Develop Reference

Java Parallel search in a multidimensional array using Threads

The task is to find max and min values in a MxN matrix, and do it "in parallel" using Java Threads.
I was pretty surprised to find out that my parallelSearch() method runs much slower compared to the regularSearch() method. Both of them use the same brute-force algorithm, so even though you can probably solve this problem using a better algorithm, both methods should be on the even playing field.
I'm curious to find out why: is it because creating new Thread objects is a time-expensive task, or is it because I'm doing something totally wrong in my parallelSearch() method?
import java.util.Random;
public class Task {
static int max = -1;
static int min = 99;
static int[][] matrix;
static Random rnd = new Random();
static{
matrix = new int[4][1000];
Task t = new Task();
t.fill(matrix);
}
public static void main(String[] args) throws InterruptedException {
Task t = new Task();
long cur = System.currentTimeMillis();
System.out.println(t.regularSearch(matrix));
System.out.println("regular search took "+(System.currentTimeMillis() - cur)+" millis ");
cur = System.currentTimeMillis();
System.out.println(t.parallelSearch(matrix));
System.out.println("parallel search took "+(System.currentTimeMillis() - cur)+" millis ");
}
void fill(int[][] input){
for (int i = 0; i < input.length; i++) {
for (int i1 = 0; i1 < input[i].length; i1++) {
input[i][i1] = rnd.nextInt(200);
}
}
}
String regularSearch(int[][]input){
StringBuilder result = new StringBuilder();
for (int[] anInput : input) {
for (int anAnInput : anInput) {
if(anAnInput>max){
max = anAnInput;
}
if(anAnInput<min){
min = anAnInput;
}
}
}
String rslt = result.append("max: ").append(max).append(" min: ").append(min).toString();
max = -1;
min = 200; //doing this to have a fair comparison in main()
return rslt;
}
String parallelSearch(int[][] input) throws InterruptedException {
StringBuilder result = new StringBuilder();
for (int i = 0; i < input[0].length; i++) {
int x = i;
Thread t = new Thread(()->{
for (int[] anInput : input) {
if (anInput[x] > max) {
max = anInput[x];
}
if (anInput[x] < min) {
min = anInput[x];
}
}
});
t.start();
}
Thread.sleep(10);
result.append("max: ").append(max).append(" min: ").append(min);
return result.toString();
}
}
EDIT:
As was pointed out in the comments, my parallelSearch() was creating way more Threads than necessary, and I've rewritten it to better suit my task. This one actually runs faster than the regular one on big sizes. I've tested it against a 4x500000 matrix with random.nextInt(100000) bound and it runs as fast or even up to 10 seconds faster then the regularSearch()
String parallelSearch(int[][] input) throws InterruptedException {
StringBuilder result = new StringBuilder();
for (int i = 0; i < input.length; i++) {
int x = i;
Thread t = new Thread(()->{
for(int num:input[x]){
if(num>max){
max = num;
}
if(num<min){
min = num;
}
}
});
t.start();
}
Thread.sleep(10);
result.append("max: ").append(max).append(" min: ").append(min);
return result.toString();
}

How to generate a random number without a repeat in Java

I am using this to generate a random number inside a method and return it:
int randomValue = ThreadLocalRandom.current().nextInt(0, filteredArrayList.size());
How can I make sure there are not two random numbers in a row? I don't care if I get a 3, then a 5, then a 3 again. Only if I get a 3 and then a 3.

int temp = -1; // This will be your value to be compared to random value
for(int i = 0; i < 10; i++) { // assuming your filteredArraylist is size 10
int randomValue = ThreadLocalRandom.current().nextInt(0, 10);
if(randomValue == temp) {
i--; // it will repeat the generation of random value if temp and randomValue is same
} else {
temp = randomValue; // you will put here the ne random value to be compared to next value
System.out.println(randomValue);
}

