9 ms·

Worst Bug Ever

112 points by swanson 14y ago

revelation 14y ago
Thats not so much a "nice bug" but rather a big fat warning sign for anyone trying to use MATLAB.
yuvadam 14y ago
Nice anecdote, but that's a rather uber-sensational title - not in a good way.
- cfinke 14y ago
  Agreed; I expected it to be the story of Therac-25 (http://en.wikipedia.org/wiki/Therac-25 http://en.wikipedia.org/wiki/Therac-25) It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation, approximately 100 times the intended dose. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics and software engineering.
  - swanson 14y ago
    It's linked to in the first paragraph.
dpark 14y ago
>the imaginary constant i in MATLAB was getting overwritten! The constness of i in Matlab is also imaginary, which is pretty terrible.
- Dove 14y ago
  Yeah, I'm hoping by "overwritten" he really meant "shadowed". I don't know MATLAB, but being able to change the value of i seems both useless and dangerous.
  - dpark 14y ago
    I think it's shadowed globally, which has the same effect as it being overwritten. But I'm not certain.
geuis 14y ago
Maybe change the title.
manojlds 14y ago
If that is the worst bug ever, I would die a happy programmer. Btw, why would anyone use the 3*i form when there is the much clearer 3i form?
- daeken 14y ago
  What if you're dealing in imaginary units that aren't constant? E.g. foo*i.
  - manojlds 14y ago
    Ah, right. I am surprised there is not a better syntax for it though.
    - Stwerp 14y ago
      It should still be ' foo * 1j '
  - lazugod 14y ago
    (foo)i, maybe?
jasonkester 14y ago
I had a similarly fun time back in the 90s debugging a VBA script a co-worker had written for an Excel spreadsheet. She was looping through a bunch of time values... ... so she named her variable "time" ... which, when modified, would (naturally) set the system time of the computer. I'm not sure whether they've fixed that one yet.
- ja27 14y ago
  At least once a quarter I'd have a student working on some programming assignment on one of our unix-ish systems where they named the executable "test" and couldn't figure out why it wasn't doing anything they expected. I also got to the point with FORTRAN (yes, I'm old) students where I'd cut them off and tell them to count the columns and make sure each line started with 6 blanks, not 5. My favorite bug is still some piece of C code buried in a library that was like this: if (flag == TRUE) ... Or there's the old for loop typo (much easier to detect on today's high-DPI screens): for(float x=0.0; x < 3,4; x = x + 0.1) ...
  - viraptor 14y ago
    I don't get what's wrong with the flag check apart from being a bit too verbose.
    - mpyne 14y ago
      TRUE is going to be 1. The flag value might have the value 0x2, 0x4, 0xsomething_else, etc., which will not equal exactly 1 (TRUE). What you want varies but is probably something like: if (flag & FLAG_VALUE) ... if (flag) .... if ((flag & FLAG_VALUE) == TRUE) ... that kind of thing.
      - geon 14y ago
        if ((flag & FLAG_VALUE) == TRUE) ... That would still check if flag is 1, right?
        14y ago
CodeCube 14y ago
Kind of along the same lines in the sense of being my biggest facepalm moment; many years ago in my first professional programming gig we had full SA access to our sql server. We regularly did troubleshooting via direct SQL statements (a practice I've since shied away from). One particular day, we were troubleshooting a problem in our monthly billing process. I went to delete a few rows and foolishly forgot to include a limiting statement on the WHERE clause, which resulted in most of the data being blown away. Thankfully, we were able to restore that table from a backup and rerun everything to rebuild the table's state to a correct place ... but suffice to say the experience taught me many things about attention to detail, and how debugging in production should happen. Good times!
- Erwin 14y ago
  A good practice there is to do tricky DELETE/UPDATE inside a transaction. In e.g. Postgres, executing DELETE/UDPATE tells you number of affected rows, so if you see 1243 rows were updated rather than 1 as you expected, you can ROLLBACK.
  - bartonfink 14y ago
    Another good approach is to replace 'delete' with 'select count(1)' or something similar. The where clause can remain the same, you know how many rows will be affected and there's less overhead of remembering to set up a transaction, writing an update statement, rolling back, etc.
    - CodeCube 14y ago
      Yeah, if I ever did have to troubleshoot in SQL directly, I definitely started doing this ... only changing it to update or delete when I was 100% sure it was affecting only what I wanted to affect.
- jmelloy 14y ago
  My biggest production whoops was an update gone awry ... instead of updating one row, updated all 300 million. Spotted it pretty quick, cancelled the update, but then the rollback locked the table and wouldn't allow inserts while it was rolling back ...
spalletti 14y ago
it's the most frequent error you can do when writing 'a-little-bit-more' complex scripts using MATLAB. it's sad to say but MATLAB takes too much care of backward code compatibility and they didn't fix this! all matlab's newcomers do that error. :)
carlob 14y ago
I'm baffled by the fact that the single most common name for an integer variable has a special meaning (though it's not a reserved word). I'm pretty sure this is a bug in MatLab, not in OP's code. But still I was wondering: isn't that code horribly un-idiomatic for an array based programming language?
- jarek 14y ago
  I'd bet i has been used to denote the imaginary unit long before it was used as a loop counter in programming languages. Wikipedia appears to date the former to sometime in the 18th century.
pfortuny 14y ago
