Question

I am using this to try to create a weighted random:

int choice_weight[] = { 25, 25, 25, 25 };
int num_choices = choice_weight.length;
Random random = new Random();
int i;
int sum_of_weight = 0;

for (i = 0; i < num_choices; i++) {
    sum_of_weight += choice_weight[i];
}

int rnd = random.nextInt(sum_of_weight);
System.out.println(rnd);

for (i = 0; i < num_choices; i++) {
    if (rnd < choice_weight[i])
        rnd -= choice_weight[i];
}

I am using a swtich statement to test and see if the cases are:

case 1:

case 2:

case 3:

case 4:

The problem is that random.nextInt(sum_of_weight) is generating numbers between 100 and 0 which is based on my 4 weights of 25,25,25,25. I need a number to match one of the possible cases 1-4? Or should I change up my method of testing?

This whole process is kind of confusing me, some help would be greatly appreciated.

So basically it will be a 25 percent chance a particular number will be chosen. Also, I will be changing the percentages throughout the program’s life cycle.

Answer 1

Shouldn’t you “increase” your range when testing for your number? Ie:

System.out.println(rnd);

int winner=0;
int rangeHigh=0;
for (i = 0; i < num_choices; i++) {
   rangeHigh += choice_weight[i];
   if (rnd < rangeHigh){
      winner = (i+1);
      break;
   }
}
System.out.println("lucky winner number: " + winner);

Basically you’re generating a number between 0-99 and checking if you values are:

1. < 25  //bucket 1 - [0-24]
2. < 50  //bucket 2 - [25-49]
3. < 75  //bucket 3 - [50-74]
4. < 100  //bucket 4 - [75-99]

On second look the way you have it will work too (w/ adjustments):

int winner = 0;
for (i = 0; i < num_choices; i++) {
    if (rnd < choice_weight[i]){
       winner = i+1;
       break;
    }
    rnd -= choice_weight[i];
}