Random R Notes - Factors, Rank v. Order, Unsplit

Some R issues I have run into recently...

I split a dataframe, then split it again, and the analysis was taking forever. Something was wrong. After inspection, the sub-split DF had all the factor levels from the original DF! Terrible since there were about 25,000 in one variable. It just took forever, which I don't think it should but hey I gave up on it.

I needed droplevels. You can apply it to the whole DF, removing unused levels from your old DF and then assign it (to a new one or just write over the old one).

your_new_df = droplevels(your_old_df)

(Google cannot handle the less than sign, used for "get" in R instead of =, with either the code tag or the pre tag, it just blows it up. Annoying.)

Then I could run the ordering code on dates. But no: there is, I learned, order and there is rank. There is also sort but I managed to avoid that somehow, so I won't discuss that here.

Note that for rank you need to figure out what to do with ties! (That is, when values are equal, how to rank them exactly.)

There was a really great post about it on Stackoverflow but I can't find it at the moment. This post might help, though.

Or, I made a nice little example! I use R: as the start of the input lines since the greater than symbol and blogger are not friends.

R: the_list = c('A', 'D', 'B', 'C')

R: order(the_list)

[1] 1 3 4 2

R: rank(the_list)

[1] 1 4 2 3


So, you see the two outputs are different.
Order says, put the first element first, then the third element would come next (B), then the fourth element (C), then the second element (D).
Rank says, the first element is first, the second element (D) is the fourth of them all, the third element (B) is the second overall, and the fourth element (C) is the third overall.

Edit: I called sort!

R: sort(the_list)


[1] "A" "B" "C" "D"


That's awesome.

So after that, I wanted to unsplit. But, no, I had added the rank column, so instead of 400 rows I only got 100 (I had 100 df's with 4 rows each). Unsplit does not work well (or at all?) if you add (or subtract?) items. So stackoverflow told me I needed do.call and rbind ("row bind").

rejoined_df = do.call(rbind, splitted_df)


Note that the splitted_df is the result of the split() call, which I'm not showing, so is not actually a DF, I think it's a list of DFs maybe. Or maybe it's not a typical DF. But you can call it directly, and if you use split you should familiarize yourself a bit with the resulting object.

There you have it, some random R notes.

Apple's Audio Outputs and #Fail

If you want to stream sound on your Mac to your stereo jack speakers by the computer and to your Apple Airport Express with its audio, you can't if your sound stream isn't through iTunes and is, say, via the web (like with Pandora in my case). Not at the same time, regardless of what the web says. The stereo jack output doesn't work that way, apparently. (This does work quite well if your sound source is iTunes, note.)

A lot of sites say you can do this generally, and have detailed explanations about how you make a new device through the Audio Midi Setup app. An article at MacWorld hints at the problem but isn't at all clear: "Let’s say you have an Airplay device plus a USB, ethernet, or Firewire audio interface attached to your Mac..." Right. Note the article doesn't mention the stereo out port (the one that matches your little headphones on your iPhone, Android, or oldschool Walkman), because it won't work.

The best I got was stereo on my Mac and one horribly noisy speaker on the Airport Express (through a receiver). The easy solution, although I haven't actually tried it, is to use one of the other sound outs on the Mac (probably the monitor over Firewire) instead of the speakers, but I like my speakers here. The other, better and more expensive solution, is to go with Sonos -- a friend of mine has some Sonos devices and it is an awesome setup, very easy to use and understand, streams from lots of sources, and the app on the iPhone seems great to me.

Update: You can make an "aggregate" device or a "multi-output" device, and neither works for me at all. When I tried using the Airport and the Firewire monitor speakers, which seemed like it should work via all the online help, it didn't -- the best I got was one of the two speakers on the receiver via the Airport (both of which work fine through iTunes or System Preferences when the Airport is the only sound out device) and the monitor speakers had a large amount of noise (a hiss, to be specific). This is unbelievable. Apple is usually really good about making simple things easy, but not here. Weird given how great NeXT hardware and software were at sound.

Update 2: I FOUND A SOLUTION! Via TidBITS, the little app Airfoil from Rogue Amoeba! It works! And it's only $29.00 as I am typing. Really OSX should just do this, but it doesn't, and $29 for a solution that does a lot more than what I am trying to do currently (that is, it gives me future flexibility) is great.

Amazon's Data Fail

So, I believe I have been an Amazon customer for well over ten years -- at least eight since I moved here to NYC, and I ordered stuff from them prior to that.

However, Amazon continues to ask me if I want the college student discount almost every time I check out. This is absurd. And there is no way to toggle it off, I had to get customer support in chat and not even he could do it, he had to bump it up to his supervisor.

Given all the data Amazon has on me, they should know better. It's not just a question of an algorithm, someone -- a team most likely -- was in charge of the implementation here, not just of the algorithm but of the page and its features.

They have the data, and the feature knowledge, yet they failed tremendously anyways. This is not the "Target knew a woman was pregnant before her parents did!" story.

Here is the actual screenshot I took, this is not some random illustrative image I grabbed from somewhere else on the web:

Update, Jan 21: Almost two full weeks later and I got the page again. Amazing and pathetic.