Day: May 8, 2013

VBA Internals: Performance of Fixed vs. Dynamic Arrays

Arrays in VBA can have their dimensions either fixed at compile time or modifiable at runtime. Fixed-length arrays are declared by specifying the bounds in the declaration, while dynamic arrays must be initialized using the ReDim statement before they can be used. (See Types of Arrays for more details).

Given the flexibility of dynamic arrays I have sometimes wondered why anyone would use a fixed-length array. Sometimes it is required for byte buffers or in user defined types for passing to and from API methods. But for straight VB code, is there any advantage?

In particular, what is the performance cost of using a dynamic array? I set to measure exactly that, and I present my results here. I'll give it away from the start: There is essentially no statistical difference in how long it takes to initialize or copy fixed-length arrays vs. dynamic-length arrays. My recommendation, then: Only use fixed-length arrays when an API function or other external limitation requires it!

Methodology

To compare the speed of initializing and copying fixed and dynamic arrays, I used the high-resolution performance timer described in Accurate Performance Timers in VBA. I executed each test 1,000 times and recorded each measurement. I then calculated the overall average and standard deviations of the measurements, and also graphed the histograms of the actual measurements. The code used to actually perform the tests is included at the end of this post.

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