How to compute change between List members using Java's stream?

Question

Is it possible to use Java's streams to compute the (absolute) change between list members? If so, how and would it be preferable to looping (for other than aesthetic reasons)? Currently, I'm implementing this as follows:

List<Double> series; // Populated with, e.g., time series data

List<Double> seriesChanges = new ArrayList<>();

    for (int i = 1; i < series.size(); i++) {
        seriesChanges.add(Math.abs(series.get(i-1) - series.get(i)));
    }

Yes, it is possible, but it will be more convoluted and less readable as what you have now, while still doing the same thing. — JB Nizet, Apr 06 '17 at 06:40
AFAIK, there is no easy way to [Collect successive pairs from a stream](http://stackoverflow.com/questions/20470010/collect-successive-pairs-from-a-stream). — stholzm, Apr 06 '17 at 06:49

score 2 · Accepted Answer · answered Apr 06 '17 at 07:58

Slightly different solution, which does not depend on side effects on a mutable list:

IntStream.range(1, series.size())
         .mapToObj(i -> Math.abs(series.get(i - 1) - series.get(i)))
         .collect(Collectors.toList())

I can think of only one reason to use the Streams API here: it is easier to parallelize (just throw in .parallel() and maybe use an unmodifiable list... I did not try it).

score 0 · Answer 2 · answered Apr 06 '17 at 06:52

0

Smth like this:

IntStream.range(1, series.size()).forEach(i -> seriesChanges.add(Math.abs(series.get(i - 1) - series.get(i))));

But I agree with @JB Nizet that it's really difficlut to read

answered Apr 06 '17 at 06:52

Ivan Pronin

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It would be better to map and collect than to use `forEach` to add to a collection. – RealSkeptic Apr 06 '17 at 06:58
Please, post an example of it. How would you address to `i-1` and `i` elements in this case? – Ivan Pronin Apr 06 '17 at 07:06
The same way that you did: `range(...).map(i -> Math.abs(series.get(i-1) - series.get(i))).collect(Collectors.toList())`. – RealSkeptic Apr 06 '17 at 07:11
Have you tried this code? It gives compile error `Type mismatch: cannot convert from double to int` – Ivan Pronin Apr 06 '17 at 07:14
OK, use `mapToObj` instead of `map`. – RealSkeptic Apr 06 '17 at 07:55

How to compute change between List members using Java's stream?

2 Answers2