Chad Perrin: SOB

2 October 2007

When does ( ?? == 63 ) ?

Filed under: Geek,Humor — apotheon @ 03:56

I stumbled across a capricious little bit of Ruby-fu today. The author of this fun snippet is Jay Phillips, and it looks like this:

def wazup?(a) puts "#{a} dat!" end

When you run that code, it produces the following output:

true dat!

I racked my brains for a bit trying to figure out what’s going on here. I’ve finally figured it out. I started out just reading the method definition — that’s the easy part:

def wazup?(a)
  puts "#{a} dat!"

All it does is take an argument and stuff it into an output string so that when fed foo, it outputs “food dat!” Obviously, it’s getting fed “true” when the method is called.

The next line is where the fun happens, of course. First off, it’s useful to space things out a little bit to show how code would likely be formatted if the intention here wasn’t just to make it look cool:

wazup? !!!?! ? ?1 : !!??

That’s a ternary operator, with !!!?! as the boolean tested argument, ?1 as the “if true” return value, and !!?? as the “if false” return value. This could be rewritten thusly (losing pretty much all the fun of the way it was initially written):

if !!!?!
  wazup? ?1
  wazup? !!??

Those of us who are programmers should all know that leading exclamation points translate to negation of whatever they lead — like a leading exclamation point means “not”. Those of us who are logicians should all know that “not not” is the same as no “not” at all, and “not not not” is the same as a single “not”. Thus, in further translation:

if !?!
  wazup? ?1
  wazup? ??

This is where I hit my first hitch in trying to figure out what was going on. It probably took me all of about five seconds to get this far. It took me a bit longer to get farther. It’s not difficult — it just requires one to know a feature of Ruby that I had not yet encountered (and remembered having encountered).

I played around in irb trying to figure out what was going on. I considered the fact that either ?! or ?? must be getting evaluated as “true” when passed to the wazup? method, and that whichever of them was being sent to wazup? is determined by whether ?! is true or false in a boolean context. The fact there was an exclamation point in front of ?! (making it !?!) means that the usual evaluation of the ternary operator is being reversed, so that if ?! is true it returns ?? instead of ?1, and vice versa.

Feeding each of ?!, ?1, and ?? to irb evaluates to (respectively) 33, 49, and 63:

if !33

Everything in Ruby evaluates to “true” in a boolean context except “false” and “nil”. That means that you could effectively read this, for purposes of passing the result of the ternary operator to the method wazup?, as:

if !true

. . . or:

if false

All that was missing was figuring out why (for instance) ?? evaluated to 63. I figured out that incrementing that digit in ?1 kept increasing the evaluation, so that ?2 == 50, ?3 == 51, and so on. It dawned on me that I was looking at ASCII values, then.

A little googling later, and I discovered what I was beginning to suspect: ? prepended to a single ASCII character evaluates to that character’s ASCII decimal integer value. The mystery was solved. Of course, now it has probably taken longer for me to chronicle my thoughts in the process of figuring that out than it did to think them, but such is life. I feel enlightened.

Not so difficult, but fun nonetheless:


true dat!

Python/Ruby string concatenation discussion

Filed under: Geek — apotheon @ 11:51

The discussion in response to my I learn something new every day — this time about Python and Ruby has gotten a bit lengthy. 25 comments is too many to really address everything properly within further comments — it’s rapidly approaching the point where I might just let others discuss the matter and not get involved. Rather than ignore it, though, I’ve decided to tackle it here, in a new “top level” post.

Jeremy Bowers made a good point about various languages’ implementations of a += operator. This is sort of an implementation detail, though, rather than a common linguistic design choice — and tends to be more suited to a compiler (in Java’s case, a bytecode compiler) than to an interpreter. Python provides the ability to compile source code to bytecode, as I understand it, and Perl does a JIT parse tree compilation every time you run it — to some extent, such an implementation might be appropriate in both cases, though it may also impose some new limitations on how the language itself will be designed in the future. For this reason, even though VM implementations of Ruby are starting to appear, I don’t know that such an implementation of the += operator (or method, in this case) would be a good choice for Ruby. Luckily, Ruby has << as well, and collect-before-joining approach can be otherwise implemented by the programmer if so desired.

Jeremy’s also right, as far as I’m aware, about Python “generally” outperforming Ruby on an algorithm-by-algorithm basis, for current stable releases of Python and Ruby. I certainly didn’t mean to suggest that Ruby outperforms Python consistently — only that certain language design decisions limit the ability to eke greater performance out of certain types of operations, even when the design decision in question doesn’t seem directly related. In this case, it results in a greater performance benefit to Ruby for a particular type of operation than for Python, for a roughly comparable algorithm.