That is a pretty awful MATLAB perk, using 'i' as the sqrt(-1) (and indexing the arrays from 1...). It makes polyglot programmers make a lot of silly mistakes. It would have been SO easy to call the imaginary constant just 'I' instead of 'i'...
- TeMPOraL 14y ago
  Or just make it not mutable. Throw a big, fat error when someone assigns value to i.
  - chimeracoder 14y ago
    Or better yet, do what Python does. Require a constant in front of the 'i', even if it's only '1'. Since identifiers can't begin with a numeral, there's no way to shadow a valid use of a complex number this way.
  - pfortuny 14y ago
    This would have been the best way to learn it, some kind of 'CRASH Eh! MAN your files have been formatted and all this project has been deleted because i=sqrt(-1) Sorry for the mess, hope you learn from this...' But I keep tripping over the same stone...
    - KMag 14y ago
      Presumably the parent meant to throw an exception and show a script-level stack trace, not silently exit with no diagnostic messages. Insisting on doing something, even if it's the wrong thing, is much worse in most applications than refusing to run in order to avoid producing incorrect and misleading results.
- jpwright 14y ago
  That would drive mathematicians insane, and this is coming from an electrical engineer's standpoint (where we use j instead)
wereHamster 14y ago
Oh the joys of side-effects. Wouldn't you love to have a programming language that doesn't have them?
- deleted 14y ago
  [deleted]
- SoftwareMaven 14y ago
  It would never work for something as side-effect prone as pure mathematics!
grayrest 14y ago
Further evidence that j is the one true imaginary unit. Title is still sensationalist.
fosap 14y ago
IMO Matlab is the worst programming language widely used in production. This perfectly fits my expectations of Matlab: - i is a mutable, but should be constant - It has a horrible, non obvious name because mathematicians are used to it. (And they are not used to var names longer than two chars) - It is in the global scope. - It's a feature that has a special syntax (sort of). /edit Apologies for php programmers, maybe php is worse. But I'm not sure.
- _pmf_ 14y ago
  Having worked with both Matlab and VBA, it's hard to say for me what is worse. Nowadays, whenever I have to do something (UI, non-numeric data processing) in Matlab, I tend to write a Matlab-shim that calls a Java-component (the Java interaction is fortunately very nice). The mere fact that one gets things done with less hassle in Java than with a scripting language is a quite damning verdict for said scripting language.
- nemetroid 14y ago
  > - i is a mutable, but should be constant It's not really mutable, because it isn't a variable. However, if there is a variable by that name, Matlab will use it instead of the complex constant. > - It a feature that has a special syntax (sort of). The problem is rather a lack of special syntax (or too much leniency). Python also has built-in complex numbers: >>> 1 + 1j (1+1j) but it doesn't allow you to omit the constant before the j: >>> 1 + j Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'j' is not defined You can do it the same way in Matlab to avoid this problem (1 + 1i is always OK since implicit multiplication is not allowed), so the problem is that Matlab lets you use i syntactically like a variable when it in fact isn't.
  - fosap 14y ago
    >It's not really mutable, because it isn't a variable. However, if there is a variable by that name, Matlab will use it instead of the complex constant. This is what i mean with a sort of special syntax. It's awful corner case. I don't like the Python syntax either. What would be wrong with Math.C(1,1) for 1+i? Or how about 1 + Math.I ?
SaulOfTheJungle 14y ago
Why did he get the correct results when the script was run stand-alone?
- earless1 14y ago
  He was passing the numbers directly when running stand-alone. His other code was looping through an array.
stevoski 14y ago
Matlab: the language designed so poorly it makes PHP look good.
- dubcanada 14y ago
  I love that you said that, I didn't read my daily dose of crying about PHP today. Thanks!
electic 14y ago
This is not the worst bug ever. Nice story, however, the title is way over the top.
- bascule 14y ago
  Yeah, I was thinking more like Therac-25 giving people radiation overdoses or the Patriot missile failures due to floating point roundoff error
jstanley 14y ago
Unlike many of the other commenters, I don't see much problem with having "i" mean the imaginary unit. Where MATLAB falls down though is: 1.) not having lexical scoping 2.) not having warnings when a global is overridden in a local scope these are pretty inexcusable. For anyone looking for a "better MATLAB", Python has NumPy, SciPy and Matplotlib which are pretty much everything.
- deleted 14y ago
  [deleted]
- w0utert 14y ago
  >> For anyone looking for a "better MATLAB", Python has NumPy, SciPy and Matplotlib which are pretty much everything. I hear this at work all the time, "just use Python, we're going to use Python everywhere we use Matlab from now on". It simply doesn't work like that. Usually it's programmers who are too overly enthusiastic about replacing Matlab with Python, because they are comfortable with all aspects of programming languages. The people typically writing the most complex Matlab code aren't, more often than not they are physicists or mathematicians. Python + SciPy + Matplotlib is not nearly as accessible as Matlab, and you drag in all kinds of typical 'software engineering' problems that don't exist when you stay inside the Matlab IDE and its related tools. I can conjure up a plot in Matlab and manipulate it incrementally for example, while Matplotlib often requires a lot of crazy data mangling and setup code that isn't required in Matlab (trust me, I've spent months re-implementing a library of Matlab plotting scripts in Matplotlib, and the code of some of them is completely untraceable to the original Matlab code, and full of stuff completely unrelated to plotting). Also, lots of Matlab code relies on all the crazy implicit operations and type juggling allowed by Matlab, many of which are almost fundamentally incompatible with a well-defined programming language suited for production code. This is all even ignoring the fact that Matlab comes with a pretty extensive library of toolboxes that have no equivalent in SciPy or Numpy. If you're really determined and know how to program in Python, you can basically get a lot of stuff done without requiring Matlab, but if you've always been using Matlab like a prototyping tool or a very elaborate graphical calculator (and this constitutes a very extensive part of Matlab users), Python + all its scientific libraries is no substitute.
