Develop Reference

Unique or non repeating random number generation

I am trying to generate unique random number using the below function. Whenerver I run my code in a loop of 1000, it generates duplicate number also.
Code
private static String randomize() {
int count = 10;
List<Integer> digits = createList(count);
Collections.shuffle(digits); // this re-arranges the elements in the list
return listToString(digits);
}
private static <T> String listToString(List<T> list) {
StringBuilder result = new StringBuilder();
for (T object : list) {
result.append(object);
}
return result.toString();
}
private static List<Integer> createList(int size) {
List<Integer> result = new ArrayList<Integer>(size);
for (int i = 0; i < size; i++) {
result.add(i);
}
return result;
}
public static void main(String[] args) {
for (int i = 0; i < 1000; i++) {
String strName = randomize();
System.out.println(strName);
}
I searched a lot in google and tried ThreadLocal also, it did not help me. Any help or guide on this will be really helpful. My idea is to generate 10 digit non-repeating random number within a loop.

You're not really generating random numbers, are you? You're generating random permutations of the 10 unique digits [0..9]. (e.g. "0123456789", "9834105672", etc.) Since there are only 3268800 (10!) unique permutations, you have a decent chance of hitting a duplicate with 1000 tries.
(I haven't worked out the math, but since 1000^2 is within an order of magnitude of 10!, my intuition tells me there's at least a 10% chance of a duplicate in any given set. Google "birthday paradox" for details.)
What you want to do, and what #Andrei is trying to explain, is to check/store the result every time you call randomize() to make sure you don't have duplicates. Roughly:
public static void main(String[] args) {
Set<String> results = new HashSet<>(1000);
while (results.size() < 1000) {
String strName = randomize();
if (!results.contains(strName)) {
System.out.println(strName);
results.add(strName);
}
}
}
Unless of course you actually want a set of random numbers:
public static void main(String[] args) {
Set<Long> results = new HashSet<>(1000);
while (results.size() < 1000) {
// Random number on [1E9..1E10]
long random = (long) (Random.nextDouble() * 900000000L) + 100000000L;
if (!results.contains(random)) {
System.out.println(random);
results.add(random);
}
}
}

Java & NetBeans exercise, can't get it to work

I'm a beginner in coding and wanted to train, and so I started doing exercises that I find on the internet, I finished one and was unsatisfied because of how easy it was, and created myself a challenge.
The exercise was: you type in a variable and it tells you if it is above a certain number, in this case it's 50, but here's the thing, I didn't want to type it in, I want it to be randomly generated, but I can't find a way to solve the problem, it blocks at nextInt.
public class CheckPassFail { // saved as "CheckPassFail.java"
public static void main(String[] args) {
random r = new random ();
int Low = 1;
int High = 60;
int mark = r.nextInt(High-Low)+ Low;
System.out.println("The mark is " + mark);
if (mark>50) {
System.out.println("PASS");
} else {
System.out.println("Fail");
}
}
private static class random {
public random() {
}
}
}

Use Java's Random class instead of defining your own private static class.
import java.util.Random;
public class CheckPassFail { // saved as "CheckPassFail.java"
public static void main(String[] args) {
Random r = new Random ();
int Low = 1;
int High = 60;
int mark = r.nextInt(High-Low)+ Low;
System.out.println("The mark is " + mark);
if (mark>50) {
System.out.println("PASS");
} else {
System.out.println("Fail");
}
}
}

Returning String Methods in Java?

