Generating a random number between multiple ranges

Question

I understand how to make a random number which is between two numbers:

1 + (int)(Math.random() * ((10 - 1) + 1))

or

min + (int)(Math.random() * ((max - min) + 1))

But how do I go about generating a random number which falls into multiple ranges?

For example: number can be between 1 to 10 or between 50 to 60

Answer 1

I'd go with something like this, to allow you to do it with as many ranges as you like:

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
class RandomInRanges
{
 private final List<Integer> range = new ArrayList<>();
 RandomInRanges(int min, int max)
 {
 this.addRange(min, max);
 }
 final void addRange(int min, int max)
 {
 for(int i = min; i <= max; i++)
 {
 this.range.add(i);
 }
 }
 int getRandom()
 {
 return this.range.get(new Random().nextInt(this.range.size()));
 }
 public static void main(String[] args)
 {
 RandomInRanges rir = new RandomInRanges(1, 10);
 rir.addRange(50, 60);
 System.out.println(rir.getRandom());
 }
}

• Javier: Fine for the requested ranges, but likely impractical for ranges with too many numbers.
• Karlo Delalic: This works great! Although I don't really have any clue about how it works.
• MrLore: @KarloDelalic Basically it just makes a list of all the numbers which are in the ranges you give it, then picks a random number from that list, which is therefore a random number from any of the ranges.

Answer 2

First generate an integer between 1 and 20. Then if the value is above 10, map to the second interval.

Random random = new Random();
for (int i=0;i<100;i++) {
 int r = 1 + random.nextInt(60-50+10-1);
 if (r>10) r+=(50-10);
 System.out.println(r); 
}

Answer 3

First, you need to know how many numbers are in each range. (I'm assuming you are choosing integers from a discrete range, not real values from a continuous range.) In your example, there are 10 integers in the first range, and 11 in the second. This means that 10/21 times, you should choose from the first range, and 11/21 times choose from the second. In pseudo-code

x = random(1,21)
if x in 1..10
 return random(1,10)
else
 return random(50,60)

• Halfacht: These ranges are easily replaced with variables. And calculate the total number of numbers in all ranges using variables.

Answer 4

if the list is known I think you can use something like this.

public class Test
{
 public static void main(String[] args)
 {
 int a;
 for(int i=0;i<10;i++)
 {
 a=(int) (Math.random()*((10-0)+(60-50)));
 if(a<=10)
 {
 }
 else if(a>(60-50))
 {
 a=a+50;
 }
 System.out.println(a);
 }
 }
}

Answer 5

How about the following approach: randomize picking a range an use that range to generage your random number, for that you'll have to put your ranges in some list or array

public class RandomRangeGenerator {
 class Range {
 public int min, max;
 public Range(int min, int max) { this.min = min; this.max = max; }
 }
 @Test
 public void generate() {
 List<Range> ranges = new ArrayList<>();
 ranges.add(new Range(1, 10));
 ranges.add(new Range(50, 60));
 List<Integer> randomNumbers = generateRandomNumbers(ranges, 10);
 System.out.println(randomNumbers.toString());
 for(Integer i : randomNumbers) {
 Assert.assertTrue((i >= 1 && i <= 10) || (i >= 50 && i <= 60));
 }
 }
 private List<Integer> generateRandomNumbers(List<Range> ranges, int numberOfNumbers) {
 List<Integer> randomNumbers = new ArrayList<>(numberOfNumbers+1);
 while(numberOfNumbers-- > 0) {
 Range range = ranges.get(new Random().nextInt(ranges.size()));
 randomNumbers.add(range.min + (int)(Math.random() * ((range.max - range.min) + 1)));
 }
 return randomNumbers;
 }
}

Answer 6

To generate numbers between two ranges add up the total number of possibilities. 1 - 10 gives us 10 and 50 - 60 gives us another 11, so 21 total. Then, generate a random number between 1 - 21.

int rn = (int)(1 + (Math.random() * 21));

If the random number is between 1 and 10, that is easy, you have your number. If it is between 11 - 21, then you have to do some work. First, you can use modulus to get the index of the number between 50 - 60. Since you have 11 possible values in that range, then mod the random number by 11 and add 50.

if (rn > 10) {
 rn %= 11;
 rn += 50;
}
System.out.println(rn);

This will print values between 1 - 10 and 50 - 60 inclusive.