• ray10k (unregistered)

    First off, isn't there some language builtin to fetch the largest value in the array? DRY, people! Also I agree with the rest of the article. It's amazing how much worse the code gets with the context. Can we have more subtle WTFs like this please?

  • A nonny mouse (unregistered)

    adding "const" to the parameter would be good practice, but would not reduce copying at all. Adding & (making it a reference) would remove copying.

    The real wtf is that the sorted property of the vectors isn't expressed in the type system. If it were, then the implementation of max would indeed be trivial on any sorted collection.

  • someone (unregistered)

    I, too, to the 0.03333 frames-per-second performance.

  • Greg (unregistered)

    To cut down on copy operations you want to use a const reference; simply adding const will only prevent accidental modifications to your vector.

  • (nodebb)


    • no const qualifier on the parameter (for signalling non mutation)
    • no reference qualifier on the parameter (to prevent copying)
    • misleading parameter name ("list")
    • overly specific function (could be made generic to work on any collection type via passing first and last iterators)
    • there is ALREADY a standard library function for this (std::max_element)
    • the function returns incorrect results on empty vectors
    • the function returns incorrect results on vectors containing all negatives
    • the parameter's type doesn't reflect that its elements are stored in sorted order
    • if the caller knew that this was a sorted type, caller could just call myVector.back() rather than this function (though would still be undefined on an empty vector)

    Not bad for such a small chunk of code. About the only wtf it missed was returning a reference to the element.

  • (nodebb) in reply to philipcraig

    ITYM "returning a reference to the local variable", but yes. A reference to the actual max element? No problem, although it's a bit strange, but a reference to the local variable "max"?

    And you missed one: Using array indexing rather than dragging an iterator across the elements. The code is sufficiently clear that the optimiser can probably save N-1 calls to list.size(), but...

  • djingis1 (unregistered) in reply to philipcraig

    You missed one:

    Type mismatch between int i and size_t std::vector.size()

  • Or do it correctly? (unregistered)

    The method name and parameter assume an unsorted list. Changing the method's behavior to choose the first or last value based on assumed order is a mistake unless the method is simply no longer called.

    Of course, add a model that smartly updates whenever the "list" of data changes so that it knows the min/max value at all times .... that's the way to go.

  • Or do it correctly? (unregistered) in reply to A nonny mouse

    Since it's an array, isn't it already passed in by reference already (via pointer)?

    Sorry if wrong. my c/c++ code knowledge is deprecated to 2001.

  • Or do it correctly? (unregistered) in reply to Steve_The_Cynic

    The call to index into the vector is not faster than an iterator. It's probably faster since all you are really doing is jumping to a location in memory based on index 0's location.

  • djingis1 (unregistered) in reply to Or do it correctly?

    You'd be correct if it was indeed a native array. But an std::vector<T> is not native array.

  • Parallel Thinker (unregistered)

    2 more WTFs regarding parallel threads that modify the "list"-array: 1.) There is a code duplication: list[i] accessed 2 times. In the if-condition and in the max-asignment. 2.) instead of storing the list.size() in a temp variable and using that, the size is always re-evaluated in every iteration of the for-loop What happens if somebody deletes the first element in a parallel thread? For example list=[0,1,2] becomes [1,2]. You are at i=1, your max is 1. Now you re-evaluate: size = 2. You exit the loop. max stays at value 1 which is not true. Second error: you are at i=2. The list is changed after the if-statement, but before the max assignment max=a[i]. Now this max-assignment throws an out-of-bounds exception. Or it accesses a memory cell that has an undefined value.

  • Peter (unregistered) in reply to ray10k

    @ray10k: Absolutely!

    double SomeClass::getMaxKeyValue(std::vector<double> const& keys) {
    std::vector<double>::const_iterator element = std::max_element(keys.begin(), keys.end(), std::less<double>()); return element != keys.end() ? *element : 0; }

  • Greg (unregistered) in reply to Parallel Thinker

    In the current code your parrallel thread "issues" are moot as the function works on a copy of the vector.

  • Erwin Smout (unregistered)

    The real WTF is that it returns 0 if the array contains negative numbers exclusively, no ?

  • Erwin Smout (unregistered) in reply to Erwin Smout

    Managed to miss the commentary in the article itself. Sorry.

  • Debra (unregistered) in reply to Or do it correctly?

    It actually makes zero difference. Have a look at https://godbolt.org/g/ToiWpU : the loop code is exactly the same in every case. Of course, compiling with optimisations disabled would probably have vastly different results depending on how much iterator debugging the standard library implementation does, but the final version isn't (and shouldn't be) any different.

  • M (unregistered)

    So a one line change in this method increased the framerate to the desired result. Sounds like a great code base where such minimal changes were needed.

  • Zenith (unregistered) in reply to ray10k

    You say that but there are alot of side effects to doing so.

    1. In .NET, it chains you to a specific framework version.
    2. You don't know what library code could be doing. Reading parts of the .NET reference for WinForms made it clear that Microsoft's developers aren't any smarter than I am.
    3. Some of the functions do stupid things on corner cases. Both max() and min() might return 0 on an empty list but crash on a null list. Both compare 0 elements so what's the difference? Why is 0 even a reasonable default anyway? And how is that default 0 distinguishable from a list where 0 is a legitimate max or min? The right answer here is to return null if there's nothing to compare and then have the caller recognize that and act accordingly. Unless you're in some dopey language that does truthy/falsy conflation of NULL, and 0, and ''', and who knows what else.
    4. Sometimes you're pulling in a ton of dependencies for a single call you really could've written yourself. JavaScript "developers" are notorious for this. happens on the desktop too though. You'd be surprised how much performance you can wring out of hardware if you aren't offloading anything and everything to a pile of libraries.
    5. Somebody else controls your support policies. They drop support, you drop support. Some framework developers get it in their head that it's their right to force users to think/act a certain way and use software as the club to beat that into them.

