Question

I have an array char[] Select = {'A','B','C','D','E','F','G','H','I','J'} and each element in this array has different probability to be selected. For example,

int[] Weight =  {10,30,25,60,20,70,10,80,20,30};

My requirement is to select 5 elements from this array and the element with high weight values has higher probability to be selected and these 5 elements should be different.

My plan is first sum the weight

int[] weightSum = {10, 40, 65, 125, 145, 215, 225, 305, 325, 355}

Then I use Random to generate a random number k in the range of [0,355]. Then looking for the first element which is bigger than k in the weightSum[]. This process are repeated 5 times.

The problem is that the element with high probability could be selected multiple times. I try to remove the duplicate elements at each iteration. The duplicates are removed, but element with high weight values are not selected.

How to solve this problem?

thanks.

Answer 1

don’t maintain the cumulative sum or adjust it each time: (requires O(n) for each selection though)

char[] Select = {'A','B','C','D','E','F','G','H','I','J'};
int[] Weight = {10,30,25,60,20,70,10,80,20,30};
int sum = 355;
for(int a=0;i<5;i++){
    int rand = (int)(Math.random()*sum);
    int s=0;//temp cumulative sum
    int i=0;
    while( (s+=Weight[i])<rand)i++;
    result.add(Select[i]);

    sum-=Weight[i];//total weight is lower now
    Weight[i]=0;//if weight is 0 it will never be selected

}

edit: fixed so I don’t subtract 0 from sum