## Wednesday, March 21, 2012

### Never do this:

Rename libc++.1.dylib (in /usr/lib) to something other than libc++.1.dylib. I wanted to install a fresh svn build of libc++ on my home computer. I figured I wanted to keep the original dylib file around just incase anything went horribly wrong (TM). The irony? Renaming libc++.1.dylib basically made my whole machine fail. Internet stopped working, Finder stopped working, spotlight stopped working, launchd crashed on reboot causing my computer to hang, and who know what else was broken because of it.

In hindsight, I guess it makes sense that renaming libc++.1.dylib would cause a lot of stuff to crash since libc++ is a dynamically loaded library. Though I always thought that when a dylib was loaded, it was somehow copied into the resident memory of the application. I guess not.

It does make me wonder though... what IS the correct way to install libc++ if not to replace the file? libc++'s website suggests creating links in /usr/lib but goes on to say "either way to should work" (and then never goes on to say what that other way is..). Hum.

And just to document what I did to fix it:

1. Booted into single user mode (held command-s during reboot).
2. Saw a diagnostic message saying that stuff crashed because libc++.1.dylib was missing. This is when I realized that renaming libc++.1.dylib did have such catastrophic effects.
3. Rebooted into recovery mode (held command-r during reboot).
4. cd up several directories until I found "Volumes"
5. cd into Volumes/Macintosh\ HD/usr/lib
6. renamed the file to libc++.1.dylib (using the 'mv' command).
7. Rebooted normally, and prayed that this would fix it
8. And it did!
Hopefully I can now figure out the correct way to install libc++ >.<

## Sunday, March 11, 2012

### Matrix-Vector Multiplication

I'm finishing up my spring break this week (I know - not much of a "spring" break, but you take what you can get) and I decided to start going through the MIT OpenCourseWare course on Linear Algebra. I want to be a graphics guy when I get out of school and I haven't taken a single linear algebra course yet! Shame on me.

Gilbert Strang is a great lecturer. He teaches linear algebra in a such a way that even the small points he makes makes you go "aha!" I'd never had matrix-vector multiplication explained to me; it was just something I memorized and had to take a leap of faith for. In his lectures, Strang really hits home the importance of this idea of "linear combinations."

Linear combinations are simple. If you have a bunch of vectors and a scalar coefficient for each vector, the linear combination is just the addition of each vector multiplied by its scalar.

As it turns out, that's exactly what matrix-vector multiplication is, too. Each column in your matrix is one of the vectors and each component in your vector is one of the coefficients (the first component of the vector is the coefficient to the first column, the second component of the vector is the coefficient to the second column, etc.).

The typical way to think of matrix-vector multiplication is to just consider your vector as a matrix and do the standard "take a row from the first matrix, take a column from the second matrix, perform the dot product" matrix multiplication.

With the linear combination concept, matrix-vector multiplication becomes much more intuitive (for me at least) because it's just a normal linear combination:

$\begin{bmatrix} c1_{1} & c2_{1} & c3_{1} \\ c1_{2} & c2_{2} & c3_{2} \\ c1_{3} & c2_{3} & c3_{3} \end{bmatrix}\begin{bmatrix} x_{1}\\ x_{2}\\ x_{3} \end{bmatrix} = x_{1}\begin{bmatrix} c1_{1}\\ c1_{2}\\ c1_{3} \end{bmatrix} + x_{2}\begin{bmatrix} c2_{1}\\ c2_{2}\\ c2_{3} \end{bmatrix} + x_{3}\begin{bmatrix} c3_{1}\\ c3_{2}\\ c3_{3} \end{bmatrix}$
If you work this out, you'll see this is equivalent to the dot-product technique.

The linear combination repurposing of matrix-vector multiplication makes a lot of sense if you imagine the matrix to be a rotation matrix where each column is an axis that forms a basis/coordinate system, and the vector to be a point in space. If you visualize vector addition using the "head-to-tail" concept, you can almost visualize why multiplication of a rotation matrix and a point works!

## Friday, March 2, 2012

### Computability and the Underpinnings of Computer Science

This week I concluded a course I took this quarter that covered the theory behind computer science. How comprehensive the course's coverage of a whole field's underpinnings was, I'm not qualified to speculate (but my guess would be: not very). With that said, it did cover a lot of interesting material. The course covered:

• The Chomsky hierarchy of formal languages.
• Regular languages/(non-deterministic and deterministic) finite state automata/regular expressions/left-linear and right-linear context-free grammars.
• Pumping lemma for proving non-regularity of languages
• Deterministic push-down automata (and whatever class of languages these are equivalent to)
• Context-free grammars/push-down automata
• Ambiguity
• Griebach Normal Form
• Chomsky Normal Form and membership test Cocke-Younger-Kasami (CYK) algorithm.
• Pumping lemma for proving language to be not context-free.
• An offhand mention of context-sensitive language
• Recursively enumerable languages/Turing Machines.
• Computability (unfortunately we were not able to solve does P=NP and win that new car).
• P time
• NP time
• NP-hard vs. NP-complete vs. NP
• Decidability vs. intractability
Overall I enjoyed the class a lot. I felt the course was not rigorous in its mathematical treatment of a lot of topics, but the professor did provide an alternative practical viewpoint which I appreciated (he was a compiler's guy, so he found grammars to be particularly interesting and snuck in a few question about LL parsers).

What I was disappointed about was the treatment of computability and exactly what the heck it means to be P or NP. I came away feeling only a little less confused about the problem than I was when I started the class.

As always, I went to google. I found a great explanation on stackoverflow (scroll to grom's answer) that was a good balance between layman's terms and formal terms I learned in class.

From my understanding now:
• P problems are a class of problems where there is a known algorithm that can provide an answer/solution to the problem in polynomial time. Polynomial time can be O(1), O(n), or O(n^100000000). It doesn't matter.
• NP problems are a class of problems where we can guarantee there exists a solution, but we can't compute it in polynomial time; it can only be computed in non-deterministic polynomial time. What that means is that there exists a finite, polynomial amount of steps to find the solution but we can't describe a polynomial algorithm to reach it. We have to resort to an exhaustive, "brute-force" search for the solution. This is a problem because some NP problems are intractable which means even though it's possible to find an answer it may take so long it's not even worth it.
• NP-complete problems are problems that are NP but have the special property that all other NP problems can be reduced to them in polynomial time. This "simply" means any NP problem can be transformed into the NP-complete problem using a polynomial time algorithm.
• NP-hard problems are problems that are at least as difficult to solve as the hardest NP problem(s). Note, this class of problems includes problems that are harder than any NP problem (which means they may not even be solvable in non-deterministic polynomial time).
So the big question is: does P=NP, meaning are all P problems equivalent to NP problems? If this is true, that means a lot of hard problems we haven't been able to reasonably compute would be solvable in an amount of time that's worthwhile. An example of this type of problem would be breaking cryptography and security codes which could be a severely good thing or bad thing (depending who you are). And so I guess that's why the Clay Mathematics Institute is willing to give any person who solves whether P=NP \$1 million for their efforts.