paines 14y ago
That is not what I call "worst" bug. It is a pretty cool bug. Screw Matlab for hiding this from you.
seanpont 14y ago
As a mechE, I was brought up on Matlab. We were taught to use ii, jj, kk... as our index variable names. Many years later, I still prefer them to i, j, and k.
- varjag 14y ago
  Like stuttering of a shell-shocked veteran, it's a battle scar of Matlab survivor.
agumonkey 14y ago
Colleague had a long long debugging session doing MDA based code generation, turns out JET (java/ibm/eclipse) template engine had a stupid arithmetic bug, the kind you'll only investigate after everything in the stack has been checked.
jtanderson 14y ago
Oh, the things a little syntax highlighting can prevent...
Pinckney 14y ago
Thanks for posting this -- I just went and fixed just such a loop in my own code.
- swanson 14y ago
  Ha! Glad I could help :)
  - Stwerp 14y ago
    Me too! I changed my array counter to 'j'. :P I work in matlab a lot and have had this error creep up several times. It's very annoying, to say the least.
    - swanson 14y ago
      The better fix - which I mentioned as well - is to use `2+3i` instead of `2+3*i`. The former will always use sqrt(-1).
      - Stwerp 14y ago
        I agree. I think I picked up this syntax from Python. My response was intended as tongue-in-cheek answer since Matlab allows both i and j as the complex number. I mainly see this when editing or modifying scripts written by others in my lab and have to pinpoint their counter usage.
varjag 14y ago
Fun, but there are worse things than scope shadowing bugs. At least this one is deterministic and can be zeroed in with a few trace prints.
deleted 14y ago
[deleted]
nollidge 14y ago
Just want to say thanks to all the Hyperbole Police in this thread pointing out this isn't actually the worst bug ever.
- arielserafini 14y ago
  I honestly don't think this is Hyperbole Policing - people were just expecting a lot more.
- swanson 14y ago
  Ya - I was going for a reference to Comic Book Guy: http://en.wikipedia.org/wiki/Comic_Book_Guy http://en.wikipedia.org/wiki/Comic_Book_Guy Thought the periods would make it clear. Oh well! EDIT: Looks like the periods got removed from the title - I can see why that would make the reference a bit more obtuse.
jballanc 14y ago
I know the feeling, but I think I have a bug that beats even this one... Once, I was working on the "alias" keyword in MacRuby, and getting it to work with methods defined in Obj-C. Since MacRuby objects are just extensions on Obj-C objects, some methods are defined in Obj-C and require special casing in order to alias them to a different identifier. For example, if you want to alias "Array#length" to a different name, you'd actually be aliasing "-[NSArray length]". So I do everything I should need to do to make it work: get a handle to the object, the object's class, the method defined for the class, a selector for the new name...but no matter what I tried, every time I aliased the method, and attempted to call the method by the new name, I would always get the same error: "Method not defined". Finally, I decided to dive deep with gdb and find out exactly what was going on. To my shock and amazement, I came to realize that I was properly aliasing the method but, due to the nature of NSArray's implementation as a class cluster (and the fact that Obj-C doesn't have proper virtual methods like C++), the implementation of "-[NSArray length]" was literally: - (NSInteger)length { [NSException raise:@"Method not defined." format:@"%"]; }
- eridius 14y ago
  Are you saying that when a name is aliased in MacRuby, the alias becomes a copy of the IMP from that particular class, such that subclass overrides of that method are never invoked? Because that sounds like a bad idea.
  - jballanc 14y ago
    Aliasing in MacRuby (or Obj-C for that matter) involves adding a new entry to a class's method table with the new name but a pointer to the old implementation. Method lookup from subclasses to superclasses proceeds the same as it would for any method.
    - eridius 14y ago
      Yes, but if you message the subclass with the alias, only the superclass's method gets invoked, because the alias apparently contains the IMP of the superclass's method. Or it must, if what you said before is to be believed. This is pretty nasty, because it breaks the idea of message sends and subclass overrides of methods. There's two ways you could solve it: 1. Set the IMP to a static function that looks up the method it should forward to and re-send the message there. This is hard to do in a manner that supports all arguments, and you'd need at least 2, possibly 3 versions anyway (one for regular, one for strret, and on some architectures, one for fpret). It would probably need to be written in assembly in order to forward the arguments. 2. More recently (in iOS 4.3, and whatever the corresponding OS X release was), the runtime got a nice little function called imp_implementationWithBlock(). This takes a block and returns an IMP. Bill Bumgarner has a nice blog post about it (http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementationwithblock/ http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementa...). You could use this function on every alias in order to invoke the correct method, passing along the correct arguments.