The heart of his final statement:

If this post says anything, it speaks to the dangers of how high-level languages can obscure the underlying algorithms, and therefore obscure the performance implications of them.

It’s true. On the other hand, unless you’re a core maintainer for one of these languages, the part that’s of most interest to you is likely to be how this affects the way you program. Since I’m not a core maintainer for any programming language at present, and have only ever directly contributed at all to a language by doing some scut-work for a C compiler project completely unrelated to these languages in particular, my focus was on how the algorithms used in implementing these languages affects the algorithms I’ll use when writing code.

A lot of attention was given to the choice of algorithm and how idiomatic it is to Python, of course. For instance, someone using the name “nirs” said:

The idiomatic Python code is:

s = []
for line in lines: s.append(line)
  s = ''.join(s)

Similarly, Paddy3118 said:

In Python one is taught not to concatenate strings using += but to use the join method instead.

Going back to Jeremy for a moment, he made the salient point that needs to be made here:

When you use two different algorithms in two different languages, all bets are always off; language differences are generally swamped by the differences in algorithms.

To be fair, I did ask (toward the end of my original post about string concatenation in Python and Ruby) for better ways to improve string concatenation performance in Python. On the other hand, several responses seemed to be offering counterarguments rather than improvements. These two in particular are doing something completely different from what I originally addressed — string concatenation. Instead, they say that you can get similar results with better performance by doing something else. All this means, in the end, is that when you tell your doctor “It hurts when I do this,” he should tell you “Don’t do that.” Sometimes, it is unfortunately the fact that it would be nice to be able to do that.

Of course, I just picked two names out of a hat. The same point was made as well by Simon Willison, metapundit, JamesH, Brandon Corfman, DDP, Vincent Foley, someone identified as “Anonymous”, and somewhat rudely by someone using the name Masklinn.

Someone else identified only as “Anonymous” posted the words “Python does have mutable strings. They’re called ‘lists’,” which points out that the same problem might be solved differently (as have the above-noted users of lists to avoid string concatenation), while simultaneously making a strictly incorrect statement.

I was somewhat impressed with Spacebat‘s response, in that it both suggested three different approaches to speeding up the operation in Python and discussed the downsides of each, rather than simply pretending that if you want a faster program you shouldn’t want the output you actually set out to get in the first place. It shouldn’t seem impressive that someone treated the matter reasonably, but it does.

Mark Thomas, meanwhile, pointed out that what I posted wasn’t idiomatic Ruby either — something the people complaining about the lack of Python “smarts” in the original example probably never thought to consider. I’m not really sure what qualifies for “idiomatic” Python style in this case: Mark suggests that the original example for Python was idiomatic, and I’m inclined to agree that it’s idiomatic Python for string concatenation operations, distinct from idiomatic “avoid string concatenation operations” Python. Mark’s example of idiomatic Ruby style is on the money, but would not have served as an effective comparison example to demonstrate the differences in the two languages’ handling of string concatenation (going back to the “different algorithms make a bigger difference than different languages” idea, again). Interesting (to me, at least) is the fact that, despite being visually quite different from the original example and leveraging the beautification and iteration capabilities of Ruby to positive effect, the execution optimizing piece of the code is exactly the same — namely, use of << in place of +=. Its execution performance is equivalent to the original “optimized” version as well.

Finally, there were several references to the idea of using an IO library call (by Chadwick Morris, Smel, Troy Kruthoff, one of Spacebat’s suggestions, and someone called Tom). While that’s useful to anyone thinking about writing code that behaves similarly, it also obscures the issue somewhat — after all, once we start doing library calls like that to make up for performance bottlenecks we lose any ability to compare the features of the core language. There are, I’m sure, at least a dozen Ruby libraries out there I could similarly use to change the performance characteristics of my examples.

Additional notes:

  1. Justin James made some interesting comments as well, but I think they deserve their own response.
  2. In the future, people posting code here might want to take note of the note above the comment text entry box that reads “Markdown: You can also format text using Markdown syntax.” In particular, indenting every line of code by four spaces in addition to any other spaces your code needs for indentation should provide the code formatting you need.
  3. There’s also a note below that text box, just above the “Preview” button, that reads “Please note: Comment moderation is enabled and may delay your comment. There is no need to resubmit your comment.” I’m afraid I didn’t get back to checking on this post’s comment activity for a couple days, and as such a bunch of people ended up saying roughly redundant things. This is not their fault, but the fault of the necessity of comment moderation and my own slowness to get around to dealing with comment moderation.

All original content Copyright Chad Perrin: Distributed under the terms of the Open Works License