I'm in a beginning programming class, and a lot of this had made sense to me up until this point, where we've started working with methods and I'm not entirely sure I understand the "static," "void," and "return" statements.
For this assignment in particular, I thought I had it all figured out, but it says it "can not find symbol histogram" on the line in the main method, although I'm clearly returning it from another method. Anyone able to help me out?
Assignment: "You see that you may need histograms often in writing your programs so you decide for this program to use your program 310a2 Histograms. You may add to this program or use it as a class. You will also write a class (or method) that will generate random number in various ranges. You may want to have the asterisks represent different values (1, 100, or 1000 units). You may also wish to use a character other than the asterisk such as the $ to represent the units of your graph. Run the program sufficient number of times to illustrate the programs various abilities.
Statements Required: output, loop control, decision making, class (optional), methods.
Sample Output:
Sales for October
Day Daily Sales Graph
2 37081 *************************************
3 28355 ****************************
4 39158 ***************************************
5 24904 ************************
6 28879 ****************************
7 13348 *************
"
Here's what I have:
import java.util.Random;
public class prog310t
{
public static int randInt(int randomNum) //determines the random value for the day
{
Random rand = new Random();
randomNum = rand.nextInt((40000 - 1000) + 1) + 10000;
return randomNum;
}
public String histogram (int randomNum) //creates the histogram string
{
String histogram = "";
int roundedRandom = (randomNum/1000);
int ceiling = roundedRandom;
for (int k = 1; k < ceiling; k++)
{
histogram = histogram + "*";
}
return histogram;
}
public void main(String[] Args)
{
System.out.println("Sales for October\n");
System.out.println("Day Daily Sales Graph");
for (int k = 2; k < 31; k++)
{
if (k == 8 || k == 15 || k == 22 || k == 29)
{
k++;
}
System.out.print(k + " ");
int randomNum = 0;
randInt(randomNum);
System.out.print(randomNum + " ");
histogram (randomNum);
System.out.print(histogram + "\n");
}
}
}
Edit: Thanks to you guys, now I've figured out what static means. Now I have a new problem; the program runs, but histogram is returning as empty. Can someone help me understand why? New Code:
import java.util.Random;
public class prog310t
{
public static int randInt(int randomNum) //determines the random value for the day
{
Random rand = new Random();
randomNum = rand.nextInt((40000 - 1000) + 1) + 10000;
return randomNum;
}
public static String histogram (int marketValue) //creates the histogram string
{
String histogram = "";
int roundedRandom = (marketValue/1000);
int ceiling = roundedRandom;
for (int k = 1; k < ceiling; k++)
{
histogram = histogram + "*";
}
return histogram;
}
public static void main(String[] Args)
{
System.out.println("Sales for October\n");
System.out.println("Day Daily Sales Graph");
for (int k = 2; k < 31; k++)
{
if (k == 8 || k == 15 || k == 22 || k == 29)
{
k++;
}
System.out.print(k + " ");
int randomNum = 0;
int marketValue = randInt(randomNum);
System.out.print(marketValue + " ");
String newHistogram = histogram (randomNum);
System.out.print(newHistogram + "\n");
}
}
}

You're correct that your issues are rooted in not understanding static. There are many resources on this, but suffice to say here that something static belongs to a Class whereas something that isn't static belogns to a specific instance. That means that
public class A{
public static int b;
public int x;
public int doStuff(){
return x;
}
public static void main(String[] args){
System.out.println(b); //Valid. Who's b? A (the class we are in)'s b.
System.out.println(x); //Error. Who's x? no instance provided, so we don't know.
doStuff(); //Error. Who are we calling doStuff() on? Which instance?
A a = new A();
System.out.println(a.x); //Valid. Who's x? a (an instance of A)'s x.
}
}
So related to that your method histogram isn't static, so you need an instance to call it. You shouldn't need an instance though; just make the method static:
Change public String histogram(int randomNum) to public static String histogram(int randomNum).
With that done, the line histogram(randomNum); becomes valid. However, you'll still get an error on System.out.print(histogram + "\n");, because histogram as defined here is a function, not a variable. This is related to the return statement. When something says return x (for any value of x), it is saying to terminate the current method call and yield the value x to whoever called the method.
For example, consider the expression 2 + 3. If you were to say int x = 2 + 3, you would expect x to have value 5 afterwards. Now consider a method:
public static int plus(int a, int b){
return a + b;
}
And the statement: int x = plus(2, 3);. Same here, we would expect x to have value 5 afterwards. The computation is done, and whoever is waiting on that value (of type int) receives and uses the value however a single value of that type would be used in place of it. For example:
int x = plus(plus(1,2),plus(3,plus(4,1)); -> x has value 11.
Back to your example: you need to assign a variable to the String value returned from histogram(randomNum);, as such:
Change histogram(randomNum) to String s = histogram(randomNum).
This will make it all compile, but you'll hit one final roadblock: The thing won't run! This is because a runnable main method needs to be static. So change your main method to have the signature:
public static void main(String[] args){...}
Then hit the green button!

