1.2.3 Vector Types: Array, List

The vector data types are Array and List. A List is a simple ordering of elements of a single type. An Array arranges the elements so that they can be easily accessed with one or more indices.

Array

Instances of Array have different semantics than arrays in C. They are multi-dimensional data structures which contain N instances of some data type. Each instance in the array can be referred to by an integer index between 0 and N-1. Multidimensional arrays are declared using C's syntax of a sequence of bracketed integer values: Int32 a[10][20] declares an array of 10 arrays of 20 integers. However, unlike C arrays, the Array class supports named dimensions. In the preceding example, the array could have been declared: Int32 a[row = 10][col = 20] where row and col are the names of the first and second dimension, respectively. You can use the dimension_name and dimension_size member functions of the Array class to determine the name and size for the i^th dimension.

Array, like all of the compound types, contains a reference to a component variable. In the preceding example, the instance of Array would contain information about the dimensions of the array (10 by 20), but not the type of the elements (Int32). The element type information is stored in the component variable which the instance of Array references.

When creating an array, the dimension sizes (and optionally their names) must be set. Regardless of the shape of the array, it is always stored as a vector. In order to access the element of a multidimensional array it is necessary to calculate the offset for a given element.

 

List

A List is an ordered collection of elements of unknown length. This is in contrast to the Array, whose size is always known in advance. When a List type is declared no size information is supplied, but in order to transmit a List object the length of that list must be known. Thus, internally the number of elements in the current value is stored.