jblock 14y ago
For some reason, I've found more bugs like this in MATLAB than in any other domain. Ever.
danialtz 14y ago
I was using a very well-known popular computation software direct competitor to ANSYS Multiphysics. I was using it since early version 3 days, through 4.x over a period of 3 years in my phd in the field of fluid-structure interaction (e.g. fish in flow). The computations usually took over several days on many cpus for 1 second of real-life simulation, making testing quite painful. Hence, many studies do not involve testing like in software field. I was calculating a parameter enhancement where my calculations were showing a great 180% enhancement; this result would make it into very high ranking (15+ IF) paper right away. By then, several people had written numerous papers on this matter using this multiphysics software in well-known journals. But my gut feeling didn't let me. Eventhough my professors were pressing me to publish, I took 6 painful months (toward the end of my phd) to rigorously test the software itself, having nothing to do with my work, since it was closed-source. After 6 months, I found out that a velocity term used in their stabilization algorithm does not use relative velocity, but absolute velocity, leading to enormous amount of fake diffusion into the model. I had to travel to another city in europe to convince them that there is such bug. Every single work in my field that used this software till then had been severely invalid. And what the developers did: they fixed the bug in the next sub-release without putting it in the changelog like nothing happened.
- KMag 14y ago
  I don't suppose you were able to publish a peer-reviewed journal article calling previous results into question, were you?
seanhandley 14y ago
"those where" </grammar nazi>
- swanson 14y ago
  Fixed, thanks. https://github.com/swanson/swanson.github.com/commit/81c14ed32a5d603bff95ccac48f5e405902c4514 https://github.com/swanson/swanson.github.com/commit/81c14ed...
why-el 14y ago
Don't trust it too much: http://cm.bell-labs.com/who/ken/trust.html http://cm.bell-labs.com/who/ken/trust.html
untog 14y ago
I made a similar error once working in Adobe Air (if I recall): for (x = 0; x < thing.length; x++) { } I didn't put a "var" before the x- so Air used the window.x object, and shifted the entire contents of the window to the right by a seemingly random number of pixels. To look at it like that, it's blindingly obvious. But it was in the middle of a whole lot of other code, and of course, the for loop ran perfectly fine.
teilo 14y ago
I don't know what I would do without list comprehensions. I can hardly imagine using iteration loops and counters anymore, and this is one among many reasons why.
- recursive 14y ago
  List comprehensions don't really handle performing a list of actions for their side effects alone.
  - teilo 14y ago
    The example in the code was not a side-effect. They were creating a new list by invoking a function on every member of the old list. Here's the same thing re-written in Python: d = [compute_diffraction_at_wavelength(x, WAVELENGTH) for x in LensLayers] Or oldschool Python (and still faster, I think): d = map(compute_diffraction_at_wavelength(WAVELENGTH), LensLayers)
antsam 14y ago
I spent a few hours debugging an OpenGL program I was working on only to realize that I forgot to account for normalized coordinates. All I had to do was individually multiply x and y by the canvas size. Always start with the small, obvious stuff first!
erikb 14y ago
Who upvoted this?
solox3 14y ago
If you call this the worst bug ever (t = [-infty, +infty]), then what's this? https://github.com/MrMEEE/bumblebee-Old-and-abbandoned/commit/a047be85247755cdbe0acce6f1dafc8beb84f2ac https://github.com/MrMEEE/bumblebee-Old-and-abbandoned/commi...
martinced 14y ago
Nice article but there's a lesson here. First I'll come with an old bug of mine (early nineties). The bug was very hard to reproduce because it was very very occasional (but terrible in consequences). I simply couldn't find it. I knew I was probably smashing some memory somewhere (I was not that experienced in C) but couldn't find where. So what did I do after weeks of hunting the bug? I moved my design away to an entirely deterministic one and started recording all the inputs the program received and then I could replay them. At one point, logically, the bug happened. And so was it "recorded". Because my program was now deterministic, I could simply feed it the inputs (and the time at which they happened) and, surely enough, the bug was there. And then finding the cause of the bug was of course trivial. What's the lesson? Well, first obviously bugs that you cannot easily reproduce because you cannot "reproduce the state" are typically kinda hard to track down. The less "state" in your program, the easier it is to reason about your program and the easier it is to reproduce the state. The lesson? Functional programming rocks.
geon 14y ago
On my first programming job (2004), I was adding shopping cart functionality to a proprietary CMS written in PHP. For some reason I couldn't get a simple item listing to work. I had spent several hours debugging it before I found the problem. The CMS had a database query result class with a method named something like "numberOfRecords". It returned the number of records minus one... I told my boss about this, and his answer was "Yeah I know, but we don't want to fix it because a lot of code depend on that bug."
anonymouz 14y ago
Python (before version 3) leaks variables from list comprehension, while it seems natural to assume that the comprehension has its own scope. Makes for some funny bugs: i = ... ... # lots of code [ do_stuff(i) for i in range(n) ] ... # whoops, our outside i has now been modified

