	 * @return
	 */
	public static int randomSeven() {
		int random;

		while (true) {
			/**
			 * When you call randomFive() two times, there are a lot of possible
			 * combinations.25 to be exact. (1,1) (1,2) (1,3) ... (5,5)
			 * 
			 * We need to call it two times and use the formula below to make
			 * sure we generate numbers from 1-7. Otherwise, it will not
			 * generate some numbers.
			 */
			random = (randomFive() - 1) * 3 + (randomFive() + 1) * 2;
			return random % 7 + 1;
		}
	}

	/**
	 * This method generates a random number 1 through 5. It uses Math.random()
	 * method for randomizing.
	 * Note: Math.ceil(0.0) is 1
	 * @return
	 */
	public static int randomFive() {
		return (int) (Math.ceil(Math.random() * 5));
	}

}

Here is the output of running above program:

Count of Generated Random Numbers 1- 9:[127, 127, 63, 131, 154, 35, 153, 129, 81]
Count of Generated Random Numbers 1- 7:[114, 172, 115, 167, 162, 107, 163]

What is DeadLock? What Situations Cause Deadlock?

DeadLock Definition:
DeadLock is a situation where two or more processes wait for the other one to finish, thus neither every does. If there are two processes A and B, if there is a deadlock, A would be waiting on B to complete while B would be waiting on A to complete.

What are the necessary conditions for the deadlock to occur?
For a deadlock to occur, all of the following four conditions, known as Coffman conditions, must hold.

  1. Mutual Exclusion:
    Only on process can access a resource at a time.
  2. Hold and Wait:
    A process is currently holding a resource and is requesting another resource hold by another process.
  3. No Pre-emption:
    The holding process does not voluntarily release the resource, and one process cannot force another process to release the resource.
  4. Circular Wait:
    Two or more processes form a circular chain, waiting on the other resource on the chain.

How to prevent deadlocks?
This can be done by avoiding one of the conditions above. The most common method is to avoid circular wait.