What's better for performance, cell arrays of objects or heterogeneous arrays?

Question

Suppose I have some classes foo < handle, and bar < foo, baz < foo, and maybe qux < foo. There are a couple ways I can store an array of these objects:

• As a cell array: A = {foo bar baz qux} % A(1) would be a cell, A{1} gives me a foo object

• Starting with R2011a, I can make foo <matlab.mixin.Heterogeneous, and then build an array directy: A = [foo bar baz qux] % A(1) directly gives me a foo object

The way I see it, from a maintenance perspective it would be better to use the second method rather than the first, this way it removes ambiguity about how to access A. Namely, when we need to dereference elements of the cell array (cell A(1) vs foo object A{1}, which lives inside A(1)).

But is there any kind of memory or performance penalty (or benefit) to using one syntax vs the other?

Coif

I would be surprised if you could see the difference in performance. Create both objects and do a whos to see their respective sizes. – Psoas

@Psoas I did the above and it looks like the heterogeneous array implementation takes up 104 bytes, and the cell array takes up a whopping 864! However, defining a foo object by itself seems to also take up 104 bytes. So I guess don't really understand what is going on inside the heterogeneous implementation... – Coif

The curse of matlab. No one really knows what is going on behind the curtain and matlab have never been to interested in telling people their secrets. – Bridoon

Answer 1

I did a small experiment (source) on the memory and running time of the cell array, containers.Map and a Heterogeneous array. In my method I preallocated each array with N=65535 elements (the max array size for Map and Heterogeneous array), then began assigning each element a uint32, and measured the time and memory. My Heterogeneous Class was a simple class with a single public property, and a constructor which assigned that property. The containers.Map had uint32 key/value pairs.

Maps took 9.17917e-01 seconds.
Cells took 5.81220e-02 seconds.
Heterogeneous array took 4.95336e+00 seconds.

**Name**     **Size**         **Bytes**       **Class**   
map          65535x1           112          containers.Map              
cellArr      65535x1           7602060      cell               
hArr         1x65535           262244       SomeHeterogeneousClass   

Immediately note that the size of the mapArray is not accurate. It is hidden behind the containers.Map class implementation, most likley the 112 bytes reported is the memory assigned to the map itself, excluding the data. I approximate the true size to be at minimum (112+65535*(sizeof(uint32)*2)) = 524392 bytes. This value is almost exactly double the hArr size, which makes me think it is quite accurate, since the map must store twice as much data (for key AND value) as the hArr.

The results are straightforward:

• Time: cell Array < Map < Heterogeneous Array
• Memory: Heterogeneous Array < Map < cell Array

I repeated the experiment with N=30 to test for small arrays, the results were similar. God only knows why cells take up so much memory and Heterogeneous arrays are so slow.

Mochun