Worst Bug Ever

112 points by swanson 14y ago

revelation 14y ago

Thats not so much a "nice bug" but rather a big fat warning sign for anyone trying to use MATLAB.

yuvadam 14y ago

Nice anecdote, but that's a rather uber-sensational title - not in a good way.

cfinke 14y ago
Agreed; I expected it to be the story of Therac-25 (http://en.wikipedia.org/wiki/Therac-25 http://en.wikipedia.org/wiki/Therac-25) It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation, approximately 100 times the intended dose. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics and software engineering.
- swanson 14y ago
  It's linked to in the first paragraph.

dpark 14y ago

>the imaginary constant i in MATLAB was getting overwritten! The constness of i in Matlab is also imaginary, which is pretty terrible.

Dove 14y ago
Yeah, I'm hoping by "overwritten" he really meant "shadowed". I don't know MATLAB, but being able to change the value of i seems both useless and dangerous.
- dpark 14y ago
  I think it's shadowed globally, which has the same effect as it being overwritten. But I'm not certain.

geuis 14y ago

Maybe change the title.

manojlds 14y ago

If that is the worst bug ever, I would die a happy programmer. Btw, why would anyone use the 3*i form when there is the much clearer 3i form?

daeken 14y ago
What if you're dealing in imaginary units that aren't constant? E.g. foo*i.
- manojlds 14y ago
  Ah, right. I am surprised there is not a better syntax for it though.
  - Stwerp 14y ago
    It should still be ' foo * 1j '
- lazugod 14y ago
  (foo)i, maybe?

jasonkester 14y ago

I had a similarly fun time back in the 90s debugging a VBA script a co-worker had written for an Excel spreadsheet. She was looping through a bunch of time values... ... so she named her variable "time" ... which, when modified, would (naturally) set the system time of the computer. I'm not sure whether they've fixed that one yet.

ja27 14y ago
At least once a quarter I'd have a student working on some programming assignment on one of our unix-ish systems where they named the executable "test" and couldn't figure out why it wasn't doing anything they expected. I also got to the point with FORTRAN (yes, I'm old) students where I'd cut them off and tell them to count the columns and make sure each line started with 6 blanks, not 5. My favorite bug is still some piece of C code buried in a library that was like this: if (flag == TRUE) ... Or there's the old for loop typo (much easier to detect on today's high-DPI screens): for(float x=0.0; x < 3,4; x = x + 0.1) ...
- viraptor 14y ago
  I don't get what's wrong with the flag check apart from being a bit too verbose.
  - mpyne 14y ago
    TRUE is going to be 1. The flag value might have the value 0x2, 0x4, 0xsomething_else, etc., which will not equal exactly 1 (TRUE). What you want varies but is probably something like: if (flag & FLAG_VALUE) ... if (flag) .... if ((flag & FLAG_VALUE) == TRUE) ... that kind of thing.
    - geon 14y ago
      if ((flag & FLAG_VALUE) == TRUE) ... That would still check if flag is 1, right?
      - 14y ago

CodeCube 14y ago

Kind of along the same lines in the sense of being my biggest facepalm moment; many years ago in my first professional programming gig we had full SA access to our sql server. We regularly did troubleshooting via direct SQL statements (a practice I've since shied away from). One particular day, we were troubleshooting a problem in our monthly billing process. I went to delete a few rows and foolishly forgot to include a limiting statement on the WHERE clause, which resulted in most of the data being blown away. Thankfully, we were able to restore that table from a backup and rerun everything to rebuild the table's state to a correct place ... but suffice to say the experience taught me many things about attention to detail, and how debugging in production should happen. Good times!

Erwin 14y ago
A good practice there is to do tricky DELETE/UPDATE inside a transaction. In e.g. Postgres, executing DELETE/UDPATE tells you number of affected rows, so if you see 1243 rows were updated rather than 1 as you expected, you can ROLLBACK.
- bartonfink 14y ago
  Another good approach is to replace 'delete' with 'select count(1)' or something similar. The where clause can remain the same, you know how many rows will be affected and there's less overhead of remembering to set up a transaction, writing an update statement, rolling back, etc.
  - CodeCube 14y ago
    Yeah, if I ever did have to troubleshoot in SQL directly, I definitely started doing this ... only changing it to update or delete when I was 100% sure it was affecting only what I wanted to affect.
jmelloy 14y ago
My biggest production whoops was an update gone awry ... instead of updating one row, updated all 300 million. Spotted it pretty quick, cancelled the update, but then the rollback locked the table and wouldn't allow inserts while it was rolling back ...

spalletti 14y ago

it's the most frequent error you can do when writing 'a-little-bit-more' complex scripts using MATLAB. it's sad to say but MATLAB takes too much care of backward code compatibility and they didn't fix this! all matlab's newcomers do that error. :)

carlob 14y ago

I'm baffled by the fact that the single most common name for an integer variable has a special meaning (though it's not a reserved word). I'm pretty sure this is a bug in MatLab, not in OP's code. But still I was wondering: isn't that code horribly un-idiomatic for an array based programming language?

jarek 14y ago
I'd bet i has been used to denote the imaginary unit long before it was used as a loop counter in programming languages. Wikipedia appears to date the former to sometime in the 18th century.

pfortuny 14y ago

That is a pretty awful MATLAB perk, using 'i' as the sqrt(-1) (and indexing the arrays from 1...). It makes polyglot programmers make a lot of silly mistakes. It would have been SO easy to call the imaginary constant just 'I' instead of 'i'...

