Darren's Weekly Nugget 10/09/2006

[Darren]

The following tip has been on my list of possible nuggets for a while, but since it has recently been discussed on LAVA, I figured I should go ahead and mention it before it becomes old news.  🙂  It's basically a faster way to accomplish the following:

You see here that we have a data structure where objects are identified by their names.  We could probably make this code easier to read (and possibly faster) by maintaining a separate array of the names and simply searching that array with Search 1D Array:

Both of these methods are inherently linear searches.  It turns out that the following method of accessing the data is much faster:

I had never given much thought to Variant Attributes until this trick was shown to me.  You can basically store each data object as a different attribute of some dummy variant.  I say it's a "dummy" variant because its actual value and datatype are inconsequential...we're purely using the variant to gain access to its attributes as a data store.  I'm no computer scientist, but apparently the variant attributes are stored using a "red/black tree" algorithm, which ends up being an order log(n) search...this is much faster than an order n search, which is what the While Loop or Search 1D Array approach would be.

So the next time you find yourself creating a string array lookup similar to one of the two traditional approaches above, try giving the Variant Attribute approach a try.  Note that this approach is recommended for LabVIEW 8.0 and later, due to some performance improvements with variants that did not exist in LabVIEW 7.1 and previous.

-D

P.S. - Check out past nuggets here.

Message Edited by Darren on 10-09-2006 11:14 AM

[JeffBoettcher]

Interesting, but unless I missed something (entirely possible, mind you) a few details remain to be explained.  Does this assume that the Object Data cluster contains a string?  If so, does said string have to be at the "top-level" of the cluster?  If there are multiple strings in Object Data, does it use the first one in the cluster order, the highest-level one, etc?

I also don't quite understand, the first two find an item with a particular name in an array of clusters.  The variant approach looks like it is doesn't have any arrays in it, so what is it searching through?

Message Edited by Jeff B on 10-09-2006 11:27 AM

[Darren]

Object Data can contain whatever you want...it's just that whenever you add the object data to the variant attributes, you're associating a specific name (the attribute name) with that data for looking up later.  You can check out the LAVA discussion I linked above for more details if it's still unclear.

-D

[Ben]

Is there anyone out there who would like to translate this nugget for the "variant diabled" (like me!) ?

Given the "Old way" what would I have to do to convert my code to the new way?

Ben

[Jarrod_S.]

I'm attaching a simple example that should help a little. The best way I can phrase this is to think of the dummy variant Darren described more as a refnum to some storage. The refnum itself is meaningless, but it allows us to call functions (like Get/Set) on that specific storage in memory.

So we can store any type of data object as a name/value pair inside the variant and then retrieve those values by name. You pass around the variant data like you would a refnum. The big difference in this analogy is that if you copy the variant, you copy the entire storage, which is not what happens with a refnum or any other type of reference.

Message Edited by Jarrod S. on 10-09-2006 01:10 PM