Arrays.fill with multidimensional array in Java

How can I fill a multidimensional array in Java without using a loop? I've tried:

double[][] arr = new double[20][4];
Arrays.fill(arr, 0);

This results in java.lang.ArrayStoreException: java.lang.Double

This is because a double[][] is an array of double[] which you can't assign 0.0 to (it would be like doing double[] vector = 0.0). In fact, Java has no true multidimensional arrays.

As it happens, 0.0 is the default value for doubles in Java, thus the matrix will actually already be filled with zeros when you get it from new. However, if you wanted to fill it with, say, 1.0 you could do the following:

I don't believe the API provides a method to solve this without using a loop. It's simple enough however to do it with a for-each loop.

double[][] matrix = new double[20][4];

// Fill each row with 1.0
for (double[] row: matrix)
    Arrays.fill(row, 1.0);
    

double[][] arr = new double[20][4];
Arrays.fill(arr[0], 0);
Arrays.fill(arr[1], 0);
Arrays.fill(arr[2], 0);
Arrays.fill(arr[3], 0);
Arrays.fill(arr[4], 0);
Arrays.fill(arr[5], 0);
Arrays.fill(arr[6], 0);
Arrays.fill(arr[7], 0);
Arrays.fill(arr[8], 0);
Arrays.fill(arr[9], 0);
Arrays.fill(arr[10], 0);
Arrays.fill(arr[11], 0);
Arrays.fill(arr[12], 0);
Arrays.fill(arr[13], 0);
Arrays.fill(arr[14], 0);
Arrays.fill(arr[15], 0);
Arrays.fill(arr[16], 0);
Arrays.fill(arr[17], 0);
Arrays.fill(arr[18], 0);
Arrays.fill(arr[19], 0);

As Per Java 8, we can use this way.

double[][] arr = new double[20][4];
Arrays.stream(arr).forEach(a -> Arrays.fill(a, 0));

We can initialize a value in multidimensional array in a nicer and smart way.

The OP asked how to solve this problem without a loop! For some reason it is fashionable these days to avoid loops. Why is this? Probably there is a realization that using map, reduce, filter, and friends, and methods like each hide loops and cut down on program verbage and are kind of cool. The same goes for really sweet Unix pipelines. Or jQuery code. Things just look great without loops.

But does Java have a map method? Not really, but we could define one with a Function interface with an eval or exec method. It isn't too hard and would be a good exercise. It might be expensive and not used in practice.

Another way to do this without a loop is to use tail recursion. Yes, it is kind of silly and no one would use it in practice either, but it does show, maybe, that loops are fine in this case. Nevertheless, just to show "yet another loop free example" and to have fun, here is:

import java.util.Arrays;
public class FillExample {
    private static void fillRowsWithZeros(double[][] a, int rows, int cols) {
        if (rows >= 0) {
            double[] row = new double[cols];
            Arrays.fill(row, 0.0);
            a[rows] = row;
            fillRowsWithZeros(a, rows - 1, cols);
        }
    }

    public static void main(String[] args) {
        double[][] arr = new double[20][4];
        fillRowsWithZeros(arr, arr.length - 1, arr[0].length);
        System.out.println(Arrays.deepToString(arr));
    }
}

It isn't pretty, but in answer to the OP's question, there are no explicit loops.

>how can I fill a multidimensional array in Java without using a loop?

Multidimensional arrays are just arrays of arrays and fill(...) doesn't check the type of the array and the value you pass in (this responsibility is upon the developer).

Thus you can't fill a multidimensional array reasonably well without using a loop.

Be aware of the fact that, unlike languages like C or C++, Java arrays are objects and in multidimensional arrays all but the last level contain references to other Array objects. I'm not 100% sure about this, but most likely they are distributed in memory, thus you can't just fill a contiguous block without a loop, like C/C++ would allow you to do.

As an extension to the answer, I found this post but was looking to fill a 4 dimensional array. The original example is only a two dimensional array, but the question says "multidimensional". I didn't want to post a new question for this...