TeMPOraL 14y ago
Or just make it not mutable. Throw a big, fat error when someone assigns value to i.
- chimeracoder 14y ago
  Or better yet, do what Python does. Require a constant in front of the 'i', even if it's only '1'. Since identifiers can't begin with a numeral, there's no way to shadow a valid use of a complex number this way.
- pfortuny 14y ago
  This would have been the best way to learn it, some kind of 'CRASH Eh! MAN your files have been formatted and all this project has been deleted because i=sqrt(-1) Sorry for the mess, hope you learn from this...' But I keep tripping over the same stone...
  - KMag 14y ago
    Presumably the parent meant to throw an exception and show a script-level stack trace, not silently exit with no diagnostic messages. Insisting on doing something, even if it's the wrong thing, is much worse in most applications than refusing to run in order to avoid producing incorrect and misleading results.
jpwright 14y ago
That would drive mathematicians insane, and this is coming from an electrical engineer's standpoint (where we use j instead)

wereHamster 14y ago

Oh the joys of side-effects. Wouldn't you love to have a programming language that doesn't have them?

deleted 14y ago
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SoftwareMaven 14y ago
It would never work for something as side-effect prone as pure mathematics!

grayrest 14y ago

Further evidence that j is the one true imaginary unit. Title is still sensationalist.

fosap 14y ago

IMO Matlab is the worst programming language widely used in production. This perfectly fits my expectations of Matlab: - i is a mutable, but should be constant - It has a horrible, non obvious name because mathematicians are used to it. (And they are not used to var names longer than two chars) - It is in the global scope. - It's a feature that has a special syntax (sort of). /edit Apologies for php programmers, maybe php is worse. But I'm not sure.

_pmf_ 14y ago
Having worked with both Matlab and VBA, it's hard to say for me what is worse. Nowadays, whenever I have to do something (UI, non-numeric data processing) in Matlab, I tend to write a Matlab-shim that calls a Java-component (the Java interaction is fortunately very nice). The mere fact that one gets things done with less hassle in Java than with a scripting language is a quite damning verdict for said scripting language.
nemetroid 14y ago
> - i is a mutable, but should be constant It's not really mutable, because it isn't a variable. However, if there is a variable by that name, Matlab will use it instead of the complex constant. > - It a feature that has a special syntax (sort of). The problem is rather a lack of special syntax (or too much leniency). Python also has built-in complex numbers: >>> 1 + 1j (1+1j) but it doesn't allow you to omit the constant before the j: >>> 1 + j Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'j' is not defined You can do it the same way in Matlab to avoid this problem (1 + 1i is always OK since implicit multiplication is not allowed), so the problem is that Matlab lets you use i syntactically like a variable when it in fact isn't.
- fosap 14y ago
  >It's not really mutable, because it isn't a variable. However, if there is a variable by that name, Matlab will use it instead of the complex constant. This is what i mean with a sort of special syntax. It's awful corner case. I don't like the Python syntax either. What would be wrong with Math.C(1,1) for 1+i? Or how about 1 + Math.I ?

SaulOfTheJungle 14y ago

Why did he get the correct results when the script was run stand-alone?

earless1 14y ago
He was passing the numbers directly when running stand-alone. His other code was looping through an array.

stevoski 14y ago

Matlab: the language designed so poorly it makes PHP look good.

dubcanada 14y ago
I love that you said that, I didn't read my daily dose of crying about PHP today. Thanks!

electic 14y ago

This is not the worst bug ever. Nice story, however, the title is way over the top.

bascule 14y ago
Yeah, I was thinking more like Therac-25 giving people radiation overdoses or the Patriot missile failures due to floating point roundoff error

jstanley 14y ago

Unlike many of the other commenters, I don't see much problem with having "i" mean the imaginary unit. Where MATLAB falls down though is: 1.) not having lexical scoping 2.) not having warnings when a global is overridden in a local scope these are pretty inexcusable. For anyone looking for a "better MATLAB", Python has NumPy, SciPy and Matplotlib which are pretty much everything.