Try following sample
import java.util.Random;
public class RandomTest {
public static void main(String[] args) {
for(int i = 0; i < 10; i++) {
System.out.println("####### " + RandomUtil.getRandomInt(10));
}
}
}
class RandomUtil {
private static int lastInt = -1;
private static Random random = new Random();
public synchronized static int getRandomInt(int upperBound) {
return getRandomInt(0, upperBound);
}
public synchronized static int getRandomInt(int lowerBound, int upperBound) {
int newInt = -1;
while( (newInt = lowerBound + random.nextInt(upperBound - lowerBound)) == lastInt) {
//Keep looping
}
lastInt = newInt;
return newInt;
}
}

As Scary Wombat said, you'll want to compare the previous value to the newly randomized value in a loop (which I assume you are using, since we only see the one line.
Something like this...
int prevRandomNumber, currRandomNumber;
while (notFinished) {
currRandomNumber = getRandom(); // your random number generator
if (prevRandomNumber == currRandomNumber) { // if there would be two in a row
continue; // try again
} else { // otherwise, add to array
addNumberToArray(currRandomNumber);
prevRandomNumber = currRandomNumber;
}
}

Just remember the last generated value and if equals then reject. Here is an example:
int lastRandomValue = -1;
boolean stop = false;
int attempts = 0;
final int maxAttempts = 100_000;
while (!stop) {
int currentRandomValue = ThreadLocalRandom.current().nextInt(0, filteredArrayList.size());
if (currentRandomValue != lastRandomValue) {
// Use the value
...
// Reset the counter
attempts = 0;
...
// Stop the generation process if generated enough values
if (...) {
stop = true;
}
} else {
// Increment a counter
attempts++;
}
if (attempts >= maxAttempts) {
stop = true;
}
}

Edited:
I'd do something like this: (requires the last random value/a non-reachable number for the first time, e.g. -1)
private int notPreviousRandom(int previousRandomValue) {
int randomValue;
do {
randomValue = ThreadLocalRandom.current().nextInt(0, filteredArrayList.size());
} while (randomValue == previousRandomValue);
return randomValue;
}
Alternatively you could define previousRandomValue as an attribute:
// class
private int previousRandomValue = -1;
// ...
private int notPreviousRandom() {
int randomValue;
do {
randomValue = ThreadLocalRandom.current().nextInt(0, filteredArrayList.size());
} while (randomValue == previousRandomValue);
previousRandomValue = randomValue; // for the next time you're using the
// method
return randomValue;
}

How to create dynamic array in java with unclear and diffrent inpu INDEXes?

I am new to Java and I needed dynamic Array ... all of thing I found that's for dynamic Array we should use "Array List' that's ok but when I want the indexes to be the power of X that given from input , I face ERORR ! .. the indexes are unclear and the are not specified what is the first or 2th power ! .... can anyone help me how solve it?
public static void main(String[] args) throws Exception {
Scanner Reader = new Scanner(System.in);
ArrayList<Float> Zarayeb = new ArrayList<Float>();
Float s ;
int m;
System.out.print("Add Count of equation Sentences : ");
int N = Reader.nextInt();
if (N == 0)
return;
for (int i = 0; i < N ; i++) {
s = Reader.nextFloat() ;
System.out.print("x^");
m = Reader.nextInt();
if (Zarayeb.get(m)== null)
Zarayeb.add(0 , s);
else{
Float l ;
l = Zarayeb.get(m);
Zarayeb.add (m , l+s);
}
if (i < N-1)
System.out.print("\r+");
}
System.out.print("Add Count of equation Sentences : ");
N = Reader.nextInt();
if (N == 0)
return;
for (int i = 0; i < N ; i++) {
s = Reader.nextFloat() ;
System.out.print("x^");
m = Reader.nextInt();
if (Zarayeb.get(m)== null)
Zarayeb.add(m , s);
else{
Float l ;
l = Zarayeb.get(m);
Zarayeb.add (m , l+s);
}
if (i < N-1)
System.out.print("\r+");
}
System.out.print("Enter X: ");
float X = Reader.nextFloat();
float Sum = 0;
for (int i = 0; i < Zarayeb.size();i++) {
Sum += (Zarayeb.get(i) * Math.pow(X,i));
}
System.out.println("\nThe final answer is : " + Sum);

