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Arithmetic operation on multidimensional array using Streams

 
Rancher
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I found this thread https://coderanch.com/t/679348/java/Java-Streams-Convert-multidiamentional-array which gives a nice example of converting a multidimensional array to a nested List. I am interested in a similar application but rather applying mathematical operations to arrays of higher dimension.

Here is my best shot. The idea is just to double each entry of a 2D array using streams.

import java.util.Arrays;



Error is "bad return type in lambda expression". Thanks for any help.
 
Marshal
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Remembering that Java® doesn's actually support 2D arrays; it supports arrays of arrays, which are better . . .
Don't know, but have a few hints:-
  • 1: Indent Streams code differently. Every time you call a method on a Stream after the first call, push the enter key and indent the code so the dots align vertically.
  • 2: Use an IDE. If you hover your mouse over the method call you will get a popup with its return type shown e.g. Stream<Foo>. At least that is what Eclipse does, so I expect NetBeans and IntelliJ will have similar functionality.
  • 3: Consider not using arrays of primitives; maybe use Double[]s instead.
  • 4: I am 99% sure that flatMap takes a Stream<Foo[]> and creates a Stream<Foo>, so it is probably doing the exact opposite of what you want.
  • 5: Rather than assigning the arrays in lines 4 and 6, consider printing them with System.out.println(Arrays.deepToString(...
  • 6: Use the peek() method to view the contents of your Streams whilst in development for debugging.
  • In your line 4, you are creating a DoubleStream, which has a no-args toArray() method returning a double[]. If you had a Double[] you would be better off the Stream#toArray() method with a parameter: myStream.toArray(Double[]::new)

    What is wrong with writing a method which takes a double[] as its parameter and returns a new double[] with changed values? Call that method with a λ from the map() method.

    Actually, by going through by hand and working out the types, there is quite a simple solution and you are not that far off. Here it is I think a few misspellings have been introduced in copying but you can retain about half of your line 6 and change the other half.

    Addition:-

    java LambdaTest
    [2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0, 0.0]
    [[1.0, 2.0, 3.0], [1.0, 2.0, 3.0]]
    [[2.0, 4.0, 6.0], [2.0, 4.0, 6.0]]

     
    Eric Barnhill
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    Thank you very much for the tips. Use of Double[] was key, then this works:



    Although as you also suggested, calling a 1D method within the stream will make more design sense most of the time.
     
    Campbell Ritchie
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    Well done

    This is my solution minus the “spelling errors”, and I shall let you work out what mistake I made.
     
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    I would suggest this:



    Edit: Oops! Sorry, I should have refresh the page before posting my answer, which i the same as yours!

    Edit2: Not exactly the same in fact.
     
    Campbell Ritchie
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    That picks up my mistake; because I forgot to write double[][]::new in line 16, I got an Object[] instead of a double[][].

    double[] array1D = new double[]{1, 2, 3, 4, 5, 6, 7, 8, 9, 0};

    You realise you are allowed to miss out new double[] because you are declaring and initialising the array on the same line?
     
    Pierre-Yves Saumont
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    Not sure to understand what you mean. Where should "new double[]" go?
     
    Campbell Ritchie
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  • 1: I missed out double[][]::new in my last toArray() call, so I got an Object[] rather than a double[][].
  • 2: In OP's original code it says double[] array1D = new double[]{...}; Because the declaration and initialisation are on the same line that can be shortened to double[] array1D = {...};
  • Sorry, I have confused you by not being clear that second part was directed at the OP.
     
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