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The below commands helped me:

I suspect this may have been an accident, though I prefer the new behavior.

The new behavior is a consequence of a change to how the bytecode for * arguments works. The change is in the changelog under Python 3.9.0 alpha 3:

bpo-39320: Replace four complex bytecodes for building sequences with three simpler ones.

The following four bytecodes have been removed:

• BUILD_LIST_UNPACK
• BUILD_TUPLE_UNPACK
• BUILD_SET_UNPACK
• BUILD_TUPLE_UNPACK_WITH_CALL

The following three bytecodes have been added:

• LIST_TO_TUPLE
• LIST_EXTEND
• SET_UPDATE

On Python 3.8, the bytecode for f(*a, a.pop()) looks like this:

To prove that it's correct, you have to find some sort of invariant. Something that's true during every pass of the loop.

Looking at it, after the very first pass of the inner loop, the largest element of the list will actually be in the first position.

Now in the second pass of the inner loop, i = 1, and the very first comparison is between i = 1 and j = 0. So, the largest element was in position 0, and after this comparison, it will be swapped to position 1.

In general, then it's not hard to see that after each step of the outer loop, the largest element will have moved one to the right. So after the full steps, we know at least the largest element will be in the correct position.

What about all the rest? Let's say the second-largest element sits at position i of the current loop. We know that the largest element sits at position i-1 as per the previous discussion. Counter j starts at 0. So now we're looking for the first A[j] such that it's A[j] > A[i]. Well, the A[i] is the second largest element, so the first time that happens is when j = i-1, at the first largest element. Thus, they're adjacent and get swapped, and are now in the "right" order. Now A[i] again points to the largest element, and hence for the rest of the inner loop no more swaps are performed.

So we can say: Once the outer loop index has moved past the location of the second largest element, the second and first largest elements will be in the right order. They will now slide up together, in every iteration of the outer loop, so we know that at the end of the algorithm both the first and second-largest elements will be in the right position.

What about the third-largest element? Well, we can use the same logic again: Once the outer loop counter i is at the position of the third-largest element, it'll be swapped such that it'll be just below the second largest element (if we have found that one already!) or otherwise just below the first largest element.

Ah. And here we now have our invariant: After k iterations of the outer loop, the k-length sequence of elements, ending at position k-1, will be in sorted order:

After the 1st iteration, the 1-length sequence, at position 0, will be in the correct order. That's trivial.

After the 2nd iteration, we know the largest element is at position 1, so obviously the sequence A[0], A[1] is in the correct order.

Now let's assume we're at step k, so all the elements up to position k-1 will be in order. Now i = k and we iterate over j. What this does is basically find the position at which the new element needs to be slotted into the existing sorted sequence so that it'll be properly sorted. Once that happens, the rest of the elements "bubble one up" until now the largest element sits at position i = k and no further swaps happen.

Thus finally at the end of step N, all the elements up to position N-1 are in the correct order, QED.

The code shown is valid (all C++ Standard versions, I believe). The similar restrictions are all listed in [reserved.names]. Since read is not declared in the C++ standard library, nor in the C standard library, nor in older versions of the standard libraries, and is not otherwise listed there, it's fair game as a name in the global namespace.

So is it an implementation defect that it won't link with -static? (Not a "compiler bug" - the compiler piece of the toolchain is fine, and there's nothing forbidding a warning on valid code.) It does at least work with default settings (though because of how the GNU linker doesn't mind duplicated symbols in an unused object of a dynamic library), and one could argue that's all that's needed for Standard compliance.

We also have at [intro.compliance]/8

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that use such extensions that are ill-formed according to this International Standard. Having done so, however, they can compile and execute such programs.

We can consider POSIX functions such an extension. This is intentionally vague on when or how such extensions are enabled. The g++ driver of the GCC toolset links a number of libraries by default, and we can consider that as adding not only the availability of non-standard #include headers but also adding additional translation units to the program. In theory, different arguments to the g++ driver might make it work without the underlying link step using libc.so. But good luck - one could argue it's a problem that there's no simple way to link only names from the C++ and C standard libraries without including other unreserved names.

(Does not altering a well-formed program even mean that an implementation extension can't use non-reserved names for the additional libraries? I hope not, but I could see a strict reading implying that.)

So I haven't claimed a definitive answer to the question, but the practical situation is unlikely to change, and a Standard Defect Report would in my opinion be more nit-picking than a useful clarification.

Yes, alloca is functionally equivalent to a local variable length array, i.e. this:

std::memcpy(arr_copy, arr, sizeof arr); (your example) is well-defined.

std::memcpy(arr_copy, arr[0], sizeof arr);, on the other hand, causes undefined behavior (at least in C++; not entirely sure about C).

Multidimensional arrays are 1D arrays of arrays. As far as I know, they don't get much (if any) special treatment compared to true 1D arrays (i.e. arrays with elements of non-array type).

Consider an example with a 1D array:

The __enter__ method should return the context object. with ... as ... uses the return value of __enter__ to determine what object to give you. Since your __enter__ returns nothing, it implicitly returns None, so test is None.

I don't think there is a single right answer to this, but here is a couple of points to consider.

• First, I think real-world functional code often has a "sandwich structure" with some input handling, followed by purely functional transformation and some output handling. The I/O parts in F# often involve interfacing with imperative and OO .NET libraries. So, the key lesson is to keep the I/O to the outside and keep the core functional handling separate from that. In other words, it makes perfect sense to use some imperative OO code on the outside for input handling.

• Second, I think the idea of decoupling is something that is more valuable in OO code where you expect to have complex interfaces with intertwined logic. In functional code, this is (I think) much less of a concern. In other words, I think it is perfectly reasonable not to worry about this for I/O, because it is only the outer side of the "sandwich structure". If you need to change it, you can just change it without touching the core functional transformation logic (which you can test independently of I/O).

• Third, on the practical side, it is perfectly reasonable to use interfaces in F#. If you really wanted to do the decoupling, you could just define an interface:

Objects in C++ often have destructors that need to run at the end of their lifetime. delete[] makes sure the destructors of each element of the array are called. But doing this has unspecified overhead, while delete does not. One for arrays, which pays the overhead and one for single objects which does not.

In order to only have one version, an implementation would need a mechanism for tracking extra information about every pointer. But one of the founding principles of C++ is that the user shouldn't be forced to pay a cost that they don't absolutely have to.

Always delete what you new and always delete[] what you new[]. But in modern C++, new and new[] are generally not used anymore. Use std::make_unique, std::make_shared, std::vector or other more expressive and safer alternatives.