Java 8 lambda expression evaluation

Question

I have a method similar to the following one:

public double[] foo(double[] doubleArray) { 
    DoubleStream stream = Arrays.stream(doubleArray);

    return stream.map(s -> s / stream.sum()).toArray();
}

What is the complexity of this method? How many times will DoubleStream's sum method be executed? Once or O(n) times, with n = doubleArray.length?

this will still fail... you need to create the stream again from the source — Eugene
– Eugene, Commented Aug 1, 2017 at 9:00

Eran · Accepted Answer · 2017-08-01 08:54:29Z

7

This code will throw an exception, since you can't consume the same Stream more than once. You can only execute one terminal operation on a Stream.

If you change the code to:

public double[] foo(double[] doubleArray) { 
    return Arrays.stream(doubleArray).map(s -> s / Arrays.stream(doubleArray).sum()).toArray();
}

it will work, but the running time will be quadratic (O(n^2)), since the sum will be computed n times.

A better approach would be to compute the sum just once:

public double[] foo(double[] doubleArray) { 
    double sum = Arrays.stream(doubleArray).sum();
    return Arrays.stream(doubleArray).map(s -> s / sum).toArray();
}

This will run in linear time.

answered Aug 1, 2017 at 8:54

Eran

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1 Comment

Eran Over a year ago

@Eugene You can mention it, or you can just write linear time as I did. It doesn't matter if it's n or 2n or cn. All are linear time.

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