Installing windows

Episode 34 will be delayed again — sorry! — because once again the time I had set aside for writing this weekend got consumed by a real-world task that could not wait. (I will try for Thursday of this week.)

Some friends who are moving had a handyman failure; as is often the case when renovating a house to be sold, they have a set of build dependencies that required this window to be replaced in a hurry in order to not slip the schedule for other renovations, so I volunteered to take care of it.


Living in a 112 year old house myself, I am used to doing archaeological investigations of the strange decisions made by previous owners. This window, though obviously an old single-paned window, did not look like it was original to the 120-year-old house. It was the wrong size for the rough opening; the hinges looked more modern than turn-of-the-century, and so on.

Sure enough, when disassembled there was a gap behind the trim that was insulated with crumpled newspapers from 1957. Oddly enough they were Pittsburgh newspapers from different days; perhaps the owners of the house in 1957 moved from Pittsburgh, replaced a window, and insulated the gaps with the packing paper they moved with? It’s a mystery.

Having zero haircuts since quarantine began has done wonders for my hair.

New window in and trimmed — obviously the paint will need to be redone but that’s why the window had to go in before the painters arrived.

And the interior needs a little more drywalling and priming before it is ready for painting, but it is 1000000x better than before at least.

The neighbours in the blue house apparently asked my friends for my contact information as they also have a window that needs replacing. I am quite chuffed. I had my friends pass along that I only do windows as a favour, but I would be happy to design them a programming language for hire should they need one of those.

Next time: Gosper’s algorithm, finally!


Implementing a full fence

Episode 34 of my ongoing series will be slightly delayed because I spent the time on the weekend I normally spend writing instead rebuilding one of my backyard fences.

I forgot to take a before picture, but believe me, it was ruinous when I bought the place in 1997 and to the point in 2020 where it was actually falling to pieces in a stiff breeze.

I replaced it with an identical design and materials:

…and divided and re-homed some sixteen dozen irises in the process.

It’s nice implementing something that requires no typing every now and then.


Approximate results may vary

Part 33 of my ongoing series is coming but I did not get all the code written that I wanted to this week, so it will be delayed. In the meanwhile:

Living in Canada as a child, of course I grew up learning the metric system (with some familiarity with the imperial and US systems of course). If you want to know how many milliliters of liquid a cubic box holds, you just compute the volume of the box in cubic centimeters and you’re done, because a milliliter is by definition the same volume as a cubic centimeter.

Despite having lived in Seattle for over 25 years, I still sometimes do not have an intuitive sense of conversions of US units; for a particular project I knew that I needed an amount of liquid equal to about the volume of a two-inch cube, but the bottle was measured in fluid ounces. Fortunately we have this operation in every browser:

Thanks Bing for those important eight digits after the decimal place there, manufactured from the zero digits after the decimal place of the input. For context, that extra 0043 on the end represents a volume equivalent to roughly the size of a dozen specs of microscopic dust.

But the punchline that made me LOL was “for an approximate result, divide the volume by 1.8046875”, because of course when I am quickly approximating the volume of eight cubic inches in fluid ounces, the natural operation that immediately comes to mind is divide by 1.8046875.

I have some questions.

That was Bing; what does Google do with the same query?

OK, that’s an improvement in that the amount of precision is still unnecessary, but not outright absurd. But the other problems are all the same.

Why division? Maybe it is just me, but I find division by uneven quantities significantly harder mental arithmetic than multiplication; assuming we want the over-precision, would it not be better to say “for an approximate result, multiply the volume by 0.5541125″? 

Why say “to approximate…” and then give an absurd amount of precision in the conversion factor? Approximation is by definition about deliberately not computing an over-precise value.

Surely the right way to say this is “for an approximate result, multiply the volume by 5/9” or even better, “divide by two“. When I saw “divide by 1.8046875” the first thing I thought after “wow that’s so over-precise” was “1.8 is 18/10 is 9/5, so multiply by 5/9“.

I’m going to get there eventually; software can shorten that journey. And I’m going to remember that 5/9ths of a cubic inch is a fluid ounce much more easily than I remember to divide by 1.8046875.