First I refactored your code a bit to make sense of it:
Main class with the top level logic:
import java.util.Scanner;
public class Main {
private Scanner scanner;
private final Totals totals = new Totals();
public static void main(final String[] args) {
final Main app = new Main();
app.run();
}
private void run() {
scanner = new Scanner(System.in);
try {
readAndProcessEquationSentences();
} finally {
scanner.close();
}
}
private void readAndProcessEquationSentences() {
readSentences(true);
readSentences(false);
System.out.println("The final answer is : " + totals.calculateSum(readBaseInput()));
}
private void readSentences(final boolean useInitialLogic) {
System.out.print("Enter number of equation sentences:");
final int numberOfSentences = scanner.nextInt();
if (numberOfSentences == 0) {
throw new RuntimeException("No sentences");
}
for (int i = 0; i < numberOfSentences; i++) {
Sentence sentence = Sentence.read(scanner);
if (useInitialLogic) {
totals.addInitialSentence(sentence);
} else {
totals.addNextSentence(sentence);
}
if (i < numberOfSentences - 1) {
System.out.print("\r+");
}
}
}
private float readBaseInput() {
System.out.print("Enter base: ");
return scanner.nextFloat();
}
}
Sentence class which represents one equation sentence entered by the user:
import java.util.Scanner;
public class Sentence {
private Float x;
private int y;
public static Sentence read(final Scanner scanner) {
final Sentence sentence = new Sentence();
System.out.println("Enter x^y");
System.out.print("x=");
sentence.x = scanner.nextFloat();
System.out.println();
System.out.print("y=");
sentence.y = scanner.nextInt();
System.out.println();
return sentence;
}
public Float getX() {
return x;
}
public int getY() {
return y;
}
}
Totals class which keeps track of the totals:
import java.util.ArrayList;
import java.util.List;
public class Totals {
private final List<Float> values = new ArrayList<Float>();
public void addInitialSentence(final Sentence sentence) {
if (values.size() <= sentence.getY()) {
addToStart(sentence);
} else {
addToValue(sentence);
}
}
private void addToStart(final Sentence sentence) {
values.add(0, sentence.getX());
}
public void addNextSentence(final Sentence sentence) {
if (values.size() <= sentence.getY()) {
values.add(sentence.getY(), sentence.getX());
} else {
addToValue(sentence);
}
}
private void addToValue(final Sentence sentence) {
Float total = values.get(sentence.getY());
total = total + sentence.getX();
values.add(sentence.getY(), total);
}
public float calculateSum(final float base) {
float sum = 0;
for (int i = 0; i < values.size(); i++) {
sum += (values.get(i) * Math.pow(base, i));
}
return sum;
}
}
I don't have the foggiest idea what this is supposed to do. I named the variables according to this foggy idea.
You are letting the user input values in two separate loops, with a slightly different logic I called 'initial' and 'next'.
In the initial loop you were doing this:
if (Zarayeb.get(m) == null)
Zarayeb.add(0 , s);
In the next loop this:
if (Zarayeb.get(m) == null)
Zarayeb.add(m , s);
There are problems with this because the ArrayList.get(m) will throw an IndexOutOfBoundException if m is out or range. So I changed that to the equivalent of:
if (Zarayeb.size() <= m) {
....
}
However, in the 'next' case this still does not solve it. What should happen in the second loop when an 'm' value is entered for which no element yet exists in the ArrayList?
Why do you need to enter sentences in two loops?
What is the logic supposed to achieve exactly?

Adding results from loop into Array?

