Houston secrets

The Engines of our Ingenuity episode Secret City discusses little-known city features. I was particularly interested in the Houston locations the show mentioned. I’ve been through the downtown tunnels John Leinhard mentions, but I was not familiar with the 1940 Air Terminal Museum or the Gable Street Power Station.

Here are three Houston-area attractions that I find many Houstonians don’t know about.

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Houston Deco
Robert’s Rule of Order and Galveston flooding

Rosenbrock’s banana function

Rosenbrock’s banana function is a famous test case for optimization software. It’s called the banana function because of its curved contours.

The definition of the function is

f(x, y) = (1 - x)^2 + 100(y - x^2)^2

The function has a global minimum at (1, 1). If an optimization method starts at the point (-1.2, 1), it has to find its way to the other side of a flat, curved valley to find the optimal point.

Rosenbrock’s banana function is just one of the canonical test functions in the paper “Testing Unconstrained Optimization Software” by Moré, Garbow, and Hillstrom in ACM Transactions on Mathematical Software Vol. 7, No. 1, March 1981, pp 17-41. The starting point (-1.2, 1) mentioned above comes from the starting point required in that paper.

I mentioned a while back that I was trying out Python alternatives for tasks I have done in Mathematica. I tried making contour plots with Python using matplotlib. This was challenging to figure out. The plots did not look very good, though I imagine I could have made satisfactory plots if I had explored the options. Instead, I fired up Mathematica and produced the plot above easily using the following code.

Banana[x_, y_] := (1 - x)^2 + 100 (y - x*x)^2
ContourPlot[Banana[x, y], {x, -1.5, 1.5}, {y, 0.7, 2.0}]
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Miscellaneous Emacs adventures

I recently found out there’s an Emacs command M-x woman that’s a pun on “w/o man”, i.e. a way to read online help without using the usual man command.

* * *

I tried to edit a 1.2 GB text file with Emacs the other day. I got an error saying that Emacs has a 500 MB file size limit on 32-bit systems. This was on a 32-bit Windows XP machine, so the warning was reasonable. I appreciated that it promptly said it could not open the file.

I tried again on my 64-bit Windows 7 machine and got the same message; I believe I’m running a 32-bit version of Emacs on my 64-bit PC.

I also tried opening the file with Emacs on a 64-bit Red Hat Enterprise Linux box with 16 GB of memory. This time I did not get a file size message. Instead, Emacs crashed.

The file opened quickly on Windows, even my 32-bit XP box, using Notepad++.

* * *

When I set up Emacs on Windows, I created an “Open with Emacs” context menu following the instructions here. That has worked well except for two problems.

  1. Every file is opened in a new instance of Emacs.
  2. It stopped working on one of my computers for reasons unknown.

I’ve been meaning to solve the first problem, and when the second happened I decided it was time. According to the Emacs documentation,

The recommended way to associate files is to associate them with
emacsclientw.exe. In order for this to work when Emacs is not yet started, you will also need to set the environment variable ALTERNATE_EDITOR to runemacs.exe. To open files in a running instance of Emacs, you will need to add the following to your init file: (server-start)

I had to tinker with that a little to make it work. I know I had to add the path to my Emacs bin directory to my PATH environment variable but I don’t remember what else I did.

The key thing to note is that you should associate files with emacsclientw.exe rather than with runemacs.exe. I used the instructions in this article to remove the association with the latter. Thanks to Mark for pointing out the article.

(The Windows “Open with” feature remains a mystery. The article above explains how to remove a program from the “Open with” list, and the directions worked for removing runemacs.exe. But they did not work for removing other programs. I’m also unclear how you add something to the “Open with” list. If you right-click on a file and select “Open with -> Choose program …” the program you select may appear in the “Open with” list next time. Or not. This did add emacsclientw.exe to the list.)

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Remapping Caps Lock

I remap the Caps Lock key to be a control key on every computer that I use regularly. Here’s why.

  1. Caps Lock is a nearly worthless key taking up valuable real estate.
  2. I’m more likely to use Caps Lock accidentally than intentionally.
  3. I use the Control key far more often than the Caps Lock key.
  4. The Caps Lock key is in a consistent position on all keyboards but the left Control key isn’t.

Some people swap the Caps Lock and and left Control keys. I prefer to just disable the left Control key. On desktop keyboards, I map the useless Scroll Lock key to act as a Caps Lock key for those rare instances when I actually want to type a long sequence of capital letters.

KeyTweak is a convenient program for remapping a Windows keyboard.