Once you start to see this pattern of over-precision in conversions, it’s like the FedEx arrow: you can’t stop seeing it. Let’s ask the browser how much does an American robin weigh?

64.8 grams. An underweight robin is not 64.5 grams, and not 65.0 grams but 64.8 grams.

To be fair, it looks like this over-precision was the fault of a human author (and their editor) not thinking clearly about how to communicate the fact at hand, rather than bad software this time; if you’re converting “two and a third ounces” to grams it would be perfectly reasonable to round to 65, or even 60. (That third of an ounce is already suspect; surely “two to three ounces” is just fine.) Most odd though is that the computations are not even correct! 2 and 1/3rd ounces is 66.15 grams, and 3 ounces is 85.05 grams, making it rather mysterious where the extra few hundred milligrams went.

I was wondering how many earthworms a robin would have to eat to make up a discrepancy of 0.2 grams. A largish earthworm has got to weigh on the order of a gram, right?

Wow! (For my metric readers out there: 1.5 pounds is 680.388555 grams according to Bing.)

Again: what the heck, Bing? I did not ask for “world’s largest earthworm” or “unusually large earthworms” or even “Australian earthworms”. You know where I live, Bing. (And Google search does no better.)

For some reason I am reminded of Janelle Shane’s “AI Weirdness” tweets; the ones about animal facts are particularly entertaining. These earthworm facts at least have the benefit of being both interesting and correct, but this is hardly the useful result about normal garden-variety annelids that I wanted.

I am also reminded of my favourite animal fact: the hippopotamus can jump higher than a house. It sounds impressive until you remember that houses can’t jump at all.

Obviously all these issues are silly and unimportant, which is why I chose them for my fun-for-Friday blog. And the fact that I can type “8 cubic inches in oz” into my browser or say “how much does a robin weigh?” into a smart speaker and instantly get the answer is already a user interface triumph; I don’t want to minimize that great work. But there is still work to do! Unwarranted extra precision is certainly not the worst sort of fallacious reasoning we see on the internet, but it is one of the most easily mitigated by human-focused software design. I’d love to see improvements to these search functions that show even more attention to what the human user really needs.

Socially distant abbreviated summer vacation

Normally this time of year I would be visiting friends and family in Canada, but obviously that’s impossible right now. Instead we took a long weekend at a rental on Bainbridge Island and strolled around some parks in a socially distant manner instead.

The rental had a balcony at tree level, so at least I got to indulge in my vacation pastime of taking pictures of birds. There were a lot of common feeder birds in the neighbourhood — house finches, house sparrows, a few different species of chickadees, that sort of thing — and I got a few shots in. This little female Rufous hummingbirb posed for glamour shots in a birch tree all day long:

(Click on any picture for a larger image)

She was vexed that a female Anna’s hummingbird kept horning in on her territory, hence her ticked-off expression there.

This super fuzzy juvenile black-capped chickadee also spent days posing:

despite being harassed all day long by the noisy gang of teenager Stellar jays on the rooftop next door.

And of course once again I was completely incapable of taking a non-blurry picture of a belted kingfisher despite many attempts.

Sigh. Better luck next time.


I went out to Shilshole Bay Marina Tuesday night to get a few photos of the comet; it is quite spectacular! If you’re going stargazing this week, bring binoculars, look to the northwest about an hour after sunset, below the Big Dipper.

And don’t forget to turn around and look the other way because Jupiter and Saturn are in opposition and very bright, just left of Sagittarius.

Click on photos for a higher resolution version.

Report thy feat unto Lord British

Welcome to yet another working-from-home pandemic episode of Fun For Friday Fabulous Adventures. Over the past while I’ve gradually been looking for music, movies and games I enjoyed as a teenager and seeing how they hold up. So I am pleased to announce that it took me only 35 years to finally complete a game I started on my Commodore 64 in 1985:


If you are unfamiliar with Ultima IV: it was an incredibly influential game that moved the state of the art forwards in “swords and sorcery” style games. The world was, for its time, enormously detailed, full of characters you could interact with who gave you clues as to the various puzzles you had to solve to win the game. Those puzzles largely took the form of “fetch quests” — you need eight magic stones hidden in six dungeons, and the mantras of eight shrines and the Bell of Courage, and so on; there’s quests built upon quests upon quests.

