CODEWITHSUNDEEP
javaaveragejava-8
Sam
Sam

Reputation: 405

Java: grouping together multiple Lists to one list

Is there a quick way to take a List< List<Double> > and average them so I only have one Double list where each column is the average of all the original lists at that column?

Upvotes: 0

Views: 220

Answers (1)

assylias
assylias

Reputation: 328737

With Java 8 you can write something like this - it is not really short though...

List<List<Double>> list = Arrays.asList(Arrays.asList(1d, 2d, 3d),
                                        Arrays.asList(2d, 3d, 4d, 5d));

//the largest list size
int maxSize = list.stream().mapToInt(List::size).max().orElse(0);

//for each index in the final list, we calculate the average across "valid" lists
//i.e. lists that are large enough
List<Double> averages = IntStream.range(0, maxSize)
                        .mapToObj(i -> list.stream()
                                            .filter(sublist -> sublist.size() > i)
                                            .mapToDouble(sublist -> sublist.get(i))
                                            .average().getAsDouble())
                        .collect(toList());

//prints averages = [1.5, 2.5, 3.5, 5.0]
System.out.println("averages = " + averages);

Upvotes: 7

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