On Linux, you can use the xmodmap utility.The following commands disable the left Control key and turn the Caps Lock key into a control key.

xmodmap -e "remove Lock = Caps_Lock"
xmodmap -e "remove Control = Control_L"
xmodmap -e "keysym Caps_Lock = Control_L"
xmodmap -e "add Control = Control_L"

See Dave Richeson’s comment below for Mac instructions.

Related post41 dumb things to check

Sine approximation for small angles

For small angles, sin(θ) is approximately θ. This post takes a close look at this familiar approximation.

I was confused when I first heard that sin(θ) ≈ θ for small θ. My thought was “Of course they’re approximately equal. All small numbers are approximately equal to each other.” I also was confused by the footnote “The angle θ must be measured in radians.” If the approximation is just saying that small angles have small sines, it doesn’t matter whether you measure in radians or degrees. I missed the point, as I imagine most students do. I run into people who have completed math or science degrees without understanding this tidbit from their freshman year.

The point is that the error in the approximation sin(θ) ≈ θ is small, even relative to the size of θ. If θ is small, the error in the approximation sin(θ) ≈ θ is really, really small. I use the phrase “really, really” in a precise sense; I’m not just talking like an eight-year-old child. If θ is small, θ2 is really small and θ3 is really, really small.

For small values of θ, say θ < 1, the difference between sin(θ) and θ is very nearly θ3/6 and so the absolute error is really, really small. The relative error is about θ2/6. If the absolute error is really, really small, the relative error is really small.

The approximation sin(θ) ≈ θ comes from a Taylor series, but there’s more going on than that. For example, the approximation log(1 + x) ≈ x also comes from a Taylor series, but that approximation is not nearly as accurate.

The Taylor series for log(1 + x) is

xx2/2 + x3/3 – …

This means that for small x, log(1 + x) is approximately x, and the error in the approximation is roughly x2/2. So if x is small, the absolute error in the approximation is really small and the relative error is small. Notice this is one less “really” than the analogous statement about sines.

The reason the sine approximation is one “really” better than the log approximation is that sine is an odd function, so its Taylor series has only odd terms. The error is approximately the first term left out. The series for sin(θ) is

θ – θ3 / 3! + θ5/5! – …

so sin(θ) – θ is on the order of θ3, not just θ2.

We can say even more. Not only is the error in sin(θ) ≈ θ approximately θ3/6, it is in fact bounded by θ3/6 for small, positive θ. This comes from the alternating series theorem. When you make an approximation from an alternating Taylor series, the error is bounded by the first term you leave out. The argument has to be small enough that the terms of the series decrease in absolute value. For example, if you  stick  θ = 1 into the series for sine, each term of the series is smaller than the previous one. But if you stick in θ = 2 then the terms get larger before they get smaller. So the precise meaning of “small enough” is small enough that the terms of the alternating series decrease in absolute value.

The series for log(1 + x) also alternates, so the error estimate is also an upper bound on the error in that case. Not all series are alternating series. But when we do have an alternating series, we get a bonus as far as being able to control the error in approximations.

It’s also true that for small θ, cos(θ) is approximately 1. At first this may seem analogous to saying that for small θ, sin(θ) is approximately 0. While both are true, the former is much better.

The series for cosine is

1 – θ2/2! + θ4/4! – …

and so the error in approximating cos(θ) with 1 is approximately θ2/2. In fact, since the series alternates, the error is bound by θ2/2, if θ is small enough. And in this case the relative error is approximately equal to the absolute error. This says that for small θ, the approximation cos(θ) ≈ 1 is really, really good. By contrast, saying that sin(θ) ≈ 0 for small θ is a bad approximation: the absolute error is approximately θ and the relative error is 100%. Rounding small values of θ down to 0 produces a very good approximation in one context and a poor approximation in another context.

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Windows XP and Ubuntu start-up music

I just realized that the start-up music for Ubuntu is a variation on the start-up music for Windows XP. (You can hear the Ubuntu theme in this video at around 0:10 [Update: video has been removed]. The Windows XP theme is in this video at around 1:16.) If you don’t hear the similarity, concentrate on the rhythm rather than the melody.

The Ubuntu music style is African, like the word ubuntu. It was influenced by Windows, like the Ubuntu user interface, but it’s a new composition.

I’ve been running Ubuntu on a virtual machine. The sound quality was so bad that I never clearly heard Ubuntu start up. But I recently installed Ubuntu on a physical machine and heard the start-up music clearly for the first time.