But the most interesting thing about Ultima IV was that it was the first game I played, and maybe the first game ever, to not just build in a moral system, but to make it the object of the game. The central design element is to not just reward, but require the player to be just, honest, compassionate, honorable and humble within the context of the game, and the quest is ultimately for wisdom. It was an astonishing achievement in a world where the object of all previous fantasy games was “kill the big bad guy”.

Other aspects of the game design are interesting by contrasting them to game design today. The game assumes, for example, that you have read the detailed (and well-produced) manuals that came in the box. It assumes that you’re reading the (cloth!) map and deciphering the runic lettering on it. This would not typically fly today; we expect games to introduce players gradually, to have a tutorial mode built into the game itself, and to not rely on anything exogenous to the software.

It’s a hard game to win without help. I remembered a great many details from my teenage years, and I knew that I could just look up all the mantras and locations of secret items and whatnot on the internet, but I decided to play it more or less like I did not have any of that information ahead of time. I did read some walkthroughs of the harder dungeon levels, because figuring out where all the secret door triggers are is tedious.

Ultima IV is available for free from GOG, and the rest of the series is cheap, so I encourage you to give it a shot if you want some of the retro gaming experience that informed modern fantasy game tropes.

Spoiler ahead:

If you do, there’s a bit of a cheat that I wish I’d remembered: once you are an Avatar and have the Mystic Robes, you can sell them, use the proceeds to buy magic wands and bows, and then find the robes again in the same place you found it the first time. That takes out a lot of the tedium of trying to get enough gold to afford magic ranged weapons. And don’t waste your money on magic armor; just get the Mystic Robes sooner rather than later!

The game ends with an exhortation to contact game author Lord British, also known as Richard Garriott, who amazingly is still making fantasy games that come with cloth maps in the box. I’ll have to order a copy and see how the series has evolved in the last 35 years.

Back in the 1980s if you did contact Lord British after winning an Ultima game, he would send you an autographed certificate of completion, but sadly I suspect I will not be getting one.

2020-04-06 (4).png

I also had Ultima V for my Amiga but I never got very far in it. Perhaps I’ll give it a shot next.


New grad vs senior dev

A student who I used to tutor in CS occasionally sent me a meme yesterday which showed “NEW GRAD vs SENIOR DEVELOPER”; the new grad is all caps yelling


and the senior developer responds

Ha ha, nested for loops go brrrrrrrrrrrr…

OK, that’s silly and juvenile, but… oh no, I feel a flashback coming on.

It is 1994 and I am a second-year CS student at my first internship at Microsoft on the Visual Basic compiler team, reading the source code for InStr for the first time. InStr is the function in Visual Basic that takes two strings, call them source and query, and tells you the index at which query first appears as a substring of source, and the implementation is naive-brute-force.

I am shocked to learn this! Shocked, I tell you!

Let me digress slightly here and say what the naive brute force algorithm is for this problem.

Aside: To keep it simple we’ll ignore all the difficulties inherent in this problem entailed by the fact that VB was the first Microsoft product where one version worked everywhere in the world on every version of Windows no matter how Windows was localized; systems that used Chinese DBCS character encodings ran the same VB binary as systems that used European code pages, and we had to support all these encodings plus Unicode UTF-16. As you might imagine, the string code was a bit of a mess. (And cleaning it up in VBScript was one of my first jobs as an FTE in 1996!)

Today for simplicity we’ll just assume we have a flat, zero-terminated array of chars, one character per char as was originally intended.

The extremely naive algorithm for finding a string in another goes something like this pseudo-C algorithm:

bool starts(char *source, char *query)
  int i = 0;
  while (query[i] != '\0')
    if (source[i] != query[i])
      return false;
    i = i + 1;
  return true;
int find(char *source, char *query)
  int i = 0;
  while (source[i] != '\0')
    if (starts(source + i, query))
      return i;
    i = i + 1;
  return -1;  

The attentive reader will note that this is the aforementioned nested for loop; I’ve just extracted the nested loop into its own helper method. The extremely attentive reader will have already noticed that I wrote a few bugs into the algorithm above; what are they?

