NdArray

public interface NdArray
Known Indirect Subclasses

A data structure of N-dimensions.

The `NdArray` interface creates an abstraction between the physical storage of a data record, which can be linear or segmented, and its logical representation. In general, they achieve better performances than standard multi-dimensional arrays in Java by mapping directly linear data segments in memory.

Like DataBuffer , NdArray instances support 64-bits indexing so they can be used to map very large data records. They also support special coordinates that allow traversing their values in any direction or to select only a subset of them.

Example of usage:

// Creates a 2x3x2 matrix (of rank 3)
    FloatNdArray matrix3d = NdArrays.ofFloats(shape(2, 3, 2));

    // Initialize sub-matrices data with vectors
    matrix.set(NdArrays.vectorOf(1.0f, 2.0f), 0, 0)
          .set(NdArrays.vectorOf(3.0f, 4.0f), 0, 1)
          .set(NdArrays.vectorOf(5.0f, 6.0f), 0, 2)
          .set(NdArrays.vectorOf(7.0f, 8.0f), 1, 0)
          .set(NdArrays.vectorOf(9.0f, 10.0f), 1, 1)
          .set(NdArrays.vectorOf(11.0f, 12.0f), 1, 2);

    // Access the second 3x2 matrix (of rank 2)
    FloatNdArray matrix = matrix3d.get(1);

    // Access directly the float value at (1, 0) from the second matrix
    assertEquals(9.0f, matrix.getFloat(1, 0));
 

Public Methods

abstract NdArray <T>
copyTo ( NdArray <T> dst)
Copy the content of this array to the destination array.
abstract NdArraySequence <? extends NdArray <T>>
elements (int dimensionIdx)
Returns a sequence of all elements at a given dimension.
abstract boolean
equals (Object obj)
Checks equality between n-dimensional arrays.
abstract NdArray <T>
get (long... coordinates)
Returns the N-dimensional element of this array at the given coordinates.
abstract T
getObject (long... coordinates)
Returns the value of the scalar found at the given coordinates.
abstract NdArray <T>
read ( DataBuffer <T> dst)
Read the content of this N-dimensional array into the destination buffer.
abstract NdArraySequence <? extends NdArray <T>>
scalars ()
Returns a sequence of all scalars in this array.
abstract NdArray <T>
set ( NdArray <T> src, long... coordinates)
Assigns the value of the N-dimensional element found at the given coordinates.
abstract NdArray <T>
setObject (T value, long... coordinates)
Assigns the value of the scalar found at the given coordinates.
abstract NdArray <T>
slice ( Index... indices)
Creates a multi-dimensional view (or slice) of this array by mapping one or more dimensions to the given index selectors.
abstract NdArray <T>
write ( DataBuffer <T> src)
Write the content of this N-dimensional array from the source buffer.

Inherited Methods

Public Methods

public abstract NdArray <T> copyTo ( NdArray <T> dst)

Copy the content of this array to the destination array.

The shape() of the destination array must be equal to the shape of this array, or an exception is thrown. After the copy, the content of both arrays can be altered independently, without affecting each other.

Parameters
dst array to receive a copy of the content of this array
Returns
  • this array
Throws
IllegalArgumentException if the shape of dst is not equal to the shape of this array

public abstract NdArraySequence <? extends NdArray <T>> elements (int dimensionIdx)

Returns a sequence of all elements at a given dimension.

Logically, the N-dimensional array can be flatten in a single vector, where the scalars of the (n - 1) th element precedes those of the (n) th element, for a total of size() values.

For example, given a n x m matrix on the [x, y] axes, elements are iterated in the following order:

x 0 y 0 , x 0 y 1 , ..., x 0 y m-1 , x 1 y 0 , x 1 y 1 , ..., x n-1 y m-1

The returned sequence can then be iterated to visit each elements, either by calling forEach(Consumer) or forEachIndexed(BiConsumer) .

