Category Archives: Computation

Basics: Binary Search

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For the basics, I wrote a bunch of stuff about sorting. It seems worth taking a moment
to talk about something related: binary search. Binary search is one of the most important
and fundamental algorithms, and it shows up in sorts of places.

It also has the amazing property that despite being simple and ubiquitous, it’s virtually
always written wrong. There’s a bit of subtlety in implementing it correctly, and virtually
everyone manages to put off-by-one indexing errors into their implementations. (Including me; last time I implemented a binary search, the first version included one of the classic errors.) The errors are so ubiquitous that even in a textbook that discusses the fact that most programmers get it wrong, they got it wrong in their example code!

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More π-Calculus Games

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Ok. So I’m still tweaking syntax, to try to find a way of writing π-calculus in a way that’s
easy for me to write in my editor, and for you to read in your browser. Here’s the latest version:

  1. Sequential Composition: Process1.Process2.
  2. Send expressions: !channel(tuple).Process
  3. Receive expressions: ?channel(tuple).Process
  4. New channel expression new(name,...) { Process }
  5. Process duplication expression: *(Process)
  6. Parallel composition: Process1 | Process2.
  7. Choice composition: Process1 + Process2.
  8. Null process: ∅

So, in this syntax, the final version of the storage cell from yesterdays post
is:

NewCell[creator,initval]=new(read,write){ (Cell[read,write,initval]
| !creator(read,write)) }
Cell[read,write,val]=( !read(val).Cell[read,write,val]
+ ?write(v).Cell[read,write,v])

Now, let’s try to use π-calculus to do something that actually involves real concurrency.

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Building Things in π-Calculus: Storage Cells

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As a refresher for me, and to give some examples to help you guys understand it, I’m going to go through a couple of examples of interesting things you can build with π-calculus. We’ll start with a simple way of building mutable storage.

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Not Quite Basics: Sorting Algorithms

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Multiple people have written to me, after seeing yesterday’s algorithms basics post, asking me to say more about sorting algorithms. I have to say that it’s not my favorite topic – sorting is one of those old bugaboos that you can’t avoid, but which gets really dull after a while. But there is a kernel of interest to it – sorting can be used to demonstrate a lot of interesting ideas about
computational complexity.

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Quantum Computation Complexity: BQP

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What started me on this whole complexity theory series was a question in
the comments about the difference between quantum computers and classical
computers. Taking the broadest possible view, in theory, a quantum computer
is a kind of non-deterministic machine – so in the best possible case,
a quantum machine can solve NP-complete problems in polynomial time. The set
of things computable on a quantum machine is not different from the set of
things computable on a classical machine – but the things that are tractable (solvable in a reasonable amount of time) on a quantum
computer may be different.

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Probabilistic Complexity

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As I’ve mentioned in the past, complexity theory isn’t really one of my favorite topics. As a professional computer scientist, knowing and loving complexity up to the level of NP-completeness
is practically a requirement. But once you start to get beyond P and NP, there are thousands of complexity classes that have been proposed for one reason or another, and really understanding all of them can get remarkably difficult, and what’s worse, can feel like an utterly pointless exercise,
particularly if the writer/teacher you’re learning from isn’t very good.

I’m not going to write a long series of posts on the more esoteric complexity classes. But there
are a few that are interesting, and might even have some relevance to actual practical problems in
selecting algorithms for a software system.

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Basic Complexity Classes: P and NP

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Now that we’ve gone through a very basic introduction to computational complexity, we’re ready
to take a high-level glimpse at some of the more interesting things that arise from it. The one
that you’ll hear about most often is “P vs NP”, which is what I’m going to write about today.

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