Of course there are many nano-optimizations one can perform on this algorithm if you know a few C tips and tricks; again, we’ll ignore those. It’s the algorithmic complexity I’m interested in here.

The action of the algorithm is straightforward. If we want to know if query “banana” is inside source “apple banana orange” then we ask:

  • does “apple banana orange” start with “banana”? No.
  • does “pple banana orange” start with “banana”? No.
  • does “ple banana orange” start with “banana”? No.
  • does “banana orange” start with “banana”? Yes! We’re done.

It might not be clear why the naive algorithm is bad. The key is to think about what the worst case is. The worst case would have to be one where there is no match, because that means we have to check the most possible substrings. Of the no-match cases, what are the worst ones? The ones where starts does the most work to return false.  For example, suppose source is “aaaaaaaaaaaaaaaaaaaa” — twenty characters — and query is “aaaab”. What does the naive algorithm do?

  • Does “aaaaaaaaaaaaaaaaaaaa” start with “aaaab”? No, but it takes five comparisons to determine that.
  • Does “aaaaaaaaaaaaaaaaaaa” start with “aaaab”? No, but it takes five comparisons to determine that.
  • … and so on.

In the majority of attempts it takes us the maximum number of comparisons to determine that the source substring does not start with the query. The naive algorithm’s worst case is O(n*m) where n is the length of source and m is the length of the query.

There are a lot of obvious ways to make minor improvements to the extremely naive version above, and in fact the implementation in VB was slightly better. The implementation in VB was basically this:

char* skipto(char *source, char c)
  char *result = source;
  while (*result != '\0' && *result != c)
    result = result + 1;
  return result;
int find(char *source, char *query)
  char *current = skipto(source, query[0]);
  while (*current != '\0;)
    if (starts(current, query))
      return current - source;
    current = skipto(current + 1, query[0]);
  return -1;

(WOW, EVEN MORE BUGS! Can you spot them? It’s maybe easier this time.)

This is more complicated but not actually better algorithmically; all we’ve done is moved the initial check in starts that checks for equality of the first letters into its own helper method. In fact, what the heck, this code looks worse. It does more work and is more complicated. What’s going on here? We’ll come back to this.

As I said, I was a second year CS student and (no surprise) a bit of a keener; I had read ahead and knew that there were string finding algorithms that are considerably better than O(n*m). The basic strategy of these better algorithms is to do some preprocessing of the strings to look for interesting features that allow you to “skip over” regions of the source string that you know cannot possibly contain the query string.

This is a heavily-studied problem because, first off, obviously it is a “foundational” problem; finding substrings is useful in many other algorithms, and second, because we genuinely do have extremely difficult problems to solve in this space. “Find this DNA fragment inside this genome”, for example, involves strings that may be billions of characters long with lots of partial matches.

I’m not going to go into the various different algorithms that are available to solve this problem and their many pros and cons; you can read about them on Wikipedia if you’re interested.

Anyways, where was I, oh yes, CS student summer intern vs Senior Developer.

I read this code and was outraged that it was not the most asymptotically efficient possible code, so I got a meeting with Tim Paterson, who had written much of the string library and had the office next to me.

Let me repeat that for those youngsters in the audience here, TIM FREAKIN’ PATERSON. Tim “QDOS” Paterson, who one fine day wrote an operating system, sold it to BillG, and that became MS-DOS, the most popular operating system in the world. As I’ve mentioned before, Tim was very intimidating to young me and did not always suffer foolish questions gladly, but it turned out that in this case he was very patient with all-caps THIS IS INEFFICIENT Eric. More patient than I likely deserved.

As Tim explained to me, first off, the reason why VB does this seemingly bizarre “find the first character match, then check if query is a prefix of source” logic is because the skipto method is not written in the naive fashion that I showed here. The skipto method is a single x86 machine instruction. (REPNE SCASB, maybe? My x86 machine code knowledge was never very good. It was something in the REP family at least.) It is blazingly fast. It harnesses the power of purpose-built hardware to solve the problem of “where’s that first character at?”

