如何从各种元素中随机选择

Question

my question is pretty simple in the concept : I have an array of 24 booleans which I've set all to false initially. I can choose randomly an element of this array with the code:

Random r = new Random();
int i1=r.nextInt(24);

Now from time to time I change the value of some elements to true, let say to the element number 14 and 22. How can I choose now randomly only between the other elements who are still false and not consider the one who are true? Thank you in advance for the answer.

Edit : The code who works was :

Random r = new Random ();
int rand = r.nextInt (24);

while (array[rand]) {
          rand = r.nextInt (24);

        }

Answer 1

You can keep track of the indices in which the elements have the value of true in an ArrayList for example. When generating a random number, check whether it's equal to one of the already true numbers (which should be excluded from the random number generation/selection) and re-generate a random number, until you get one of the array indices which is still at false .

Answer 2

int numberTrue=0; //adjust this in your other code accordingly
Random r = new Random ();
int rand = r.nextInt (24);
// loop until not true found or stop if all items are true
while (array [rand] && numberTrue < 24) {
  rand = r.nextInt (24);
  count++;
}

I thought about it for a bit, and without knowing your code here are a couple of suggestions to avoid that infinite loop when they are all true (assuming that condition may exist):
1. keep a count of how many true are set (example adjusted for this)
2. Store a second array of the true indexes, as suggested in the other answer

The first is probably the simplest to implement (adding a numberTrue++; and numberTrue--; in the appropriate code spots) - just a note here that if it's not possible to keep this count, you can achieve the same thing by running an initial for loop over your array and counting the number of true elements.
The second way would lend itself to utilizing one of the answers from the other post:

public int generateRandom(int start, int end, ArrayList<Integer> excludeRows) {
    Random rand = new Random();
    int range = end - start + 1;

    int random = rand.nextInt(range) + 1;
    while(excludeRows.contains(random)) {
        random = rand.nextInt(range) + 1;
    }

    return random;
}

(I believe their code should actually read rand.nextInt(range) + start
So you would run:

if (trueIndexes.length < 24) {
      generateRandom(0, 23, trueIndexes);
    }

Answer 3

您可以创建具有“ false”的布尔元素列表，并从列表中随机选择元素。