I am currently just mocking about with Java. To get some learning in, and of course the easiest way to learn is by asking.
In this section I've created a loop to give me 50 random numbers. What I want to do, is to compare these numbers later on. That's why I want to move all the numbers into an array. I have no clue how. I've tried different stuff, but my syntax is wrong. Can somebody tell me how to do this?
Code:
package project.main;
import java.util.Random;
public class Main {
public static int numbers[];
public static final void main(String []args) {
getRandom();
recheckNumbers();
}
public static void getRandom() {
Random RandomNumber = new Random();
for(int i = 1; i <= 50; ++i){
int randomInt = RandomNumber.nextInt(100);
System.out.println("Generated : " + randomInt);
}
}
public static void recheckNumbers() {
if(numbers[0] < numbers[1]) {
System.out.println("numbers[0] is biggest");
} else {
System.out.println("numbers[1] is biggest");
}
}
}
I just rewrote it a bit. Im now running into another issue at line 14. which is the numbers[i] = randomInt part.
Heres the new code..
package project.main;
import java.util.Random;
public class Main {
public static int numbers[];
public static final void main(String []args) {
Random RandomNumber = new Random();
for(int i = 0; i <= 49; ++i){
int randomInt = RandomNumber.nextInt(100);
numbers[i] = randomInt;
}
}
}

for(int i = 0; i <= 49; ++i){
int randomInt = RandomNumber.nextInt(100);
numbers[i] = randomInt;
System.out.println("Generated : " + randomInt);
}
After that you can loop through to get number
for(int i = 0; i <= 49; ++i){
System.out.println("Generated : " + numbers[i]);
}
Solution to new question
import java.util.Random;
public class Main {
public static int[] numbers = new int[50];
public static void main(String[] args) {
Random RandomNumber = new Random();
for(int i = 0; i <= 49; ++i){
int randomInt = RandomNumber.nextInt(100);
numbers[i] = randomInt;
}
}
}

The solution lemon provided is correct.
Additionally I want to point you to a mistake you did in your recheckNumbers method. You check if numbers[0] is smaller than numbers[1] and print out that numbers[0] is the biggest in the if block. You should switch the output from the if and else block to return the correct answer.

You need to say the size of the array. Here is my solution to your problem:
public class Main {
public static int numbers[] = new int[50];
public static void main(String[] args) {
getRandom();
recheckNumbers();
}
public static void getRandom(){
Random randomNumber = new Random(); // variables should start with lower case
for(int i = 0; i < 50; i++){
int randomInt = randomNumber.nextInt(100); // generate a random integer in the (0-99) interval
System.out.println("Generated: " + randomInt); // print the generated number
numbers[i] = randomInt; // put it in the array
}
}
public static void recheckNumbers(){
if(numbers[0] > numbers[1]){
System.out.println("numbers[0] is bigger");
} else {
System.out.println("numbers[1] is bigger");
}
}
}
Hope it helps :)

Aid with quick sort algorithm in java (netbeans)

I am quite new to programming and I am just starting using java. My task was to write a program using quick sort, I managed to write it but it always gives me an index out of bounds. Could anyone take a look at my code and help me by identifying what I am doing wrong? Thanks
This is the code for the main class.
package quicksort;
public class Quicksort {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
int[] x = {5,3,10,1,9,8,7,4,2,6,0};
quicksort_class q = new quicksort_class(x);
q.sort();
for(int i = 0; i < 11-1; i++)
{
System.out.println(x[i]);
}
}
}
This is the code for quicksort_class.
public class quicksort_class {
int[] array1 = new int[11];
public quicksort_class(int[] w)
{
array1 = w;
}
public void partitionstep(int leftlimit, int rightlimit)
{
int LPointer = leftlimit;
int RPointer = rightlimit;
Random random = new Random();
int midpoint = random.nextInt(11);
int checknumber = array1[midpoint];
while(LPointer < RPointer)
{
while(array1[LPointer] <= checknumber)
{
LPointer = LPointer + 1;
}
while(array1[RPointer] >= checknumber)
{
RPointer = RPointer --;
}
swap(LPointer, RPointer);
partitionstep(leftlimit, midpoint - 1);
partitionstep(midpoint + 1, rightlimit);
}
}
public void swap(int x, int y)
{
int temp = array1[x];
array1[x] = array1[y];
array1[y] = temp;
}
public void sort()
{
partitionstep(0, array1.length - 1);
}
}

Your midpoint value should be calculated based on your leftLimit and rightLimit. It should not be a random value based off of the fixed value 11.