That explains that; it genuinely is a big perf win to let the hardware do the heavy lifting here. But what about the asymptotic problem? Well, as Tim patiently explained to me, guess what? Most VB developers are NOT asking if “aaaab” can be found in “aaaaaaa…”. The vast majority of VB developers are asking is “London” anywhere in this address, or similar problems where the strings are normal human-language strings without a lot of repetitions, and both the source and query strings are short.  Like, very short. Less than 100 characters short. Fits into a cache line short.

Think about it this way; most source strings that VB developers are searching have any given character in them maybe 2% of the time, and so for whatever the start character is of the query string, the skipto step is going to find those 2% of possible matches very quickly. And then the starts step is the vast majority of the time going to very quickly identify false matches. In practice the naive brute force algorithm is almost always O(n + m). 

Moreover, Tim explained to me, any solution that involves allocating a table, preprocessing strings, and so on, is going to take longer to do all that stuff than the blazingly-fast-99.9999%-of-the-time brute force algorithm takes to just give you the answer. The additional complexity is simply not worth it in scenarios that are relevant to VB developers. VB developers are developing line-of-business solutions, and their line of business is not typically genomics; if it is, they have special-purpose libraries for those problems; they’re not using InStr.

And we’re back in 2020. I hope you enjoyed that trip down memory lane.

It turns out that yes, fresh grads and keener interns do complain to senior developers about asymptotic efficiency, and senior developers do say “but nested for loops go brrrrrrr” — yes, they go brrrrrr extremely quickly much of the time, and senior developers know that!

And now I am the senior developer, and I try to be patient with the fresh grads as my mentors were patient with me.

UPDATE: Welcome, Hacker News readers. I always know when I’m linked from Hacker News because of the huge but short-lived spike in traffic. The original author of the meme that inspired this post has weighed in. Thanks for inspiring this trip back to a simpler time!

Bandits, victims and idiots

I don’t enjoy politics, I don’t know enough about it, and my privilege greatly insulates me from its negative effects, and so I don’t talk about it much on this blog. My intention in creating the blog lo these decades ago was to make a friendly, human, competent public face to my team at Microsoft, and to get information out about languages that was not in the official documentation; it was not ever intended to be a soapbox. Only a couple times have I commented on political situations, and today will be the third apparently.

I have been thinking many times these last four years, and much more these last few days about the late Italian economic historian Carlo Cipolla. Not because of his economic theories, of which I know very little, but rather because of his theory of stupidity. You can read the principles in brief for yourself at the link above, or the original paper here, but I can summarize thus:

  • Powerful smart people take actions that benefit both themselves and others.
  • Victims lack the power to protect themselves. They are unable to find actions that benefit themselves, and are victimized to the benefit of others.
  • Bandits take actions that benefit themselves at the expense of victims.
  • Stupid idiots take actions that benefit neither themselves nor others.

These are value-laden terms so let’s be clear here that neither I nor Cipolla are suggesting that victims, bandits or idiots are not intelligent:

  • No matter how intelligent you are and how many precautions you take, you can be victimized by a bandit or an idiot. Victims are not to blame for their victimization. We’ll come back to this in a moment.
  • Bandits are often very intelligent; they just use their skills to victimize others. Whether that’s because they are genuinely not intelligent enough to make a living helping others, or because they are that intelligent but psychologically enjoy being a bandit, or are bandits for other reasons, it doesn’t matter for our purposes. Assume that bandits are extremely intelligent and devious, but motivated by gain.
  • Idiots, ironically, are often very intelligent; a great many idiots have fancy degrees from excellent colleges. As Cipolla points out in his paper, there is no characteristic that identifies idiots other than their inability to act in a way that benefits anyone including themselves. That includes intelligence or lack thereof.

Some key consequences of this model have been on my mind these last few days:

  • Bandits, even the psychopaths, are motivated by self-interest and recognize actions that benefit themselves. You can reason with a bandit, but more importantly, you can reason about a bandit, and therefore you can make use of a bandit. You can make an offer to a powerful bandit and count on them to take it up if it maximizes their gain.
  • You cannot reason with an idiot. You can’t negotiate with them to anyone’s advantage because they will take positions that harm themselves at the same time as they harm others. There are no “useful idiots”; any attempt to use an idiot to benefit yourself will backfire horribly as they manage to find a way for everyone to lose.
  • When the idiots are in power, there is no bright line separating the smart from the victims; rather, there is just a spectrum of more or less power and privilege. Victims by definition lack the power to defend themselves, and the more privileged have no lever to pull to change the course of the idiot, who will act with such brazen disregard for the well-being of everyone including themself that it is hard to devise a strategy.

