Iteratively split an array to subarrays according to a given length

Question

How do I create a subarray of an array in Java. Given a number 3 and x [2,4,6,30,9], the answer is X1[2,4,6] X2[4,6,30] X3[6,30,9]

This is what I have so far, but it only prints the first three elements. I also have to print the median of every subArray.


double[] floatsArr = new double[3]; 

// floats is the array with all the elements in

for(int i = 0; i < floats.length; i++){
            
     for(int k = 1; k < (filterSize+1); k++){
                 floatsArr[k] = floats[k];  
                 System.out.println(floatsArr[k]);

    }
}

Use [Arrays.copyOfRange()](https://docs.oracle.com/en/java/javase/16/docs/api/java.base/java/util/Arrays.html#copyOfRange(float%5B%5D,int,int)) — , Aug 12 '21 at 21:57

RaffleBuffle · Accepted Answer · 2021-08-13T09:33:31.677

It seems you want to create all subarrays of a given length from an array.

You can do it by hand:

static double[][] subArrays(double[] arr, int k)
{
    double[][] subArrs = new double[arr.length-k+1][k];
    
    for(int i=0; i<subArrs.length; i++)
        for(int j=0; j<k; j++)
            subArrs[i][j] = arr[i+j];
    
    return subArrs;
}

Or use the built-in method Arrays.copyOfRange:

static double[][] subArrays(double[] arr, int k)
{
    double[][] subArrs = new double[arr.length-k+1][];
    
    for(int i=0; i<subArrs.length; i++)
        subArrs[i] = Arrays.copyOfRange(arr, i, i+k);
    
    return subArrs;
}

Test:

double[] arr = {2, 4, 6, 30, 9};
    
for(double[] subArr : subArrays(arr, 3))
    System.out.println(Arrays.toString(subArr));

Output:

[2.0, 4.0, 6.0]
[4.0, 6.0, 30.0]
[6.0, 30.0, 9.0]

Thanks so much! However I want the output to be a double instead so that I can sort it and find the median — ItsWhoYouThinkItIs, Aug 13 '21 at 07:45

WJS · Answer 2 · 2021-08-13T00:09:18.383

Here is a convenient way to do it.

Given a group sub array size and source array

int groupSize = 3;
double[] arr = { 2, 4, 6, 30, 9 };

first stream ints from 0 to arr.length - groupSize. This will serve as the starting index to create each subarray.
then Stream the actual indices of the source array to create the subarray.
and map the double values using the previous index into the stream and then gather into an array.
then collect those arrays into a multi-array array.

double[][] result =
        IntStream.rangeClosed(0, arr.length - groupSize)
                .mapToObj(i -> IntStream.range(i, groupSize + i)
                        .mapToDouble(k -> arr[k]).toArray())
                .toArray(double[][]::new);


for (double[] a : result) {
    System.out.println(Arrays.toString(a));
}

Prints

[2.0, 4.0, 6.0]
[4.0, 6.0, 30.0]
[6.0, 30.0, 9.0]

And even shorter is to use Arrays.copyOfRange() in place of the second stream.

double[][] result = IntStream
        .rangeClosed(0, arr.length - groupSize)
        .mapToObj(i -> Arrays.copyOfRange(arr, i, i + groupSize))
        .toArray(double[][]::new);

Iteratively split an array to subarrays according to a given length

2 Answers2