Dynamic Arrays 


 

  • Arrays that grow and shrink at run-time are called dynamic arrays (also called redimmable arrays).

  • Arrays in VB.Net are all dynamic, even if declared with an initial size, like
             Dim dynamicArray (9) as Integer

  • Arrays in most programming languages are static, and cannot be resized.

  • Dynamic arrays are more flexible than fixed-sized arrays, because they can be re­sized anytime to accommodate new data.

════════════════════

A dynamic array's size can be changed anytime with ReDim. For example, the line 

ReDim dynamicArray (19) 

changes the number of elements allocated from 10 to 20.    

Note: Resizing dynamic arrays consumes processor time and can slow a program's execution, so ReDim as infrequently as possible. See note at end of page.

════════════════════

The total number of dimensions cannot be changed by ReDim (i.e., if the array is a two-dimensional array, it cannot become a three-dimensional array).   The ReDim statement cannot change the data type of an array variable or provide new initialization values for the array elements.

════════════════════

When ReDim is executed, all values contained in the array are lost. Numeric values are reset to zero and Strings are reset to a zero-length String. Values can be retained by using keyword Preserve with ReDim. For example, the line 

ReDim Preserve dynamicArray(19) 

resizes dynamicArray, preserving (i.e., retaining) the original values in the array. 

════════════════════

Memory allocated for a dynamic array can be deallocated (released) at run-time using keyword Erase. A dynamic array that has been deallocated must be redimensioned with ReDim before it can be used again.    

        Erase dynamicArray

════════════════════

Array resizing should be used sparingly.  When you call ReDim Preserve dynamicArray(19), VB.NET actually takes the array as it stands before that statement, creates a copy of it with the additional positions needed (or a smaller size), then destroys the original copy.  This is because VB.NET arrays are not truly dynamic. If you need a structure that can actually grow and shrink on the fly, use an ArrayList.

An ArrayList tutorial.  And another.