Replace for loop with java 8 foreach for updating values

Question

I'm looking to replace the following for loop with an elegant java 8 stream or lambda solution. Is there anything concise and efficient?

    public static void main(String[] args) {
        ArrayList<Integer> myList = new ArrayList<>( Arrays.asList( 10,-3,5));

        // add 1/2 of previous element to each element
        for(int i =1 ;i < myList.size();  ++i )
            myList.set(i, myList.get(i)+myList.get(i-1)/2);

        // myList.skip(1).forEach( e -> e + prevE/2 );  // looking for something in this spirit
    }

Answer 1

Your loop evaluation has a dependency to the result of the previous evaluation. It is equivalent to

for(int i = 1, value = myList.get(0); i < myList.size(); i++ ) {
    value = myList.get(i) + value/2;
    myList.set(i, value);
}

There is no real simplification by using the Stream API or lambda expressions possible. In fact, I would prefer the variant shown above, even if it’s bigger rather than smaller, as it makes clear what actually happens (and may be slightly more efficient by avoiding multiple List lookups).

It also allows you to program position independent, if you create a new List:

List<Integer> srcList = Arrays.asList(10, -3, 5), dstList = new ArrayList<>();

int value = 0;
for(Integer i: srcList) {
    value = i + value/2;
    dstList.add(value);
}

Answer 2

There is no way to access previous element in the java stream. You might use IntStream#range but it doesn't look elegant:

IntStream.range(1, myList.size())
        .forEachOrdered(i -> myList.set(i, myList.get(i) + myList.get(i - 1) / 2));

Answer 3

Yes you can achieve this using this line of code:

IntStream.range(1,myList.size())
    .forEachOrdered(i->myList.set(i,(myList.get(i)+myList.get(i-1)/2)));
System.out.println(myList);