ReDimkeyword

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 [, ...]]) [, ...]

or:

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

Parameters

Name		Description
`symbolname`		A new or existing array identifier.
`expression`		or
`(expression)`		An expression referring to an existing array. This can be used to resize arrays which are members of user-defined types. In some cases, it is necessary to specify parentheses around the expression (especially if the array expression itself contains parentheses) - see the examples below.
`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.

When used with in a user-defined type, ReDim creates variable-length arrays while being able to pre-size them with constant subscripts.

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 (if resizing is not possible, the whole original data block is first shifted to another memory location).
- 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 (because multidimensional arrays are stored in row-major order : values differing only in the last index are contiguous).
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 procedure: FreeBASIC cannot prevent you trying this at compile-time, but generates an error at run-time.
ReDim cannot be used inside a member procedure if the array contains as element the instance itself of the object, because that could cause horrible crashes:
- If the array data are moved into memory by ReDim, the passed This reference becomes inconsistent, in the same way as a dangling pointer.
- In that case, all subsequent accesses to any non-static member field from this member procedure will be erroneous, except if the passed This reference would be readjusted (by means of @This = @array(...)) immediately after executing ReDim in the body of this member procedure.

Remarks

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.

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)



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)



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)



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