如何在Java中获取链接列表中元素的所有索引

Question

I have a linked list. I want to set a weight factor p^i for each position i of this list. Then I want to get a weighted value for the elements of the list as the sum of these factors.

Let me give an example to make things concrete. Let us say that my list has size 5, ie it has elements from index 0 to index 4. Let us say that p = 0.5 (ie p = 1/2). Hence, the weighting factor of position 0 is p^0 = 1, of position 1 is p^1 = 0.5, of position 2 is p^2 = 0.25, of position 3 is p^3 = 0.125, and finally of position 4 is p^4 = 0.0625.

Let us now assume that I have the following items in the list, from position 0 to position 4 (I give the IDs of these items): 4 | 4 | 5 | 4 | 5. Therefore, since the item with ID = 4 is in the positions 0, 1, and 3, its value should be the sum of these weighting factors, that is, p^0 + p^1 + p^3 = 1 + 0.5 + 0.125 = 1.625. Similarly, for the item with ID = 5 we have p^2 + p^4 = 0.25 + 0.0625 = 0.3125.

My problem is: How to get ALL the indexes where these elements (ID 4 and ID 5) are located in the list in order to calculate the value of these elements as the sum of the associated with these indexes weighting factors? indexOf(Object o) returns the index of the first occurence of the specified element in the list, I need the index of all occurences. Any suggestion?

EDIT: Updated to a geometrically decreasing sum instead of increasing

I did a modification to my code, instead of having a geometrically increasing sum in my list (which i call window and takes objects of type Request) to have a geometrically decreasing sum. My code (which I post it here exactly as it is in my program) works just fine if a I have a small number of requests (eg 100 - I set the number of requests at the main method of my Main class - the entrypoint of the code). However, if I use for example 1000 requests, I get an error:

    java.lang.IllegalArgumentException: n must be positive 

which refers to another point in my code where I have:

if(reqToBeRemoved == null) {

    reqToBeRemoved = 
minKeyList.get((new Random(System.currentTimeMillis()).nextInt(minKeyList.size())));

} 

Here is my code, I hope what I post here to be usefull (obviously I cannot post the whole code here, neither it is required I guess):

public void doWindowLookup(Request request) {

    int index = 0;
    double sum = 0;
    double initVal = 0;

    for(Request r : window) {

        if(r.equals(request)) {

            if(index == 0) {

                initVal = 1;

            }

            else {

                request.weight = Math.pow(p, index);
                sum += request.weight;

            }

            request.weightedWinFreq = initVal - sum;

        }

        index++;
    }

    logger.info("TEST: Item: " + request.reqID + " has weightedWinFreq: " +
                 request.getWeightedWinFreq());

}

What this error message means? Which general item n should be positive??? I assume when I call random.nextInt() I pass a zero argument (as indicated by the previous thread Exception in thread "main" java.lang.IllegalArgumentException: n must be positive ), but how is this possible, since my argument is minKeyList.size()?

(If my question needs additional clarification, please ask me to provide the required info.)

EDIT: Regarding the Previous Update to My Question: It Was My Fault in Another Part of the Program. Question Considered to be Answered.

Answer 1

You can always loop through the list manually:

double weight = 0.0;
Object target = new Integer(4);
int index = 1;
for (Object elem : linkedList) {
    if (elem.equals(target)) {
        weight += Math.pow(p, index);
    }
    index++;
}

Answer 2

it's better to use iterator

    int n = 1;
    List<Integer> list = new LinkedList<Integer>(Arrays.asList(1, 2, 3, 1));
    ListIterator<Integer> it = list.listIterator();
    List<Integer> indexes = new ArrayList<Integer>();
    while (it.hasNext()) {
        if (it.next().equals(n)) {
            indexes.add(it.nextIndex() - 1);
        }
    }
    System.out.println(indexes);