Excluding numbers from interval, retrieving the rest

Question

I want an interval of numbers between 0 and 9 using random generator. For example, if I already received 0,2,7 i don't want those numbers again, instead i want one of the rest between the given interval[1 or 3 or 4 or 5 or 6 or 8 or 9].

This isn't random. You want to take the numbers `[1,9]` in a `Collection` and shuffle it. The pull from the top. — Boris the Spider, Dec 01 '14 at 23:36

score 3 · Accepted Answer · answered Dec 01 '14 at 23:41

As Boris the Spider says:

// We want numbers between 0 and 9 inclusive
int min = 0, max = 9; 
// We need a Collection, lets use a List, could use any ordered collection here
List<Integer> nums = new ArrayList<>();
// Put the numbers in the collection
for (int n=min; n<=max; n++) { nums.add(n); }

// Randomly sort (shuffle) the collection
Collections.shuffle(nums);

// Pull numbers from the collection (the order should be random now)
for (int count=0; count<nums.length; count++) {
    System.out.println("Number " + count + " is " + nums.get(count));
}

score 0 · Answer 2 · answered Dec 01 '14 at 23:51

This solution runs in better time performance with math.random.

LinkedList<Integer> numbers = new LinkedList<>();
numbers.add(1);
numbers.add(2); //or more

while (!numbers.isEmpty()) {
    System.out.println(numbers.remove((int) (Math.random() * numbers.size())));
}

score 0 · Answer 3 · edited May 23 '17 at 11:50

0

With java 8 you can do the following

List<Integer> integers = IntStream.range(0, 10).boxed().collect(Collectors.toList());
Collections.shuffle(integers);

As shown in this answer.

edited May 23 '17 at 11:50

Community

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answered Dec 02 '14 at 00:40

mike

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score -1 · Answer 4 · answered Dec 01 '14 at 23:43

This is an alternative approach to Java.util.Collections, which uses the Java.util.Random class.

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{

    final static int RAND_CAP = 9;

    public static void main (String[] args) throws java.lang.Exception
    {
        ArrayList<Integer> usedNumbers = new ArrayList<Integer>();
        Random rand = new Random(System.currentTimeMillis());
        while (usedNumbers.size() < RAND_CAP) {
            int randNum = Math.abs(rand.nextInt() % RAND_CAP) + 1;
            if (!usedNumbers.contains(randNum)) {
                usedNumbers.add(randNum);
                System.out.println(randNum);
            }
        }
    }
}

run code here

This is very poor as performance degrades as you reach the end of the required numbers. In fact, there is no reason the second number should ever return - `random.nextInt` can keep returning the same number. Further `List.contains` is `O(n)`, `Set.contains` is `O(1)`. This answer shows a fundamental misunderstanding of both programmatic randomness and the Java collections API. — Boris the Spider, Dec 02 '14 at 08:17

Excluding numbers from interval, retrieving the rest

4 Answers4