What does this code do?

At the SciPy 2010 conference, a speaker showed several short code samples and asked us what each sample did. The samples were clearly written, but we had no comments to provide context. This was the last sample.

def what( x, n ):
    if n < 0:         
        n = -n         
        x = 1.0 / x     
    z = 1.0     
    while n > 0:
        if n % 2 == 1:
            z *= x
        x *= x
        n /= 2
    return z

The quiz was at the end of the day and I was tired. I couldn’t tell what the code does. Then I found out to my chagrin that the sample above implements an algorithm I know well. I’ve written the same code and I’ve even blogged about here.

This exercise changed my opinion of “self-documenting” code. Without some contextual clue, it is hard to understand the purpose of even a small piece of code.

Meaningful variable and function names would have helped, but a tiny comment might have helped even more. Not some redundant comment like explaining that the line x = 1.0 / x takes a reciprocal, but a comment explaining the problem the code is trying to solve.

For another example, what do you think this code does?

uint what()
    m_z = 36969 * (m_z & 65535) + (m_z >> 16);
    m_w = 18000 * (m_w & 65535) + (m_w >> 16);
    return (m_z << 16) + (m_w & 65535);

It’s clear enough what the code does at a low level—it’s just a few operations—but it’s not at all clear what it’s for.

Try to figure out what the code samples do before reading further. But if you give up, the first example is described here and the second example comes from here.

In an ordinary face-to-face conversation, more information is conveyed non-verbally than verbally. We may think that our literal words are most important, but so much is conveyed by voice inflection, facial expression, posture, etc. Something similar is going on with source code. When we read a piece of source code, we typically bring a huge amount of implicit knowledge with us.

Suppose a coworker Sam asks you to look at his code. The fact that the question came up at work provides a large amount of context; this isn’t just a random code fragment on the web. More specifically, you know what kinds of projects Sam works on. You know why Sam wants you to look at the code. He may be showing you something he’s proud of or he may be asking for help finding a bug. You know a lot about his code before you see it.

Now suppose you’re a contractor. Sam was hit by a bus and you’ve been asked to work on his projects until he gets out of the hospital. You may complain to his office mate that Sam’s code is an awful mess, but she can’t understand what you’re talking about. She thinks his code is perfectly clear.

Now suppose you’re a contractor on the opposite side of the world from Sam. You have even less context than if you were in his office talking to his office mate. After a great deal of agony, you send your contribution back to Sam’s company. You comment your code beautifully, but Sam’s colleagues complain that your code is poorly written and that you didn’t solve the right problem.

Institutional memory is more valuable than source code comments. It costs a great deal to replace a programmer, even one who leaves behind well-commented code.

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e-book formats

ManyBooks.net has nearly 28,000 free e-books available for download. Each book is available in 23 different formats.

I wondered what software they used to create so many formats and how popular each format is. The site answers both these questions. This page lists the software used and this page lists the popularity statistics.

PDF is not as dominant as I would have expected. It is the most popular format, but it only accounts for 20% of downloads. Here’s a bar chart of the downloads per format. Click on the graph for a larger version.

See also Wikipedia’s comparison of e-book formats.

Stupidity scales

I’m fed up with conversations that end something like this.

Yes, that would be the smart thing to do, but it won’t scale. The stupid approach is better because it scales.

We can’t treat people like people because that doesn’t scale well.

We can’t use common sense because it doesn’t fit on a form.

We can’t use a simple approach to solve the problem in front of us unless the same approach would also work on a problem 100x larger that we may never have.

If the smart thing to do doesn’t scale, maybe we shouldn’t scale.

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Covariance and contravariance in math and CS

I heard the terms “covariance” and “contravariance” used in math long before I heard them used in object oriented programming.  I was curious whether there was any connection between the two. To my surprise, they’re very similar. In fact, you could formalize the OOP use of the terms so that they’re not just analogous but actually special cases of the mathematical terms.

When I started writing this post, I intended to explain covariance and contravariance. However, the post became longer and more technical than I like to write here. Instead, I’ll just announce that a connection exists and give references for those who want to read further.

Chris Burrows describes covariance and contravariance in object oriented programming in his article New C# Features in the .NET Framework 4.

The terms covariant and contravariant were defined in category theory before computer scientists applied the terms to object oriented programming. Wikipedia has a short, readable introduction to category theory, including covariant and contravariant functors. See also A Categorical Manifesto (PostScript file).