For starters your main method should be static:
public static void main(String[] Args)
Instance methods can not be called without an instance of the class they belong to where static methods can be called without an instance. So if you want to call your other methods inside the main method they must also be static unless you create an object of type prog310t then use the object to call the methods example:
public static void main(String[] Args)
{
prog310t test = new prog310t();
test.histogram(1);
}
But in your case you probably want to do:
public static String histogram (int randomNum)
public static void main(String[] Args)
{
histogram(1);
}
Also you are not catching the return of histogram() method in your main method you should do like this:
System.out.print(histogram(randomNum) + "\n");
Or
String test = histogram(randomNum);
System.out.print(test + "\n");
Static methods are part of a class and can be called without an instance but instance methods can only be called from an instance example:
public class Test
{
public static void main(String[] args)
{
getNothingStatic();// this is ok
getNothing(); // THIS IS NOT OK IT WON'T WORK NEEDS AN INSTANCE
Test test = new Test();
test.getNothing(); // this is ok
getString(); // this is ok but you are not capturing the return value
String myString = getString(); // now the return string is stored in myString for later use
}
public void getNothing()
{
}
public static void getNothingStatic()
{
}
public static String getString()
{
return "hello";
}
}
Void means the method is not returning anything it is just doing some processing. You can return primitive or Object types in place of void but in your method you must specify a return if you don't use void.

Before calling histogrom (randomNum) you need to either make histogram static or declare the object that has histogram as a method
e.g
prog310t myClass = new prog310t();
myClass.histogram()

static Math.random change java

I don't have a problem anymore, However I want to understand the behaviour of some code. Initially I was generating some random numbers and somewhere in my code the Math.random was returning the same number for all iterations. I tried to create a minimal example with the following two Classes:
first Class:
public class randomTest {
public randomTest()
{ }
public double generateRandomNumber()
{
double r = Math.random();
return r;
}
public static void main(String args[])
{
randomTest t = new randomTest();
for (int i = 0; i < 10; i++)
System.out.println(t.generateRandomNumber());
}
}
The second class:
public class anotherClass {
private randomTest t = new randomTest();
public static void main(String args[])
{
for (int i = 0; i < 10; i++)
{
anotherClass c = new anotherClass();
System.out.println(c.t.generateRandomNumber());
}
}
}
I was trying to generate a minimal code example to track the reason why I am always getting the same random value for the whole 10 iterations. In this minimal example the results are correct and random, However In my real situation The output of the second class is the same for the whole ten iterations.
At last i was able to solve the problem by changing the method I am calling into a static method. I still don't understand how did this solve my problem, and where the original problem was.
Old Nonworking code:
...
public ImagePlus createAnImage()
{
drawBackground(c.ip);
width = ip.getWidth();
height =ip.getHeight();
createCircles(requiredCircles); // this is not creating random numbers
ArrayList<Circle> list = circlesList;
drawBoundaries(list, ip, percentage);
background.setProcessor(ip);
return background;
}
...
New Code:
...
public static ImagePlus createAnImage()
{
createCircles c = new createCircles();
c.drawBackground(c.ip);
c.width = c.ip.getWidth();
c.height =c.ip.getHeight();
c.createCircles(c.requiredCircles); // this is creating random numbers
ArrayList<Circle> list = c.circlesList;
c.drawBoundaries(list, c.ip, c.percentage);
c.background.setProcessor(c.ip);
return c.background;
}
...
In both cases I was already creating an instance of createCircles class from another class as follows:
...
private ImagePlus createRandomImage(int radius, int numberOfCircles, double minPercentage, double maxPercentage, int minBackground, int maxBackground)
{
// create the image using class createCircles
createCircles c = new createCircles();
c.setParameters(radius, radius, minBackground, maxBackground, numberOfCircles, imageWidth, imageHeight, minPercentage, maxPercentage);
ImagePlus imp = c.createAnImage(); // calling the static method works
return imp;
}
Although my problem is solved, I still need to understand the reason behind this. I suppose a better understanding of static vs. non-static methods might explain it. Anyone has a clue?
Best Regards,
M. Tleis