You can use the same method, but you have to nest them so that you eventually get to a single dimensional array.

fourDArray = new float[10][10][10][1];
// Fill each row with null
for (float[][][] row: fourDArray)
{
	for (float[][] innerRow: row)
	{
		for (float[] innerInnerRow: innerRow)
		{
		Arrays.fill(innerInnerRow, -1000);
		}
	}
};

Arrays.fill works with single dimensional array, so to fill two dimensional array we can do below

for (int i = 0, len = arr.length; i < len; i++)
	Arrays.fill(arr[i], 0);

Don't we all sometimes wish there was a
<T>void java.util.Arrays.deepFill(T[]…multiDimensional). Problems start with
Object threeByThree[][] = new Object[3][3];
threeByThree[1] = null; and
threeByThree[2][1] = new int[]{42}; being perfectly legal.
(If only Object twoDim[]final[] was legal and well defined…)
(Using one of the public methods from below keeps loops from the calling source code.
If you insist on using no loops at all, substitute the loops and the call to Arrays.fill()(!) using recursion.)

/** Fills matrix {@code m} with {@code value}.
 * @return {@code m}'s dimensionality.
 * @throws java.lang.ArrayStoreException if the component type
 *  of a subarray of non-zero length at the bottom level
 *  doesn't agree with {@code value}'s type. */
public static <T>int deepFill(Object[] m, T value) {
	Class<?> components; 
   	if (null == m ||
        null == (components = m.getClass().getComponentType()))
        return 0;
   	int dim = 0;
   	do
   		dim++;
   	while (null != (components = components.getComponentType()));
   	filler((Object[][])m, value, dim);
   	return dim;
}
/** Fills matrix {@code m} with {@code value}.
 * @throws java.lang.ArrayStoreException if the component type
 *  of a subarray of non-zero length at level {@code dimensions}
 *  doesn't agree with {@code value}'s type. */
public static <T>void fill(Object[] m, T value, int dimensions) {
	if (null != m)
		filler(m, value, dimensions);
}

static <T>void filler(Object[] m, T value, int toGo) {
	if (--toGo <= 0)
		java.util.Arrays.fill(m, value);
	else
		for (Object[] subArray : (Object[][])m)
			if (null != subArray)
				filler(subArray, value, toGo);
}

Using Java 8, you can declare and initialize a two-dimensional array without using a (explicit) loop as follows:

int x = 20; // first dimension
int y = 4; // second dimension

double[][] a = IntStream.range(0, x)
                        .mapToObj(i -> new double[y])
                        .toArray(i -> new double[x][]);

This will initialize the arrays with default values (0.0 in the case of double).

In case you want to explicitly define the fill value to be used, You can add in a DoubleStream:

int x = 20; // first dimension
int y = 4; // second dimension
double v = 5.0; // fill value

double[][] a = IntStream
        .range(0, x)
        .mapToObj(i -> DoubleStream.generate(() -> v).limit(y).toArray())
        .toArray(i -> new double[x][]);

Arrays.fill works only with one-dimensional array

Source of java.util.Arrays:

public static void fill(Object[] a, Object val) {
        int i = 0;

        for(int len = a.length; i < len; ++i) {
            a[i] = val;
        }

Use own loops for initialization array

public static Object[] fillArray(Object[] arr,Object item){
    Arrays.fill(arr, item);
    return arr;
}
Character[][] maze = new Character[10][10];
    fillArray(maze, fillArray(maze[0], '?'));
    
    for(int i = 0;i<10;i++){
        System.out.println();
        for(int j = 0;j<10;j++){
            System.out.print(maze[i][j]);
        }
    }

i hope this do well

In simple words java donot provide such an API. You need to iterate through loop and using fill method you can fill 2D array with one loop.

      int row = 5;
	  int col = 6;
	  int cache[][]=new int[row][col];
	  for(int i=0;i<=row;i++){
	      Arrays.fill(cache[i]);
	  }

Recursive Solution

A simple Recursive Solution to fill every row of a 2d array with any given value.

double[][] arr = new double[20][4];
int n=arr.length;
fillArrRecursively(arr, n-1, 10); //Recursion call

---

//Recursive method to fill every row of 'arr' with the passed variable 'val'

public static int fillArrRecursively(double arr[][], int n, int val){
    if(n<0) return 1;
    Arrays.fill(arr[n], val);
    return fillArrRecursively(arr, n-1, val);
}

Arrays.fill(arr, new double[4]);