Computer scientists have been interested in category theory for some time, so it’s not too surprising that category theory terms would filter down into practical programming. The real surprise was hearing category terminology used outside of math. It was like the feeling you get when you run into a coworker at a family reunion or a neighbor at a restaurant in another city.

Update (3 Jan 2011): See also Liskov Substitution Principle is Contravariance

Update (28 Feb 2013): See also how covariance and contravariance are used in the opposite sense with vector fields.

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Origin of Mythical Man-Month

The August 2010 issue of Wired has an interview with Fred Brooks. The interviewer, Kevin Kelly, asks Brooks why he wrote his popular book The Mythical Man-Month. Here’s Brooks’ response.

As I was leaving IBM, Thomas Watson, Jr. asked me, “You’ve run the hardware part of the IBM 360, and you’re run the software part; what’s the difference between running the two?” I told him that was too hard a question for an instant answer but that I would think about it. My answer was The Mythical Man-Month.

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I was looking at a word-of-the-day calendar the other day and the word was peripeteia. I didn’t remember what the word meant, but I remembered that I heard someone use it in a presentation.

Eventually I remembered that Mike Rowe had used peripeteia in his TED talk. The word means a sudden or unexpected reversal of circumstances. Rowe begins his talk by describing a peripeteia moment he had while castrating sheep. He uses this story to illustrate how our ideas about work can be very wrong.

Three surprises with bc

bc is a quirky but useful calculator. It is a standard Unix utility and is also available for Windows.

One nice feature of bc is that you can set the parameter scale to indicate the desired precision. For example, if you set scale=100, all calculations will be carried out to 100 decimal places.

The first surprise is that the default value of scale is 0. So unless you change the default option, 1/2 will return 0. This is not because it is doing integer division: 1.0/2.0 also returns 0. bc is computing 1/2 as 0.5 and displaying the default number of decimal places, i.e. none! Note also that bc doesn’t round results; it truncates.

bc has one option: -l. This option loads the math library and sets the default value of scale to 20. I always fire up bc -l rather than just bc.

The second surprise with bc is that its math library only has five elementary functions. However, you can do a lot with these five functions if you know a few identities.

The sine and cosine of x are computed by s(x) and c(x) respectively. Angles are measured in radians. There is no tangent function in bc. If you want the tangent of x, compute s(x)/c(x). (See here for an explanation of how to compute other trigonometric functions.) As minimal as bc is, it did make a minor concession to convenience: it could have been more minimal by insisting you use sin(π/2 – x) to compute a cosine.

The only inverse trigonometric function is a(x) for arctangent. This function can be bootsrapped to compute other inverse functions via these identities:

arcsin(x) = arctan(x / sqrt(1 – x2))
arccos(x) = arctan(sqrt(1 – x2 )/ x)
arccot(x) = π/2 – arctan(x)
arcsec(x) = arctan(sqrt(x2 – 1))
arccsc(x) = arctan(1/sqrt(x2 – 1))

The functions l(x) and e(x) compute (natural) logarithm and exponential respectively. bc has a power operator ^ but it can only be used for integer powers. So you could compute the fourth power of x with x^4 but you cannot compute the fourth root of x with x^0.25. To compute xy for a floating point value y, use e(l(x)*y). Also, you can use the identity logb(x) = log(x) / log(b) to find logarithms to other bases. For example, you could compute the log base 2 of x using l(x)/l(2).

Not only is bc surprising for the functions it does not contain, such as no tangent function, it is also surprising for what it does contain. The third surprise is that in addition to its five elementary functions, the bc math library has a function j(n,x) t0 compute the nth Bessel function of x where n is an integer. (You can pass in a floating point value of n but bc will lop off the fractional part.)

I don’t know the history of bc, but it seems someone must have needed Bessel functions and convinced the author to add them. Without j, the library consists entirely of elementary functions of one argument and the names of the functions spell out “scale.” The function j breaks this pattern.

If I could include one advanced function in a calculator, it would be the gamma function, not Bessel functions. (Actually, the logarithm of the gamma function is more useful than the gamma function itself, as I explain here.) Bessel functions are important in applications but I would expect more demand for the gamma function.

Update: Right after I posted this, I got an email saying bc -l was following me on Twitter. Since when do Unix commands have Twitter accounts? Well,  Hilary Mason has created a Twitter account bc_l that will run bc -l on anything you send it via DM.

Update (November 14, 2011): The account bc_l is currently not responding to requests. Hilary says that the account stopped working when Twitter changed the OAuth protocol. She says she intends to repair it soon.

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