// Iterate matrix for initializing each of its vectors
    matrixOfFloats.elements(0).forEach(v -> {
      v.set(vector(1.0f, 2.0f, 3.0f));
    );

    // Iterate a vector for reading each of its scalar
    vectorOfFloats.scalars().forEachIdx((coords, s) -> {
      System.out.println("Value " + s.getFloat() + " found at " + coords);
    });
 }

Parameters
dimensionIdx index of the dimension
Returns
  • an NdArray sequence
Throws
IllegalArgumentException if dimensionIdx is greater or equal to the total number of dimensions of this array

public abstract boolean equals (Object obj)

Checks equality between n-dimensional arrays.

An array is equal to another object if this object is another NdArray of the same shape, type and the elements are equal and in the same order. For example:

IntNdArray array = NdArrays.ofInts(Shape.of(2, 2))
    .set(NdArrays.vectorOf(1, 2), 0)
    .set(NdArrays.vectorOf(3, 4), 1);

 assertEquals(array, StdArrays.ndCopyOf(new int[][] { {1, 2, {3, 4} }));  // true
 assertEquals(array, StdArrays.ndCopyOf(new Integer[][] { {1, 2}, {3, 4} }));  // true, as Integers are equal to ints
 assertNotEquals(array, NdArrays.vectorOf(1, 2, 3, 4));  // false, different shapes
 assertNotEquals(array, StdArrays.ndCopyOf(new int[][] { {3, 4}, {1, 2} }));  // false, different order
 assertNotEquals(array, StdArrays.ndCopyOf(new long[][] { {1L, 2L}, {3L, 4L} }));  // false, different types
 }

Note that the computation required to verify equality between two arrays can be expensive in some cases and therefore, it is recommended to not use this method in a critical path where performances matter.

Parameters
obj object to compare this array with
Returns
  • true if this array is equal to the provided object

public abstract NdArray <T> get (long... coordinates)

Returns the N-dimensional element of this array at the given coordinates.

Elements of any of the dimensions of this array can be retrieved. For example, if the number of coordinates is equal to the number of dimensions of this array, then a rank-0 (scalar) array is returned, which value can then be obtained by calling `array.getObject()`.

Any changes applied to the returned elements affect the data of this array as well, as there is no copy involved.

Note that invoking this method is an equivalent and more efficient way to slice this array on single scalar, i.e. array.get(x, y, z) is equal to array.slice(at(x), at(y), at(z))

Parameters
coordinates coordinates of the element to access, none will return this array
Returns
  • the element at this index
Throws
IndexOutOfBoundsException if some coordinates are outside the limits of their respective dimension

public abstract T getObject (long... coordinates)

Returns the value of the scalar found at the given coordinates.

To access the scalar element, the number of coordinates provided must be equal to the number of dimensions of this array (i.e. its rank). For example:

FloatNdArray matrix = NdArrays.ofFloats(shape(2, 2));  // matrix rank = 2
  matrix.getObject(0, 1);  // succeeds, returns 0.0f
  matrix.getObject(0);  // throws IllegalRankException

  FloatNdArray scalar = matrix.get(0, 1);  // scalar rank = 0
  scalar.getObject();  // succeeds, returns 0.0f
 
Note: if this array stores values of a primitive type, prefer the usage of the specialized method in the subclass for that type. For example, floatArray.getFloat(0); .

Parameters
coordinates coordinates of the scalar to resolve
Returns
  • value of that scalar
Throws
IndexOutOfBoundsException if some coordinates are outside the limits of their respective dimension
IllegalRankException if number of coordinates is not sufficient to access a scalar element

public abstract NdArray <T> read ( DataBuffer <T> dst)

Read the content of this N-dimensional array into the destination buffer.

The size of the buffer must be equal or greater to the size() of this array, or an exception is thrown. After the copy, content of the buffer and of the array can be altered independently, without affecting each other.

