ReDim

FreeBASIC

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ReDim

Defines or resizes a variable-length array

Syntax

Declaring a Dynamic Array:

ReDim [ Shared ] symbolname([subscript [, ...]]) As datatype [, ...]
ReDim [ Shared ] As datatype symbolname([subscript [, ...]]) [, ...]

Resizing a Dynamic Array:

ReDim [ Preserve ] symbolname([subscript [, ...]]) [, ...]

Parameters

Shared

Specifies shared (file-scope) access to the array throughout the module.

Preserve

When used with an existing array, the contents of the array will be preserved during the resize. Note that in some cases Preserve will not preserve data at its original index, see below.

symbolname

A new or existing array id.

subscript: [ lowerbound To ] upperbound

The lower and upper bound range for a dimension of the array. Lower bound defaults to zero (0), or the default Base, if not specified.

datatype

The type of elements contained in the array.

Description

ReDim can be used to define new variable-length arrays, or resize existing variable-length arrays while keeping the same number of dimensions. ReDim always produces variable-length arrays, so, unlike Dim, variable-length arrays can be defined with constant subscripts.

When defining a new variable-length array, its elements are default constructed. For simple data types like Integer or Double, the elements are initialized to zero (0). For user-defined types with a default constructor, that will be called.

NOTES:

ReDim Preserve may not work as expected in all cases:

Preserve's current behavior is to keep the original data contiguous in memory, and only expand or truncate the size of the memory.
Its behavior (with a single dimension) is well-defined only when the upper bound is changed. If the lower bound is changed, the current result is that the data is in effect shifted to start at the new lower bound.
With multiple dimensions, only the upper bound of only the first dimension may be safely increased. If the first dimension is reduced, the existing mappable data may be lost. If lower-order dimensions are resized at all, the effects can be hard to predict.

ReDim cannot be used on fixed-size arrays - i.e. arrays with constant bounds made with Dim. This includes the fixed-size arrays contained in UDTs (user-defined Types). This also includes fixed-length arrays passed as parameters in a function. FreeBASIC cannot prevent you trying this at compile-time, but the results at run-time will be undefined.
Using ReDim within a member procedure with an array that contains an instance of the object class is undefined, and will [hopefully] result in horrible crashes.
For use of ReDim (resizing) with a complex expression, (especially if the array expression itself contains parentheses), the array expression must be enclosed in parentheses in order to solve the parsing ambiguity.

Example

'' Define a variable-length array with 5 elements '' ReDim array(0 To 4) As Integer For index As Integer = LBound(array) To UBound(array) array(index) = index Next '' Resize a variable-length array with 10 elements '' (the lower bound should be kept the same) ReDim Preserve array(0 To 9) As Integer Print "index", "value" For index As Integer = LBound(array) To UBound(array) Print index, array(index) Next
This program will produce the following output:

index         value
 0             0
 1             1
 2             2
 3             3
 4             4
 5             0
 6             0
 7             0
 8             0
 9             0

'' Define a variable-length array Dim array() As Integer '' ReDim array to have 3*4 elements ReDim array(1 To 3, 1 To 4) Dim As Integer n = 1, i, j Print "3 * 4:" Print For i = LBound(array, 1) To UBound(array, 1) For j = LBound(array, 2) To UBound(array, 2) array(i, j) = n Print Using "## "; array(i, j); n += 1 Next Print Next Print '' ReDim Preserve array to have 4*4 elements, preserving the contents '' (only the first upper bound should be changed) ReDim Preserve array(1 To 4, 1 To 4) As Integer Print "4 * 4:" Print For i = LBound(array, 1) To UBound(array, 1) For j = LBound(array, 2) To UBound(array, 2) Print Using "## "; array(i, j); Next Print Next Print '' ReDim Preserve array to have 2*4 elements, preserving but trancating the contents '' (only the first upper bound should be changed) ReDim Preserve array(1 To 2, 1 To 4) As Integer Print "2 * 4:" Print For i = LBound(array, 1) To UBound(array, 1) For j = LBound(array, 2) To UBound(array, 2) Print Using "## "; array(i, j); Next Print Next Print
This program will produce the following output:

3 * 4:

 1   2   3   4
 5   6   7   8
 9  10  11  12

4 * 4:

 1   2   3   4
 5   6   7   8
 9  10  11  12
 0   0   0   0

2 * 4:

 1   2   3   4
 5   6   7   8

Differences from QB

Preserve was in Visual Basic, but not in QBASIC.
Multi-dimensional arrays in FreeBASIC are in row-major order, rather than column-major order.