restrictions on randomly generated values?

[busdude]

Is the a command in Java where you can disallow certain numbers from the range to be obtained with the Random class?


So if you want to generate 5 random integers between 1-50, but you do not want any of them to be 15 or 20, for example, is there a way to block them or do you have to make some sort of loop where you get a new number if one of the disallowed ones is obtained?

[Fubarable]

I think a simple loop would be the way to go here.

[Supamagier]

What Fubarable says, just use a while loop. (no for)

[Eranga]

Yes, use a while loop with check the appropriate condition you want to find.

[Paul Richards]

Is the a command in Java where you can disallow certain numbers from the range to be obtained with the Random class?


So if you want to generate 5 random integers between 1-50, but you do not want any of them to be 15 or 20, for example, is there a way to block them or do you have to make some sort of loop where you get a new number if one of the disallowed ones is obtained?

For your example, I would do as the others have suggested, and just have a loop which generates the number inside it and repeats until that number is neither 15 nor 20.

However, to take the general case, for me it would really depend on the circumstances; i.e. whether the code is part of an algorithm that has to be very fast, and what proportion of the range was excluded.

For example, if your range was 1-50, inclusive, but you wanted to exclude numbers in the range 5-44, then I would probably take a different approach. in this case, since there are only 10 permitted values then I would generate a random number between 1-10 and use a calculation to map this number onto the allowed values.

To take another example, suppose you wanted to exclude odd numbers, then I would simply generate a number between 1-25 and double it.

[Eranga]

For the first suggestion; my question is how to map numbers into the allowed values. I'm not clear that how you going to do it.

For the second; I don't think it's a good choice actually. If you double the value, it's not a random value in sense, is it? Because you generating the second number, but effect with the initial value.

[Paul Richards]

For the first suggestion; my question is how to map numbers into the allowed values. I'm not clear that how you going to do it.

For the second; I don't think it's a good choice actually. If you double the value, it's not a random value in sense, is it? Because you generating the second number, but effect with the initial value.

busdude was asking a general question, but using a specific example, as I said in this example a loop would be the best way, but I give two other examples where I think it would not.

In my first example, let me make it clear how you would do the mapping:

Random r = new Random();
int random = r.nextInt(10);
int mapped = (random<=3) ? random+1 : random + 41;
System.out.println(mapped);

And what it does is this:

random-->mapped
0-->1
1-->2
2-->3
3-->4
4-->45
5-->46
6-->47
7-->48
8-->49
9-->50


In my second example, I said that odd numbers have to be excluded.

Random r = new Random();
int random = r.nextInt(25);
int mapped = (random+1)*2

And it maps like this:

random-->mapped
0-->2
1-->4
......
23-->48
24-->50

By definition, if any numbers are excluded then the distribution is not uniform.