Variable-length Arrays
Resizable homogeneous data structures.
Variable-length arrays are arrays that can, during program execution, either be resized to hold more or less elements, or have their dimension[s] use a different subscript range. The memory used by a variable-length array to store its elements is allocated at runtime, in the free store (the heap), as opposed to fixed-length arrays whose data is either allocated on the program stack or in the .BSS or .DATA sections of the executable, depending on whether they were defined with Static.
A variable-length array is declared with the Extern keyword, followed by a variable identifier, a parenthesized list of zero (0) or more subscript ranges and an element datatype. For an array to be declared variable-length, either an empty subscript range list can be used, or one or more of the subscript ranges must have non-constant values - that is, the ranges must be specified by variables or objects convertible to Integer.
A variable-length array is defined with either the Dim or ReDim keywords, following the same form as declaration. The ReDim keyword always defines variable-length arrays, whether the subscript ranges are constant or not. Variable-length arrays cannot be defined with static storage using Static.
Since the memory for variable-length arrays is allocated at runtime, when they are defined, the elements of variable-length arrays declared with Extern cannot be accessed until the array is defined. Similarly, variable-length arrays defined with empty subscript range lists have zero size - they hold zero elements - so they too must be resized before any elements can be accessed.
Resizing a variable-length array refers to "redefining" the array with different values for its subscript range[s], allowing the array to grow or shrink. Elements outside the new subscript range[s] are erased; object elements will be destroyed. If the array is resized to a larger size, new elements are added initialized with a zero or null value; object elements are default-constructed. Variable-length arrays are resized using the ReDim keyword following the same form as definition.
By default, element values of a variable-length array are lost when resized - the array will contain zero or more elements initialized to a zero or null value, or if the elements are objects, they will be default-constructed. To retain the previous element values during a resize, use the Preserve keyword.
Overview
Variable-length arrays are arrays that can, during program execution, either be resized to hold more or less elements, or have their dimension[s] use a different subscript range. The memory used by a variable-length array to store its elements is allocated at runtime, in the free store (the heap), as opposed to fixed-length arrays whose data is either allocated on the program stack or in the .BSS or .DATA sections of the executable, depending on whether they were defined with Static.
Declaration and definition
A variable-length array is declared with the Extern keyword, followed by a variable identifier, a parenthesized list of zero (0) or more subscript ranges and an element datatype. For an array to be declared variable-length, either an empty subscript range list can be used, or one or more of the subscript ranges must have non-constant values - that is, the ranges must be specified by variables or objects convertible to Integer.
Dim maxLowerBound As Integer = -5
Dim maxUpperBound As Integer = 10
'' Declares a two dimensional variable-length array of elements of type STRING..
Extern arrayOfStrings(maxLowerBound To maxUpperBound, 20) As String
'' Declares a one-dimensional variable-length array of elements of type BYTE..
Extern arrayOfBytes() As Byte
Dim maxUpperBound As Integer = 10
'' Declares a two dimensional variable-length array of elements of type STRING..
Extern arrayOfStrings(maxLowerBound To maxUpperBound, 20) As String
'' Declares a one-dimensional variable-length array of elements of type BYTE..
Extern arrayOfBytes() As Byte
A variable-length array is defined with either the Dim or ReDim keywords, following the same form as declaration. The ReDim keyword always defines variable-length arrays, whether the subscript ranges are constant or not. Variable-length arrays cannot be defined with static storage using Static.
'' Defines one-dimensional variable-length arrays of type SHORT and INTEGER..
Dim arrayOfShorts() As Short
ReDim arrayOfIntegers(9) As Integer
Dim arrayOfShorts() As Short
ReDim arrayOfIntegers(9) As Integer
Since the memory for variable-length arrays is allocated at runtime, when they are defined, the elements of variable-length arrays declared with Extern cannot be accessed until the array is defined. Similarly, variable-length arrays defined with empty subscript range lists have zero size - they hold zero elements - so they too must be resized before any elements can be accessed.
Resizing
Resizing a variable-length array refers to "redefining" the array with different values for its subscript range[s], allowing the array to grow or shrink. Elements outside the new subscript range[s] are erased; object elements will be destroyed. If the array is resized to a larger size, new elements are added initialized with a zero or null value; object elements are default-constructed. Variable-length arrays are resized using the ReDim keyword following the same form as definition.
'' Define an empty variable-length array of SINGLE elements..
Dim array() As Single
'' Resize the array to hold ten (10) SINGLE elements..
ReDim array(9) As Single
Dim array() As Single
'' Resize the array to hold ten (10) SINGLE elements..
ReDim array(9) As Single
By default, element values of a variable-length array are lost when resized - the array will contain zero or more elements initialized to a zero or null value, or if the elements are objects, they will be default-constructed. To retain the previous element values during a resize, use the Preserve keyword.