Do not use Math.random (it produces doubles, not integers)
use the Random class to generate random integers between 0 and N.
To generate a series of random numbers as a unit, you need to use a single Random object - do not create a new Random object for each new random number.
import java.util.Random;
/** Generate 10 random integers in the range 0..99. */
public final class RandomInteger {
public static final void main(String... aArgs){
log("Generating 10 random integers in range 0..99.");
//note a single Random object is reused here
Random randomGenerator = new Random();
for (int idx = 1; idx <= 10; ++idx){
int randomInt = randomGenerator.nextInt(100);
log("Generated : " + randomInt);
}
log("Done.");
}
private static void log(String aMessage){
System.out.println(aMessage);
}
}

Java: Summing all digits of 2^1000

I'm trying to solve Project Euler problem #16, where I need to sum all the digits of 2^1000. I've gotten stuck dealing with such a big number. My program worked for any number below 10^16, but failed afterwards. This told me that my logic was correct. I went ahead and converted all variables and methods to BigDecimal, but now the program does not run properly. It compiles as it is and there is no error; it just does not terminate. Does anyone have an idea on where I went wrong here?
import java.math.BigDecimal;
import java.math.RoundingMode;
public class Powerdigitsum {
private static final BigDecimal one = new BigDecimal("1");
private static final BigDecimal ten = new BigDecimal("10");
private static BigDecimal sumofDigits(BigDecimal n){
BigDecimal sum = new BigDecimal("0");
while(n.compareTo(one) == 1 || n.compareTo(one) == 0){
sum.add(n.remainder(ten));
n.divide(ten);
n = n.setScale(0, RoundingMode.FLOOR);
}
return sum;
}
public static void main(String[] args) {
final double the_number = Math.pow(2,1000);
final double test = 15;
final BigDecimal two_to_the_thousandth_power = new BigDecimal(test);
System.out.println(sumofDigits(two_to_the_thousandth_power));
}
}

Just use BigInteger properly:
BigInteger a = new BigInteger("2").pow(1000);

The whole method is kinda wrong. See this:
private static BigInteger sumOfDigits(BigInteger n) {
BigInteger sum = BigInteger.ZERO;
while (n.compareTo(BigInteger.ZERO) == 1) {
sum = sum.add(n.remainder(ten));
n = n.divide(ten);
}
return sum;
}
You needed to compare to zero, not one. And you need to assign the values for BigIntegers and BigDecimals, their methods do nothing on their own, the instances of those classes are immutable.
For integers, it's generally better to use BigInteger. The decimal part (that gets there from dividing) is just thrown away.

final double the_number = Math.pow(2,1000);
This won't work because the_number is not large enought to take the result. You need to convert the pow call to BigInteger:
BigInteger result = new BigInteger("2").pow(1000);
But be aware.. this can take some time..

Don't use the BigDecimal(double) constructor: it is limited by the double primitive type, which cannot represent 2^1000.
You can use a BigInteger. Something along these lines should work (probably suboptimal, but...):
public static void main(final String... args)
{
// 2^1000
final BigInteger oneTo2000 = BigInteger.ONE.shiftLeft(1000);
BigInteger digitSum = BigInteger.ZERO;
// We don't want to split against the empty string, the first element would be ""
for (final String digit: oneTo2000.toString().split("(?<=.)"))
digitSum = digitSum.add(new BigInteger(digit));
System.out.println(digitSum);
}

public class SumofDigitsPow {
public static void main(String[] args) {
//2(2^1000)
String temp = BigInteger.ONE.shiftLeft(1000).toString();
int sum = 0;
for(int i=0;i<temp.length();i++){
sum+= temp.charAt(i) - '0';
}
System.out.println(Integer.toString(sum));
}
}
java.math.BigInteger.shiftLeft(int n) method returns a BigInteger whose value is (this << n),So you can get the answer by using BigInteger and LeftShift Method

import java.math.BigInteger;
public class Problem16 {
public static void main(String[] args) {
BigInteger number2 = new BigInteger("2");
BigInteger number3 = new BigInteger("0");
number3 =number2.pow(1000);
String str = number3.toString();
BigInteger sum = new BigInteger("0");
for(int i=0; i<str.length(); i++)
{
char c= str.charAt(i);
int value = Character.getNumericValue(c);
BigInteger value2 = new BigInteger(Integer.toString(value));
sum =sum.add(value2) ;
}
System.out.println(sum);
}
}