deleted 14y ago
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w0utert 14y ago
>> For anyone looking for a "better MATLAB", Python has NumPy, SciPy and Matplotlib which are pretty much everything. I hear this at work all the time, "just use Python, we're going to use Python everywhere we use Matlab from now on". It simply doesn't work like that. Usually it's programmers who are too overly enthusiastic about replacing Matlab with Python, because they are comfortable with all aspects of programming languages. The people typically writing the most complex Matlab code aren't, more often than not they are physicists or mathematicians. Python + SciPy + Matplotlib is not nearly as accessible as Matlab, and you drag in all kinds of typical 'software engineering' problems that don't exist when you stay inside the Matlab IDE and its related tools. I can conjure up a plot in Matlab and manipulate it incrementally for example, while Matplotlib often requires a lot of crazy data mangling and setup code that isn't required in Matlab (trust me, I've spent months re-implementing a library of Matlab plotting scripts in Matplotlib, and the code of some of them is completely untraceable to the original Matlab code, and full of stuff completely unrelated to plotting). Also, lots of Matlab code relies on all the crazy implicit operations and type juggling allowed by Matlab, many of which are almost fundamentally incompatible with a well-defined programming language suited for production code. This is all even ignoring the fact that Matlab comes with a pretty extensive library of toolboxes that have no equivalent in SciPy or Numpy. If you're really determined and know how to program in Python, you can basically get a lot of stuff done without requiring Matlab, but if you've always been using Matlab like a prototyping tool or a very elaborate graphical calculator (and this constitutes a very extensive part of Matlab users), Python + all its scientific libraries is no substitute.

paines 14y ago

That is not what I call "worst" bug. It is a pretty cool bug. Screw Matlab for hiding this from you.

seanpont 14y ago

As a mechE, I was brought up on Matlab. We were taught to use ii, jj, kk... as our index variable names. Many years later, I still prefer them to i, j, and k.

varjag 14y ago
Like stuttering of a shell-shocked veteran, it's a battle scar of Matlab survivor.

agumonkey 14y ago

Colleague had a long long debugging session doing MDA based code generation, turns out JET (java/ibm/eclipse) template engine had a stupid arithmetic bug, the kind you'll only investigate after everything in the stack has been checked.

jtanderson 14y ago

Oh, the things a little syntax highlighting can prevent...

Pinckney 14y ago

Thanks for posting this -- I just went and fixed just such a loop in my own code.

swanson 14y ago
Ha! Glad I could help :)
- Stwerp 14y ago
  Me too! I changed my array counter to 'j'. :P I work in matlab a lot and have had this error creep up several times. It's very annoying, to say the least.
  - swanson 14y ago
    The better fix - which I mentioned as well - is to use `2+3i` instead of `2+3*i`. The former will always use sqrt(-1).
    - Stwerp 14y ago
      I agree. I think I picked up this syntax from Python. My response was intended as tongue-in-cheek answer since Matlab allows both i and j as the complex number. I mainly see this when editing or modifying scripts written by others in my lab and have to pinpoint their counter usage.

varjag 14y ago

Fun, but there are worse things than scope shadowing bugs. At least this one is deterministic and can be zeroed in with a few trace prints.

deleted 14y ago

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nollidge 14y ago

Just want to say thanks to all the Hyperbole Police in this thread pointing out this isn't actually the worst bug ever.

arielserafini 14y ago
I honestly don't think this is Hyperbole Policing - people were just expecting a lot more.
swanson 14y ago
Ya - I was going for a reference to Comic Book Guy: http://en.wikipedia.org/wiki/Comic_Book_Guy http://en.wikipedia.org/wiki/Comic_Book_Guy Thought the periods would make it clear. Oh well! EDIT: Looks like the periods got removed from the title - I can see why that would make the reference a bit more obtuse.

jballanc 14y ago

I know the feeling, but I think I have a bug that beats even this one... Once, I was working on the "alias" keyword in MacRuby, and getting it to work with methods defined in Obj-C. Since MacRuby objects are just extensions on Obj-C objects, some methods are defined in Obj-C and require special casing in order to alias them to a different identifier. For example, if you want to alias "Array#length" to a different name, you'd actually be aliasing "-[NSArray length]". So I do everything I should need to do to make it work: get a handle to the object, the object's class, the method defined for the class, a selector for the new name...but no matter what I tried, every time I aliased the method, and attempted to call the method by the new name, I would always get the same error: "Method not defined". Finally, I decided to dive deep with gdb and find out exactly what was going on. To my shock and amazement, I came to realize that I was properly aliasing the method but, due to the nature of NSArray's implementation as a class cluster (and the fact that Obj-C doesn't have proper virtual methods like C++), the implementation of "-[NSArray length]" was literally: - (NSInteger)length { [NSException raise:@"Method not defined." format:@"%"]; }

eridius 14y ago
Are you saying that when a name is aliased in MacRuby, the alias becomes a copy of the IMP from that particular class, such that subclass overrides of that method are never invoked? Because that sounds like a bad idea.
- jballanc 14y ago
  Aliasing in MacRuby (or Obj-C for that matter) involves adding a new entry to a class's method table with the new name but a pointer to the old implementation. Method lookup from subclasses to superclasses proceeds the same as it would for any method.
  - eridius 14y ago
    Yes, but if you message the subclass with the alias, only the superclass's method gets invoked, because the alias apparently contains the IMP of the superclass's method. Or it must, if what you said before is to be believed. This is pretty nasty, because it breaks the idea of message sends and subclass overrides of methods. There's two ways you could solve it: 1. Set the IMP to a static function that looks up the method it should forward to and re-send the message there. This is hard to do in a manner that supports all arguments, and you'd need at least 2, possibly 3 versions anyway (one for regular, one for strret, and on some architectures, one for fpret). It would probably need to be written in assembly in order to forward the arguments. 2. More recently (in iOS 4.3, and whatever the corresponding OS X release was), the runtime got a nice little function called imp_implementationWithBlock(). This takes a block and returns an IMP. Bill Bumgarner has a nice blog post about it (http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementationwithblock/ http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementa...). You could use this function on every alias in order to invoke the correct method, passing along the correct arguments.

