RDFWeb and Friend of a Friend (FOAF): FOAF Contradictions

July 13, 2003

FOAF Contradictions

Q: If I can say what I like in a FOAF file, even say nothing, and if I can use any semantic web vocabularies at all, all mixed together, how can we ever know if a FOAF file is 'wrong' (broken, in error)?

A: Which answer do you want...? ;)

One part of the answer relates to the detection of inconsistencies in FOAF data.
In particular, checking for documents that contradict themselves is becoming possible, thanks to our use of W3C's Web Ontology language (OWL).

So I wrote a bit about this in the FOAF wiki, see the FoafContradictions article there. I hope to expand on it with more examples and detail about how OWL works, so am writing in wiki rather than weblog mode this time. It should be readable and hopefully useful now.

A natural topic for further attention would be the discovery of disagreements between documents. That's a rich area to explore, as it combines a variety of techniques, eg. logical (people only have one foaf:dateOfBirth) and statistical (20% of FOAF files think my surname is 'Brinkley', maybe they're right...). This is an important topic as it relates to trust strategies, to dealing with stale / dated information, and to the practical problems inherent in any 'semantic web search engine' efforts. But I didn't write about it yet. Take a look at the FoafContradictions piece and let me know if that's a useful level of detail to attempt...

Posted by danbri at July 13, 2003 03:16 AM | TrackBack

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Earlier I mentioned that variables can live in two different places. We're going to examine these two places one at a time, and we're going to start on the more familiar ground, which is called the Stack. Understanding the stack helps us understand the way programs run, and also helps us understand scope a little better.

Posted by: Martin on January 20, 2004 08:16 AM

The Stack is just what it sounds like: a tower of things that starts at the bottom and builds upward as it goes. In our case, the things in the stack are called "Stack Frames" or just "frames". We start with one stack frame at the very bottom, and we build up from there.

Posted by: Barnabas on January 20, 2004 08:16 AM

That gives us a pretty good starting point to understand a lot more about variables, and that's what we'll be examining next lesson. Those new variable types I promised last lesson will finally make an appearance, and we'll examine a few concepts that we'll use to organize our data into more meaningful structures, a sort of precursor to the objects that Cocoa works with. And we'll delve a little bit more into the fun things we can do by looking at those ever-present bits in a few new ways.

Posted by: Jerome on January 20, 2004 08:16 AM

For this program, it was a bit of overkill. It's a lot of overkill, actually. There's usually no need to store integers in the Heap, unless you're making a whole lot of them. But even in this simpler form, it gives us a little bit more flexibility than we had before, in that we can create and destroy variables as we need, without having to worry about the Stack. It also demonstrates a new variable type, the pointer, which you will use extensively throughout your programming. And it is a pattern that is ubiquitous in Cocoa, so it is a pattern you will need to understand, even though Cocoa makes it much more transparent than it is here.

Posted by: Barbara on January 20, 2004 08:16 AM

But some variables are immortal. These variables are declared outside of blocks, outside of functions. Since they don't have a block to exist in they are called global variables (as opposed to local variables), because they exist in all blocks, everywhere, and they never go out of scope. Although powerful, these kinds of variables are generally frowned upon because they encourage bad program design.

Posted by: Edward on January 20, 2004 08:16 AM

Let's see an example by converting our favoriteNumber variable from a stack variable to a heap variable. The first thing we'll do is find the project we've been working on and open it up in Project Builder. In the file, we'll start right at the top and work our way down. Under the line:

Posted by: Grace on January 20, 2004 08:16 AM

These secret identities serve a variety of purposes, and they help us to understand how variables work. In this lesson, we'll be writing a little less code than we've done in previous articles, but we'll be taking a detailed look at how variables live and work.

Posted by: Jane on January 20, 2004 08:17 AM

When a variable is finished with it's work, it does not go into retirement, and it is never mentioned again. Variables simply cease to exist, and the thirty-two bits of data that they held is released, so that some other variable may later use them.

Posted by: Beatrice on January 20, 2004 08:17 AM

Posted by: Gawen on January 20, 2004 08:17 AM

When compared to the Stack, the Heap is a simple thing to understand. All the memory that's left over is "in the Heap" (excepting some special cases and some reserve). There is little structure, but in return for this freedom of movement you must create and destroy any boundaries you need. And it is always possible that the heap might simply not have enough space for you.

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