How to get index of multiple maximum value in List

Question

In given list of Integers i want to find the multiple index of maximum value in List.

For example,

List<Integer> len=Arrays.asList(9,3,6,5,4,9);

Now i want to find the largest element which is 9 and its all index which are 0 and 5.

i have tried following code but it only gives me the first occurrence of element so from this code i am getting output only 0 not 5.

IntStream.range(0,len.size()).reduce((a, b) -> len.get(a) < len.get(b)?b:a).ifPresent(ix -> System.out.println("Index -"+ix));

So how can i achieve that ?

score 3 · Accepted Answer · answered Feb 24 '17 at 08:39

3

Something like this:

int max = len.stream().max(Integer::compareTo).get();
System.out.println(IntStream.range(0, len.size()).filter(ix -> len.get(ix).intValue() == max).boxed()
            .collect(Collectors.toList()));

answered Feb 24 '17 at 08:39

Andrei G

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1

Of course you have to handle the case when max is not present – Andrei G Feb 24 '17 at 08:40
2

but that would stream twice... interesting if it can be done with a single stream. – Eugene Feb 24 '17 at 08:41
3

`.intValue()` is unnecessary. Java will auto-unbox. – Andreas Feb 24 '17 at 08:46
1

Changing first line to `int max = len.stream().mapToInt(Integer::intValue).max().orElse(0);` would resolve issue raised by @grozandrei. – Andreas Feb 24 '17 at 08:50
1

Besides that, using `Comparator.naturalOrder()` instead of `Integer::compareTo` might be preferable… – Holger Feb 24 '17 at 11:19
2

can use int max = Collections.max(len); function to get Max value – Karthik Bharadwaj Feb 24 '17 at 13:12

score 3 · Answer 2 · answered Feb 24 '17 at 12:25

It is possible to use only one Stream pipeline:

List<Integer> len = Arrays.asList(9, 3, 6, 5, 4, 9);

List<Integer> indices = IntStream.range(0, len.size()).boxed()
    .collect(Collectors.collectingAndThen(
        Collectors.groupingBy(len::get, TreeMap::new, Collectors.toList()),
        m -> m.isEmpty() ? Collections.emptyList() : m.lastEntry().getValue()));
System.out.println(indices);

First group your indices to the respective values in your list. Then you have to decide what to do if the intermediate map is empty. Either you throw an NoSuchElementException or return an empty list. In my example I chose the latter case.

Result:

[0, 5]

score 1 · Answer 3 · answered Feb 24 '17 at 08:54

If the result doesn't have to be a Stream or Optional you could accomplish the task using this code:

IntStream.range(0, len.size()).boxed()
    .sorted(Comparator.comparing((Integer i) -> len.get(i)).reversed())
    .reduce(-1, (ixMax, ix) -> {
        if (ixMax == -1 || len.get(ixMax) == len.get(ix)) {
            System.out.println("Index -" + ix);
            return ix;
        } else {
            return ixMax;
        }
    });

Although answering the question it's probably neither concise nor very performant to be honest. With some work it should be possible to replace the if-else block with code making use of Optional.

How to get index of multiple maximum value in List

3 Answers3