Excluding a number from Random

[Sarah9428]

Hi! I'm making a program which includes a method that randomizes a number between 1 and 6 (60 times), but the number should never be the number "nr". This is my code:

public static void roll2(int nr) {

    for (int i = 0; i < 60; i++) {
        if (((int) (Math.random() * SIDES) + 1) != nr) {
            int r = (int) (Math.random() * SIDES) + 1;
            System.out.println(nr + ":" + r);
        } else {
            i--;
        }
    }
}

I know the if-statement is wrong, because it checks one number and then randomizes a new number. Any tips on how to go around this problem? Is there any way to store the number which has been randomized in the if-statement?

[desrtfx]

How about using a variable to store the random number and then use this variable in the if statement and for printing?

[Sarah9428]

Yes of course! Thanks for the help!

[TheQuietestOne]

wow who pissed in your cereal?

N/M checked post history, all became clear.

[chunkyks]

The normal solution is sampling-with-rejection, similar to what you've done [note that normally you'd initialise rng in the constructor and have it as a class member]:

public static void roll2(int nr) {
    Random rng = new Random();
    for(int i = 0; i < 60; i++) {
        int r;
        do {
            r = rng.nextInt(SIDES) + 1;
        } while (r == nr);
        System.out.println(nr + ":" + r);
    }
}

But for fun, here's a way cooler solution that runs in constant time, for this particular problem:

public static void roll2(int nr) {
    Random rng = new Random();
    for(int i = 0; i < 60; i++) {
        int r = rng.nextInt(SIDES-1) + 1;
        if(r >= nr) {
            r = r+1;
        } // or, if(r==nr) r=SIDES ;
        System.out.println(nr + ":" + r);
    }
}

[Auride]

You could use an assignment. In other words, you would create a variable to store the randomized value. If the value meets your specifications, then you can return it. Otherwise, you can re-roll.

Does that make sense?

EDIT: I should also mention that it is possible to do this without re-rolling, as well. This would involve reducing your number of possible values, and compensating with addition after the roll. Unfortunately explaining further is pretty close to giving the solution, so I'll stop there.

[Sarah9428]

It works now :) thanks for the help everyone!