    It's not that I don't like libraries but some of these arguments reek of non-programming programmerism.

  • richP (unregistered)

    raised after the fix was committed: Bug #58822: Slow motion mode no longer operates properly.

  • siciac (unregistered) in reply to Zenith

    The right answer here is to return null if there's nothing to compare and then have the caller recognize that and act accordingly.

    You may not recognize it until the code is in prod already, and the null is showing up mysteriously in another module entirely.

  • Zenith (unregistered) in reply to siciac

    As if a DivideByZeroException or ArgumentOutOfBoundsException is any better than a NullReferenceException? At least the null forces a compile-time check. Places that absolutely need a number can call GetValueOrDefault() with a reasonable default (or 0) and override the implicit warning that the null represents.

  • S. (unregistered) in reply to Or do it correctly?

    Nope. C++ standard containers have value semantics, when you pass them "as-is" it means the contents is copied. If you want reference semantics, you have to use it explicitly (ie. add a trailing & to the type). On one hand it gives a lot of control (when you know what you're doing), on the other hand it makes it easier to shoot yourself in the foot...

  • Tim! (unregistered) in reply to Parallel Thinker

    You're correct that those are WTFs when dealing with array mutation in a concurrent context. But the "parallel arrays" anti-pattern has nothing to do with concurrency. This pattern is where you keep multiple arrays of simple data types and associate the data by the accident of having the same index, instead of keeping a single array of a meaningful complex data type.

  • Fernando (unregistered)

    One more: this function doesn't use anything from its class so shouldn't be a member of the class.

  • Sole Purpose of Visit (unregistered)

    Devil's Advocate: This (awful, however "subtly" if you don't know C++) code isn't really a WTF.

    Went into production, was slow as Funk, got called out, was fixed. I'm not even sure it went into production.

    It's the ones that are never subjected to peer review, enter production, and are immortalized because "everything else depends upon this method of sorting!" that are actually the WTFs.

    Although, I'd agree: the lack of a passed-in comparison functor (by reference, natch) and what, although we are only given this as a sideline hint, appears to be a lack of generics ... is a WTF. Quite frankly, you fix those two things at the design stage, and all the pass-by-reference-to-const and what-does-zero-really-mean and the rather peculiar return-by-ref ... just vanishes.

  • Sole Purpose of Visit (unregistered) in reply to A nonny mouse

    Expressing the type concept of a sorted collection is not a very common thing. I agree: it would be useful, if quite frankly not as commonly useful as you might think.

    Scanning through my mental db of languages, I can only come up with PHP7. No, wait, bad drugs! Actually ML in general, I would think ... and I'm not even sure of that. In Haskell, for example, you could most certainly assert the requirement that the type passed in exhibits the property of "sortedness," because Haskell doesn't do classes (eg collections), it does type classes. Which are a higher-level concept.

    But then again, in real life, this hardly matters. If I want to throw the billion items in my sorted list or array or whatever into a Max function, the actual programming language is totally irrelevant. I just need to pick the right function.

    (This one ain't it.)

  • P (unregistered)

    The real wtf: the article is talking about a WTF from an algorithmic and design perspect; the comments instead spend lengths framing said WTF in all kinds of C++ language features. C++ circlejerk much?

    I bet if the code is in Java/JS the comments would've been very different.

  • Supersonic Tumbleweed (unregistered) in reply to Zenith

    ad. 3. you don't want to put extra null checks into core library functions hey, we lost enough performance to latest microcode patches due to spectre

  • Garrett (unregistered)

    This looks like a C# dev decided that C++ works pretty much the same. C# List ~= C++ Vector. The Vector doesn't absolutely need to be const (although there are probably more wtfs in the codebase around this), it needs to be a reference or a pointer.

    double SomeClass::getMaxKeyValue(std::vector<double> &list) or double SomeClass::getMaxKeyValue(std::vector<double> *list)

    The result in cases where the whole list is negative is funny though. I wonder if that just never happens or they gave up before finding out about numeric_limits or -DBL_MAX.

  • (nodebb) in reply to Zenith

    std::max_element has been in the language standard for 20 years. Its use does not tie you to any .NET version. I also never heard about any implementation bugs that were released to the public. The only improvement suggested to it (and other sequence algorithms) is to use a range based on iterator plus sentinel instead of two iterators for greater genericity and performance (Ranges TS). But that change would be source-compatible with almost all existing code.

    Bottom line is that there are about two reasons not to use it:

    • ignorance that it exists
    • terminal stages of NIH syndrome

    Of course, as has been mentioned, using it still requires a not empty check to avoid undefined behavior, as opposed to stupidly returning zero as the "maximum of nothing".

  • MiiNiPaa (unregistered)

    I have issue with claiming "parallel arrays antipattern" without providing context.

    As soon as you start hitting memory throughput issues, you will want all data, processed at once, gathered locally. It will be more cache friendly and it will simplify SIMD, if you want to use it.

    At this time all your arrays of structures will go back to N parallel arrays.

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