Calculating List of cumulative sums from List of integers with Java streams

Question

I have the following list :

 INPUT :: 4 5 8 -11 9 5 -7 4 6 -6 -8 -11 80 -32 -56 -15 5 -49 

 OUTPUT :: 4 9 17 6 15 20 13 17 23 17 9 -2 78 46 -10 -25 -20 -69

I need to calculate cumulative sum - List meaning

T(n) = T(n) + T(n-1) for n >0; 
and
T(0) = T(0)

I want to calculate that with Java stream API so that I can implement it with Spark for big data calculation. I am naive in Java Streams I have tried several expressions bt none of them is working the equivalent stuctured code should be like :

int[] numbers = {4, 5, 8, -11, 9, 5, -7, 4, 6,-6, -8, -11, 80, -32, -56, -15, 5, -49};
int temp = 0;

for (int i = 0 ; i < numbers.length ; i++) {
   temp = temp + numbers[i];
   numbers[i] = temp;
}

@Lamanus I can use this solution with some dat atype changes to JavaRDD in Spark — Nil Kulkarni, Aug 20 '20 at 13:22
@Nikolas , I am totally new to Stream API , could you help me understand why Stream wont work ? how to implement this logic in functional way ? — Nil Kulkarni, Aug 20 '20 at 13:23
This answer your question ? [Cumulative Sum using Java 8 stream API](https://stackoverflow.com/a/55265970/4207306) — Eklavya, Aug 20 '20 at 13:41
@NilKulkarni Streams will work. But there is no added benefit and in fact adds additional overhead. — WJS, Aug 20 '20 at 14:27
@Kulkarni: The Stream API is for a processing of the collection items such as filtering, mapping, reducing operations etc. It's not suitable for iterations where are two or more items involved at once. — Nikolas Charalambidis, Aug 20 '20 at 14:55

score 3 · Accepted Answer · answered Aug 20 '20 at 13:28

3

Try this.

int[] a = {4, 5, 8, -11, 9, 5, -7, 4, 6, -6, -8, -11, 80, -32, -56, -15, 5, -49};
Arrays.parallelPrefix(a, (x, y) -> x + y);
System.out.println(Arrays.toString(a));

output:

[4, 9, 17, 6, 15, 20, 13, 17, 23, 17, 9, -2, 78, 46, -10, -25, -20, -69]

answered Aug 20 '20 at 13:28

1

thanks , Cannot do upvote though ,I dont have enough reputation – Nil Kulkarni Aug 20 '20 at 13:55
1

A preferred approach over streams of course. But the OP did specify a stream solution. – WJS Aug 20 '20 at 14:17

WJS · Answer 2 · 2020-08-20T14:31:48.907

Here are two ways of doing it.

The first is very inefficient as it basically uses nested loops to accumulate the values. The first IntStream specfies the range of values and the nested IntStream creates a variable range and sums up the values from 0 to the end of that range.

int[] result1 = IntStream.range(0, vals.length).map(
        i -> IntStream.rangeClosed(0, i).map(k->vals[k]).reduce(0, (a, b) -> a + b))
        .toArray();

This one is more in line with a more conventional method. Stream a single array of 0 and then use that to accumulate a running sum of the values.

int[] result2 = Stream.of(new int[] { 0 })
        .flatMapToInt(k -> IntStream.of(vals).map(v -> {
            k[0] += v;
            return k[0];
        })).toArray();

System.out.println(Arrays.toString(result1));
System.out.println(Arrays.toString(result2));

Both print

[4, 9, 17, 6, 15, 20, 13, 17, 23, 17, 9, -2, 78, 46, -10, -25, -20, -69]
[4, 9, 17, 6, 15, 20, 13, 17, 23, 17, 9, -2, 78, 46, -10, -25, -20, -69]

But you simply can't do any better than this.

for (int i = 1; i < vals.length; i++) {
      vals[i] += vals[i-1];
}

Bottom line is to stick with what you have.

thanks . that was bit of a eye opener for me , Streams may not bring benefit every time.. — Nil Kulkarni, Aug 21 '20 at 03:03

score 0 · Answer 3 · answered Aug 21 '20 at 08:21

You can try using a custom collector.

public static void main(String[] args) {
       List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);
       List<Integer> cumulatives = integers.stream().collect(CumulativeAdd.collector());
}
     private static final class CumulativeAdd {

       List<Integer> retArray= new ArrayList<>();
       int sum = 0; 

       public void accept(Integer num) {
           sum +=num;
           retArray.add(sum);
       }

       public CumulativeAdd combine(CumulativeAdd other) {
           throw new UnsupportedOperationException("Parallel Stream not supported");
       }

       public List<Integer> finish() {
           return retArray;
       }

       public static Collector<Integer, ?, List<Integer>> collector() {
           return Collector.of(CumulativeAdd::new, CumulativeAdd::accept, CumulativeAdd::combine, CumulativeAdd::finish);
       }

   }

Calculating List of cumulative sums from List of integers with Java streams

3 Answers3