Parameters
dst the destination buffer
Returns
  • this array
Throws
BufferOverflowException if the buffer cannot hold the content of this array
See Also

public abstract NdArraySequence <? extends NdArray <T>> scalars ()

Returns a sequence of all scalars in this array.

This is equivalent to call elements(shape().numDimensions() - 1)

Returns
  • an NdArray sequence

public abstract NdArray <T> set ( NdArray <T> src, long... coordinates)

Assigns the value of the N-dimensional element found at the given coordinates.

The number of coordinates provided can be anywhere between 0 and rank - 1. For example:

FloatNdArray matrix = NdArrays.ofFloats(shape(2, 2));  // matrix rank = 2
  matrix.set(vector(10.0f, 20.0f), 0);  // success
  matrix.set(scalar(10.0f), 1, 0); // success
 

Parameters
src an array of the values to assign
coordinates coordinates of the element to assign
Returns
  • this array
Throws
IndexOutOfBoundsException if some coordinates are outside the limits of their respective dimension

public abstract NdArray <T> setObject (T value, long... coordinates)

Assigns the value of the scalar found at the given coordinates.

To access the scalar element, the number of coordinates provided must be equal to the number of dimensions of this array (i.e. its rank). For example:

FloatNdArray matrix = NdArrays.ofFloats(shape(2, 2));  // matrix rank = 2
  matrix.setObject(10.0f, 0, 1);  // succeeds
  matrix.setObject(10.0f, 0);  // throws IllegalRankException

  FloatNdArray scalar = matrix.get(0, 1);  // scalar rank = 0
  scalar.setObject(10.0f);  // succeeds
 
Note: if this array stores values of a primitive type, prefer the usage of the specialized method in the subclass for that type. For example, floatArray.setFloat(10.0f, 0);

Parameters
value the value to assign
coordinates coordinates of the scalar to assign
Returns
  • this array
Throws
IndexOutOfBoundsException if some coordinates are outside the limits of their respective dimension
IllegalRankException if number of coordinates is not sufficient to access a scalar element

public abstract NdArray <T> slice ( Index... indices)

Creates a multi-dimensional view (or slice) of this array by mapping one or more dimensions to the given index selectors.

Slices allow to traverse an N-dimensional array in any of its axis and/or to filter only elements of interest. For example, for a given matrix on the [x, y] axes, it is possible to iterate elements at y=0 for all x .

Any changes applied to the returned slice affect the data of this array as well, as there is no copy involved.

Example of usage:

FloatNdArray matrix3d = NdArrays.ofFloats(shape(3, 2, 4));  // with [x, y, z] axes

    // Iterates elements on the x axis by preserving only the 3rd value on the z axis,
    // (i.e. [x, y, 2])
    matrix3d.slice(all(), all(), at(2)).elements(0).forEach(m -> {
      assertEquals(shape(2), m); // y=2, z=0 (scalar)
    );

    // Creates a slice that contains only the last element of the y axis and elements with an
    // odd `z` coordinate.
    FloatNdArray slice = matrix3d.slice(all(), at(1), odd());
    assertEquals(shape(3, 2), slice.shape());  // x=3, y=0 (scalar), z=2 (odd coordinates)

    // Iterates backward the elements on the x axis
    matrix3d.slice(flip()).elements(0).forEach(m -> {
      assertEquals(shape(2, 4), m);  // y=2, z=4
    });
 }

Parameters
indices index selectors per dimensions, starting from dimension 0 of this array.
Returns
  • the element resulting of the index selection
Throws
IndexOutOfBoundsException if some coordinates are outside the limits of their respective dimension

public abstract NdArray <T> write ( DataBuffer <T> src)

Write the content of this N-dimensional array from the source buffer.

The size of the buffer must be equal or greater to the size() of this array, or an exception is thrown. After the copy, content of the buffer and of the array can be altered independently, without affecting each other.

Parameters
src the source buffer
Returns
  • this array
Throws
BufferUnderflowException if the buffer has not enough remaining data to write into this array
See Also