All this is by way of introduction to say: the position that I am seeing on Twitter and in the media that “soon” is a good time to “re-start the economy” is without question the stupidest, most idiotic position I have ever heard of in my life and that includes “let’s invade Afghanistan for no strategic purpose with no plan on how to ever leave”. There is no way that ends well for anyone, and that includes the billionaires who are temporarily inconvenienced by a slight dip in the flow of cash into their coffers.

I’ll leave you with how Cipolla finishes his essay, because it sums up exactly how I feel at this moment in history.

In a country which is moving downhill […] one notices among those in power an alarming proliferation of the bandits with overtones of stupidity and among those not in power an equally alarming growth in the number of helpless individuals. Such change in the composition of the non-stupid population inevitably strengthens the destructive power of the stupid fraction and makes decline a certainty. And the country goes to Hell.

Working from home

Good Friday afternoon all and welcome to this working-from-home-and-obsessively-washing-hands edition of FAIC.

I am posting today from my recently-transformed spare room which is now apparently my office. Scott Hanselman started a great twitter thread of techies showing off their home workspaces; here’s my humble contribution.


We have my work Mac hooked up to two medium-sized HP monitors, one of which cost me all of $20 at a tech thrift store. The Windows game machine is under the desk. You’ll note that I finally found a use for my VSTO 2007 book. The keyboard is the new edition of the Microsoft Natural; my original edition Natural is still on my desk at work and is not currently retrievable.

I am particularly pleased with how the desk came out. I made it myself out of 110 year old cedar fence boards; when I bought my house in 1997 the original fence was still in the back yard and falling down, so I disassembled it, removed the nails, let the boards dry out, planed them down, and figured I’d eventually do something with it. I’ve been building stuff out of it ever since, and this project finished off the last of that stock.

Here’s a better shot of the desk.


The design is my own but obviously it is just a simple mission-style desk. All the joints are dowel and glue; the only metal is the two screws that hold the two drawer knobs on. The finish is just Danish oil with a little extra linseed oil added.

To the right I have a small writing desk:


Which as you may have guessed doubles as my 1954 Kenmore Zigzag Automatic Sewing Machine:


I have not used it in a while; I used to make kites. I might start again.

The manual for this machine is unintentionally hilarious, but that’s a good topic for another day.

Finally, not shown, I’ve got a futon couch and a few plants to make it cosy.

Stay safe everyone, and hunker down.

UPDATE ONE: Obviously I’ve been spending so much time in video chat from home, which is very unusual for me. Unfortunately my setup is such that there is a south-facing window right behind me that overpowers the built-in camera on my laptop even with the curtains drawn.

I took Scott’s advice and got an inexpensive 8 inch ring light, shown here with the room otherwise dark:


The ring light is dimmable LED and has three colour temperatures, so I can now control the specularity of the light directly on my face, and also do some light shaping with the little non-dimmable desk lamp should I wish to. On a bright day the window no longer washes out the webcam image.


Photo on 3-31-20 at 1.48 PM.jpg


Photo on 3-31-20 at 1.49 PM.jpg

The window is still an almost total white-out, but at least I no longer look like a purple-faced Walking Dead extra.

And good heavens do I ever need a haircut. That’ll have to wait.

UPDATE TWO: You cannot tell in the photo above but the ring light was held on to the monitor by a jury-rigged spring clamp, which is none of stable, attractive, or good for the laptop it was clamped to. In the spirit of Adam Savage’s One Day Builds, I give you Eric Lippert’s Five Minute Build:


Yes, it is a board (left over from the desk) with three holes in it. Quarter-twenty bolt in the back, wood screws in the front, done in five minutes.


Hundred year mistakes

My manager and I got off on a tangent in our most recent one-on-one on the subject of the durability of design mistakes in programming languages. A particular favourite of mine is the worst of the operator precedence problems of C; the story of how it came about is an object lesson in how sometimes gradual evolution produces weird results. Since he was not conversant with all the details, I thought I might write it up and share the story today.

