subscript(_:)

Declaration

subscript(key: [NSNumber]) -> NSNumber { get set }

Parameters

• key:

  An Nsnumber array that represents a position in a multiarray in which each element is an index in the corresponding dimension.

Return Value

A number.

Discussion

Use this subscript to access the multiarray by using an NSNumber array in which each element is an index in the multiarray’s corresponding dimension.

// A multiarray with three dimensions.
let multiarray = try MLMultiArray(shape: [5, 29, 17], dataType: .double)

// The subscript key with three elements.
let key = [3, 5, 7] as [NSNumber]

// Set the element to π, a sentinel value.
multiarray[key] = 3.141_59

// Retrieve the element as a number.
let numberValue = multiarray[key]
print(numberValue.doubleValue)
// Prints: 3.14159

For better performance, directly access the multiarray’s underlying pointer by using a linear offset. Calculate the linear offset by summing the products of each dimension’s index with the dimension’s stride (See strides).

// Get a pointer to the multiarray’s contents.
let pointer = UnsafeMutablePointer<Double>(OpaquePointer(multiarray.dataPointer))
var linearOffset = 0

// Sum the products of each dimension’s number with its stride.
for (dimension, stride) in zip(key, multiarray.strides) {
    linearOffset += dimension.intValue * stride.intValue
}

// Check that the linear offset resolves to the same element as the subscript.
assert(pointer[linearOffset] == multiarray[key].doubleValue)

print(pointer[linearOffset])
// Prints: 3.14159

// Set the element to a different sentinel value.
pointer[linearOffset] = 2.718_28
print(pointer[linearOffset])
// Prints: 2.71828