2D Array stream reduce in java

Question

I'm new to using streams in java and I have a question on using streams. I have a double[][] in which i want to perform the summation of elements, for it I've written the following approach similar to C#Linq, but it doesn't seem to work.

Arrays.stream(myArray).reduce(0,(acc, i) -> acc + Arrays.stream(i).sum());

The error is that acc seems to be a double[], so it can't perform double[]+double. In C#Linq the accumulator is assumed to be the same type as the seed(0 in this case). What am I missing here? Thanks in advance.

Jorn Vernee · Accepted Answer · 2017-03-28 21:32:37Z

6

If you look at the signature of reduce, the type of the identity has to be the type of the stream's elements. Which would be double[] in this case. That would also give acc the type of double[].

There is an overload where you can supply an accumulator of a different type, but you also need to pass a combiner, to combine 2 accumulators.

You can do this:

double result = Arrays.stream(myArray)
    .reduce(0D, (acc, i) -> acc + Arrays.stream(i).sum(), Double::sum);

Where 0D is a double literal, and Double::sum is used to combine 2 accumulators.

Alternatively, it might be more convenient to do this:

double result = Arrays.stream(myArray)
    .flatMapToDouble(DoubleStream::of)
    .sum();

answered Mar 28, 2017 at 21:32

Jorn Vernee

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2 Comments

Bojan Petkovic Over a year ago

flatMap is the cleanest approach.

Holger Over a year ago

I’d prefer Arrays.stream(myArray) .flatMapToDouble(Arrays::stream) .sum(); as this is not a varargs use case. And Arrays::stream is even shorter than DoubleStream::of…

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