Comments for Ramblings

Comment on Beautiful Emacs (Windows Edition) by Biotrerve

Biotrerve — Fri, 19 Dec 2008 13:18:29 +0000

Hi people

As newly registered user i just want to say hi to everyone else who uses this bbs <:-)

Comment on Simple Guide to Complexity Theory by Bibi

Bibi — Tue, 02 Dec 2008 10:19:11 +0000

The problem i see with the definition of NP-hard you give (apart from its being non-standard) is that the statement “problem P is NP-hard” is a statement of our current knowledge, while with the standard definition, it is the statement of a mathematical truth. Maybe something like “as this notion is more general than NP-completeness, when one says a problem is NP-hard, it’s generally because we don’t know whether it’s in NP or because we know it’s not in NP” would make things clearer.

Comment on Simple Guide to Complexity Theory by ob

ob — Mon, 01 Dec 2008 17:45:29 +0000

Bibi, thanks for the comments. I corrected most of the errors, although if you want to be pedantic, your statement:

‘the “CPU” (head) of the TM only sees one symbol (bit) per cycle. It’s hardly possible to have a smaller bandwidth.’

is incorrect. There is no requirement that a turing machine operate on bits. The only requirement is that the alphabet contains a finite number of symbols. Thus I can define as many symbols as I want to have as much “bandwidth” as I want per step. But it seems saying “infinitely fast CPU and infinite bandwidth” was just confusing people, so I took it out.

Comment on Simple Guide to Complexity Theory by links for 2008-11-26 « My Weblog

links for 2008-11-26 « My Weblog — Thu, 27 Nov 2008 04:05:00 +0000

[...] Simple Guide to Complexity Theory | Ramblings (tags: cs) [...]

Comment on Simple Guide to Complexity Theory by Bibi

Bibi — Wed, 26 Nov 2008 20:30:24 +0000

There are some problems in the definitions.

In Turing machine, infinitely fast CPU does not make sense, and infinite bandwidth is plain wrong: the “CPU” (head) of the TM only sees one symbol (bit) per cycle. It’s hardly possible to have a smaller bandwidth.

In problem, the example given is not a decision problem, it’s a meta-question on the algorithm. This makes no sense. I’d advise not to use sorting as an example, as it is not easy to come up with a convincing decision problem associated with sorting. Shortest path in a graph seems better. Say “instead of returning the shortest path between A and B, tell whether they are at distance at most k”.

Your definition of reduction is wrong. It is generally used to prove that it is NP-hard. (By reducing from another NP-hard problem). When you do that, you don’t assume that P2 is solvable in polynomial time at all. You show that you can use the solution to P2 as a solution from P1 in polynomial time, say TR(n). Then if P2 can be solved in time T2(n), P1 can be solved in time T2(n) + TR(n). So if P2 is in P, P1 too. There’s no proof by contradiction going on here at all.

NP-hard is inexact: there’s no hypothesis that we can’t prove point 1 of NP-completeness. In particular, NP-complete problems are NP-hard.

Folks, once you have read this post, read the wp article before you go and try to impress your friends (especially around reputables universities). You might save yourselves some embarassment.

Comment on Simple Guide to Complexity Theory by ob

ob — Wed, 26 Nov 2008 19:52:52 +0000

Butch, good point. I’ve changed it to P1 and bolded all the P’s and NP’s.

Comment on Simple Guide to Complexity Theory by Butch

Butch — Wed, 26 Nov 2008 19:48:40 +0000

Nice overview. Except that calling a problem P where we already using P as a class of problem can be confusing.

Comment on Simple Guide to Complexity Theory by Mark

Mark — Wed, 26 Nov 2008 18:42:18 +0000

Time complexity T(n) refers to the number of steps it takes to solve a problem, not the amount of real time required. T(n) remains constant, regardless of the speed of the CPU.

Comment on Simple Guide to Complexity Theory by Adam

Adam — Wed, 26 Nov 2008 17:35:31 +0000

I’m not sure where the “infinitely fast CPU” part came in, but that would kind of defeat the point of analyzing the time complexity of the machine. Other than that, good stuff.

Comment on Beautiful Emacs by Cameron

Cameron — Tue, 22 Jul 2008 18:49:16 +0000

Thanks for the helpful explanation. My world is much more tolerable now.

Comment on Hidden Gems in C99 (1) by Ramblings » Blog Archive » Another useless C99 tidbit

Ramblings » Blog Archive » Another useless C99 tidbit — Mon, 14 Apr 2008 06:50:07 +0000

[...] Ramblings Stuff that’s on my mind « Hidden Gems in C99 (1) [...]

Comment on Code Tells You How Revision History Tells You Why by lm

lm — Sat, 05 Apr 2008 03:21:37 +0000

Part of the reason that we don’t have lots of comments in the BitKeeper source base is that maintaining them is hard. We go for the combination of

a) make the code simple (aka, make me understand it and I’m stupid)
b) comment the stuff that is not simple

While I can see the issue of switching gears, once you have a version control system that gives you insight you don’t see it as switching, you see it as digging deeper.

Comment on Beautiful Emacs (Windows Edition) by timjr

timjr — Fri, 01 Feb 2008 23:22:50 +0000

ob, it looks free, but it won’t install unless you have a vs license on the machine

Comment on Beautiful Emacs (Windows Edition) by Jude

Jude — Mon, 21 Jan 2008 16:32:25 +0000

Thanks for the emacs windows tips. I’ve always preferred the font “Bitstream Vera Sans Mono” for programming. It gives me an appealing “free” programmer’s font across all platforms.

http://www.gnome.org/fonts/

Thanks,
-Jude

Comment on Beautiful Emacs (Windows Edition) by ob

ob — Thu, 17 Jan 2008 02:04:16 +0000

As far as I know, the link from the webpage goes to the free Consolas font pack. You don’t need a visual studio license.