IF YOU THINK BIGINTEGER IS CHEATING AND/OR don't feel like using it/learning how to use it, this algorithm is the way to go.
Think about how you would calculate 2^1000 by hand. You'd start with 2^1 and multiply by two repeatedly. Now notice that the number of digits of powers of two increase by 1 for AT LEAST every 3 powers (could be after 4 powers like with 1024 to 8192). So make a jagged 2D array like this
int a[][]= new int[1000][];
for(int i=0;i<1000;i++)
{
a[i]= new int[1+(i/3)];
}
Then initialize a[0][0] to 2. After this, you want to write a for loop such that each row is filled from the rightmost spot. So make two variables "digit" and "carry". Digit is the number that you will input into the row you're working on, and the carry is the one you're going to take to the next calculation and add to the product of 2 and whatever digit you're multiplying it with. Be careful with the order you update digit and carry and reinitialize them to zero after every calculation. I think the hardest part is coming up with the limits for the for loop, so that it fits with the every 3 powers thing. You can make this simpler by just making a triangular jagged array that increments by one every row. I did it like this though. Here's my whole code.
import java.util.*;
public class ProjectEuler16
{
public static void main(String[] args)
{
long t1=System.currentTimeMillis();
ProjectEuler16 obj = new ProjectEuler16();
System.out.println(obj.bigNumHandler());
long t2= System.currentTimeMillis();
System.out.println(t2-t1);
}
int bigNumHandler()
{
int a[][] = new int[1000][];
for(int i=0;i<1000;i++)
{
a[i]= new int[1+(i/3)];
}
a[0][0]=2;
for(int i=1;i<1000;i++)
{
int carry=0;
int digit=0;
int f=0;
if(i%3==0)
{
f=1;
}
for(int j=a[i-1].length-1+f;j>=0;j--)
{
if(j==0&f==1)
{
a[i][0]=carry;
}
else
{
digit=((2*a[i-1][j-f])+carry)%10;
carry=((2*a[i-1][j-f])+carry)/10;
a[i][j]=digit;
}
}
}
int sum=0;
for(int k=0;k<a[999].length;k++)
{
sum=sum+a[999][k];
}
return sum;
}
}
Note that the last row lists the digits for 2^1000.I think you can figure out how to sum the digits. The program took about 5 seconds to come up with the answer.

solution::::
import java.math.BigInteger;
public class PR9 {
/**
* #param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
BigInteger zero=BigInteger.valueOf(0);
BigInteger ten=BigInteger.valueOf(10);
BigInteger sum=zero;
BigInteger a = new BigInteger("2").pow(1000);
while(a.compareTo(zero)>0){
sum=sum.add(a.mod(ten));
a=a.divide(ten);
}
System.out.println(sum);
}
}
output:::::
1366

import java.math.BigInteger;
public class P16 {
public static BigInteger digitSum(int n) {
BigInteger sum = BigInteger.ZERO;
BigInteger number = new BigInteger("2").pow(n);
while (number.compareTo(BigInteger.ZERO) == 1) {
BigInteger remainder = number.remainder(BigInteger.TEN);
sum = sum.add(remainder);
number = number.divide(BigInteger.TEN);
}
return sum;
}
public static void main(String[] args) {
final double START = System.nanoTime();
System.out.println(digitSum(Integer.parseInt(args[0])));
final double DURATION = System.nanoTime() - START;
System.out.println("Duration: " + DURATION / 1000000 + "ms.");
}
}
While there maybe a way of solving this problem without the use of BigIntegers, it is clear that they make the code run way faster.
Mine only took about 4ms to find an answer.