First off, what is the precedence of operators in C? For our purposes today we’ll consider just three operators: &&, & and ==, which I have listed in order of increasing precedence.

What is the problem? Consider:

int x = 0, y = 1, z = 0;
int r = (x & y) == z; // 1
int s = x & (y == z); // 0
int t = x & y == z;   // ?

Remember that before 1999, C had no Boolean type and that the result of a comparison is either zero for false, or one for true.

Is t supposed to equal r or s?

Many people are surprised to find out that t is equal to s! Because == is higher precedence than &, the comparison result is an input to the &, rather than the & result being an input to the comparison.

Put another way: reasonable people think that

x & y == z

should be parsed the same as

x + y == z

but it is not.

What is the origin of this egregious error that has tripped up countless C programmers? Let’s go way back in time to the very early days of C. In those days there was no && operator. Rather, if you wrote

if (x() == y & a() == b)

the compiler would generate code as though you had used the && operator; that is, this had the same semantics as

if (x() == y)
  if (a() == b)

so that a() is not called if the left hand side of the & is false. However, if you wrote

int z = q() & r();

then both sides of the & would be evaluated, and the results would be binary-anded together.

That is, the meaning of & was context sensitive; in the condition of an if or while it meant what we now call &&, the “lazy” form, and everywhere else it meant binary arithmetic, the “eager” form.

However, in either context the & operator was lower precedence than the == operator. We want

if(x() == y & a() == b())

to be

if((x() == y) & (a() == b))

and certainly not

if((x() == (y & a())) == b)

This context-sensitive design was quite rightly criticized as confusing, and so Dennis Ritchie, the designer of C, added the && operator, so that there were now separate operators for bitwise-and and short-circuit-and.

The correct thing to do at this point from a pure language design perspective would have been to make the operator precedence ordering &&, ==, &. This would mean that both

if(x() == y && a() == b())


if(x() & a() == y)

would mean exactly what users expected.

However, Ritchie pointed out that doing so would cause a potential breaking change. Any existing program that had the fragment if(a == b & c == d) would remain correct if the precedence order was &&, &, ==, but would become an incorrect program if the operator precedence was changed without also updating it to use &&.

There were several hundred kilobytes of existing C source code in the world at the time. SEVERAL HUNDRED KB. What if you made this change to the compiler and failed to update one of the & to &&, and made an existing program wrong via a precedence error? That’s a potentially disastrous breaking change.

You might say “just search all the source code for that pattern” but this was two years before grep was invented! It was as primitive as can be.

So Ritchie maintained backwards compatibility forever and made the precedence order &&, &, ==, effectively adding a little bomb to C that goes off every time someone treats & as though it parses like +, in order to maintain backwards compatibility with a version of C that only a handful of people ever used.

But wait, it gets worse.

C++, Java, JavaScript, C#, PHP and who knows how many other languages largely copied the operator precedence rules of C, so they all have this bomb in them too. (Swift, Go, Ruby and Python get it right.) Fortunately it is mitigated somewhat in languages that impose type system constraints; in C# it is an error to treat an int as a bool, but still it is vexing to require parentheses where they ought not to be necessary were there justice in the world. (And the problem is also mitigated in more modern languages by providing richer abstractions that obviate the need for frequent bit-twiddling.)

The moral of the story is: The best time to make a breaking change that involves updating existing code is now, because the bad designs that result from maintaining backwards compat unnecessarily can have repercussions for decades, and the amount of code to update is only going to get larger. It was a mistake to not take the breaking change when there were only a few tens of thousands of lines of C code in the world to update. It’s fifty years since this mistake was made, and since it has become embedded in popular successor languages we’ll be dealing with its repercussions for fifty more at least, I’d wager.

UPDATE: The most common feedback I’ve gotten from this article is “you should always use parentheses when it is unclear”. Well, obviously, yes. But that rather misses the point, which is that there is no reason for the novice developer to suppose that the expression x & y == z is under-parenthesized when x + y == z works as expected. The design of a language should lead us to naturally write correct code without having to think “will I be punished for my arrogance in believing that code actually does what it looks like it ought to?”