jblock 14y ago

For some reason, I've found more bugs like this in MATLAB than in any other domain. Ever.

danialtz 14y ago

I was using a very well-known popular computation software direct competitor to ANSYS Multiphysics. I was using it since early version 3 days, through 4.x over a period of 3 years in my phd in the field of fluid-structure interaction (e.g. fish in flow). The computations usually took over several days on many cpus for 1 second of real-life simulation, making testing quite painful. Hence, many studies do not involve testing like in software field. I was calculating a parameter enhancement where my calculations were showing a great 180% enhancement; this result would make it into very high ranking (15+ IF) paper right away. By then, several people had written numerous papers on this matter using this multiphysics software in well-known journals. But my gut feeling didn't let me. Eventhough my professors were pressing me to publish, I took 6 painful months (toward the end of my phd) to rigorously test the software itself, having nothing to do with my work, since it was closed-source. After 6 months, I found out that a velocity term used in their stabilization algorithm does not use relative velocity, but absolute velocity, leading to enormous amount of fake diffusion into the model. I had to travel to another city in europe to convince them that there is such bug. Every single work in my field that used this software till then had been severely invalid. And what the developers did: they fixed the bug in the next sub-release without putting it in the changelog like nothing happened.

KMag 14y ago
I don't suppose you were able to publish a peer-reviewed journal article calling previous results into question, were you?

seanhandley 14y ago

"those where" </grammar nazi>

swanson 14y ago
Fixed, thanks. https://github.com/swanson/swanson.github.com/commit/81c14ed32a5d603bff95ccac48f5e405902c4514 https://github.com/swanson/swanson.github.com/commit/81c14ed...

why-el 14y ago

Don't trust it too much: http://cm.bell-labs.com/who/ken/trust.html http://cm.bell-labs.com/who/ken/trust.html

untog 14y ago

I made a similar error once working in Adobe Air (if I recall): for (x = 0; x < thing.length; x++) { } I didn't put a "var" before the x- so Air used the window.x object, and shifted the entire contents of the window to the right by a seemingly random number of pixels. To look at it like that, it's blindingly obvious. But it was in the middle of a whole lot of other code, and of course, the for loop ran perfectly fine.

teilo 14y ago

I don't know what I would do without list comprehensions. I can hardly imagine using iteration loops and counters anymore, and this is one among many reasons why.

recursive 14y ago
List comprehensions don't really handle performing a list of actions for their side effects alone.
- teilo 14y ago
  The example in the code was not a side-effect. They were creating a new list by invoking a function on every member of the old list. Here's the same thing re-written in Python: d = [compute_diffraction_at_wavelength(x, WAVELENGTH) for x in LensLayers] Or oldschool Python (and still faster, I think): d = map(compute_diffraction_at_wavelength(WAVELENGTH), LensLayers)

antsam 14y ago

I spent a few hours debugging an OpenGL program I was working on only to realize that I forgot to account for normalized coordinates. All I had to do was individually multiply x and y by the canvas size. Always start with the small, obvious stuff first!

erikb 14y ago

Who upvoted this?

solox3 14y ago

If you call this the worst bug ever (t = [-infty, +infty]), then what's this? https://github.com/MrMEEE/bumblebee-Old-and-abbandoned/commit/a047be85247755cdbe0acce6f1dafc8beb84f2ac https://github.com/MrMEEE/bumblebee-Old-and-abbandoned/commi...

martinced 14y ago

Nice article but there's a lesson here. First I'll come with an old bug of mine (early nineties). The bug was very hard to reproduce because it was very very occasional (but terrible in consequences). I simply couldn't find it. I knew I was probably smashing some memory somewhere (I was not that experienced in C) but couldn't find where. So what did I do after weeks of hunting the bug? I moved my design away to an entirely deterministic one and started recording all the inputs the program received and then I could replay them. At one point, logically, the bug happened. And so was it "recorded". Because my program was now deterministic, I could simply feed it the inputs (and the time at which they happened) and, surely enough, the bug was there. And then finding the cause of the bug was of course trivial. What's the lesson? Well, first obviously bugs that you cannot easily reproduce because you cannot "reproduce the state" are typically kinda hard to track down. The less "state" in your program, the easier it is to reason about your program and the easier it is to reproduce the state. The lesson? Functional programming rocks.

geon 14y ago

On my first programming job (2004), I was adding shopping cart functionality to a proprietary CMS written in PHP. For some reason I couldn't get a simple item listing to work. I had spent several hours debugging it before I found the problem. The CMS had a database query result class with a method named something like "numberOfRecords". It returned the number of records minus one... I told my boss about this, and his answer was "Yeah I know, but we don't want to fix it because a lot of code depend on that bug."

anonymouz 14y ago

Python (before version 3) leaks variables from list comprehension, while it seems natural to assume that the comprehension has its own scope. Makes for some funny bugs: i = ... ... # lots of code [ do_stuff(i) for i in range(n) ] ... # whoops, our outside i has now been modified