double array to boolean array with logical operation in java

Question

I have an operation to make in java which I cannot find a way around without doing a for loop which I really want to avoid for computation efficiency. I used to program in Matlab and its pretty easy to do it, but in java seems a lot more tricky. My question is this... What would be the java equivalent of this Matlab code:

A = [1;-3;2;2;5-7;0];

A<1;

ans =

     0
     1
     0
     0
     1
     1

in java I tried this method while roaming the internet.

Integer[] array = {-1,2,-3,4,-5,-6};

List<Integer> result = Arrays.stream(array).filter(w ->  w < 1 )
                .collect(Collectors.toList());

In this example result = {-1,-3,-5,-6} and but I wish to have result = {1,0,1,0,1,1}

A for loop seems appropriate here. Note that using a for loop in matlab is much worse (slower) than using it in java. — m7913d, Sep 07 '17 at 12:53
@TylerNichols For loops are relative slow in matlab due to its interpreted nature. When you write `A < 1`, this _for loop_ is evaluated using an efficient C implementation, which definitely matters for large matrices. — m7913d, Sep 07 '17 at 12:57
The simple reason is that in reality I will have a vector with thousands values, and I cannot see myself let the program going on a loop just to get another vector of the same size. — kingTy, Sep 07 '17 at 12:59
@m7913d thanks for clarification I did not know that optimization occured -- still a for loop somewhere even if it goes all the way down to assembly :) (unrolling aside.....) — Tyler Nichols, Sep 07 '17 at 13:03

score 3 · Answer 1 · answered Sep 07 '17 at 12:57

3

    int[] a = {1,-3,2,2,-2,0};
    Arrays.stream(a).forEach(i -> System.out.println(i < 1 ? 1 : 0));

answered Sep 07 '17 at 12:57

Mehdi Javan

1,081
6
25

1

just to note I would benchmark this vs a for loop - I bet the results will surprise you :) see https://stackoverflow.com/questions/41278712/when-should-streams-be-preferred-over-traditional-loops-for-best-performance-do – Tyler Nichols Sep 07 '17 at 13:06
1

@TylerNichols Although I knew it might be slower, I surprised because I didn't expect such difference. An upvote for your post and comment :) – Mehdi Javan Sep 07 '17 at 13:10

score 2 · Accepted Answer · answered Sep 07 '17 at 12:53

Old school solution (assuming that X is some int value; and that `intValues' represents some sort of collection/array of ints or Integers):

List<Boolean> lessThanX = new ArrayList<>();
for (int i : intValues) {
  lessThanX.add( i < X );
}

Nowadays, using streams:

intValues.stream().map( i -> i < X ).collect(Collectors.asList());

( or something alike ... as the question contains zero effort of trying yourself - I omitted cross-checking my input - it is merely meant as inspiration to get you going )

score 1 · Answer 3 · answered Sep 07 '17 at 13:02

This does look like it should be a simple loop function.

double[] outputArray = {1, -3, 2, 2, 5, -7, 0};
boolean[] outputArray = new boolean[inputArray.length];
for (int i = 0; i < inputArray.length; i++) {
    outputArray[i] = inputArray[i]<1;
}

This can easily be changed into a method call with the outputArray as an argument, which will probably be most suited to your needs.

score 0 · Answer 4 · answered Sep 07 '17 at 13:37

0

public static int[] convertToBinary(int[] decimal, int compareInt){
  int[] binary = new int[decimal.length];
  int index = 0;
  Arrays.stream(decimal).forEach( dec -> binary[index++] = dec < compareInt ? 1 : 0 );
  return binary;
}

answered Sep 07 '17 at 13:37

Sahil Chhabra

10,621
4
63
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double array to boolean array with logical operation in java

4 Answers4