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QUESTION
Update: the root issue was a bug which was fixed in Spark 3.2.0.
Input df structures are identic in both runs, but outputs are different. Only the second run returns desired result (df6
). I know I can use aliases for dataframes which would return desired result.
The question. What is the underlying Spark mechanics in creating df3
? Spark reads df1.c1 == df2.c2
in the join
's on
clause, but it's evident that it does not pay attention to the dfs provided. What's under the hood there? How to anticipate such behaviour?
First run (incorrect df3
result):
ANSWER
Answered 2021-Sep-24 at 16:19Spark for some reason doesn't distinguish your c1
and c2
columns correctly. This is the fix for df3
to have your expected result:
QUESTION
Is the Shannon-Fano coding as described in Fano's paper The Transmission of Information (1952) really ambiguous?
In Detail:3 papers
Claude E. Shannon published his famous paper A Mathematical Theory of Communication in July 1948. In this paper he invented the term bit as we know it today and he also defined what we call Shannon entropy today. And he also proposed an entropy based data compression algorithm in this paper. But Shannon's algorithm was so weak, that under certain circumstances the "compressed" messages could be even longer than in fix length coding. A few month later (March 1949) Robert M. Fano published an improved version of Shannons algorithm in the paper The Transmission of Information. 3 years after Fano (in September 1952) his student David A. Huffman published an even better version in his paper A Method for the Construction of Minimum-Redundancy Codes. Hoffman Coding is more efficient than its two predecessors and it is still used today. But my question is about the algorithm published by Fano which usually is called Shannon-Fano-Coding.
The algorithm
This description is based on the description from Wikipedia. Sorry, I did not fully read Fano's paper. I only browsed through it. It is 37 pages long and I really tried hard to find a passage where he talks about the topic of my question, but I could not find it. So, here is how Shannon-Fano encoding works:
- Count how often each character appears in the message.
- Sort all characters by frequency, characters with highest frequency on top of the list
- Divide the list into two parts, such that the sums of frequencies in both parts are as equal as possible. Add the bit
0
to one part and the bit1
to the other part. - Repeat step 3 on each part that contains 2 or more characters until all parts consist of only 1 character.
- Concatenate all bits from all rounds. This is the Shannon-Fano-code of that character.
An example
Let's execute this on a really tiny example (I think it's the smallest message where the problem appears). Here is the message to encode:
ANSWER
Answered 2022-Mar-08 at 19:00To directly answer your question, without further elaboration about how to break ties, two different implementations of Shannon-Fano could produce different codes of different lengths for the same inputs.
As @MattTimmermans noted in the comments, Shannon-Fano does not always produce optimal prefix-free codings the way that, say, Huffman coding does. It might therefore be helpful to think of it less as an algorithm and more of a heuristic - something that likely will produce a good code but isn't guaranteed to give an optimal solution. Many heuristics suffer from similar issues, where minor tweaks in the input or how ties are broken could result in different results. A good example of this is the greedy coloring algorithm for finding vertex colorings of graphs. The linked Wikipedia article includes an example in which changing the order in which nodes are visited by the same basic algorithm yields wildly different results.
Even algorithms that produce optimal results, however, can sometimes produce different optimal results based on tiebreaks. Take Huffman coding, for example, which works by repeatedly finding the two lowest-weight trees assembled so far and merging them together. In the event that there are three or more trees at some intermediary step that are all tied for the same weight, different implementations of Huffman coding could produce different prefix-free codes based on which two they join together. The resulting trees would all be equally "good," though, in that they'd all produce outputs of the same length. (That's largely because, unlike Shannon-Fano, Huffman coding is guaranteed to produce an optimal encoding.)
That being said, it's easy to adjust Shannon-Fano so that it always produces a consistent result. For example, you could say "in the event of a tie, choose the partition that puts fewer items into the top group," at which point you would always consistently produce the same coding. It wouldn't necessarily be an optimal encoding, but, then again, since Shannon-Fano was never guaranteed to do so, this is probably not a major concern.
If, on the other hand, you're interested in the question of "when Shannon-Fano has to break a tie, how do I decide how to break the tie to produce the optimal solution?," then I'm not sure of a way to do this other than recursively trying both options and seeing which one is better, which in the worst case leads to exponentially-slow runtimes. But perhaps someone else here can find a way to do that>
QUESTION
I'm following the instructions on how to add an existing project to GitHub from GitHub docs. However, I get an 'gh' is not recognized as an internal or external command, operable program or batch file.
error message when I try to use gh repo create fs1
command in the project root directory as instructed. Here fs1
is my project name. I browsed StackOverflow but couldn't find any help yet. What am I missing?
ANSWER
Answered 2021-Aug-27 at 10:33The page you are following doesn't include the instructions to install the GitHub CLI, so it is likely you don't have it installed (it doesn't come with git as far as I know).
Installation instructions can be found here: https://cli.github.com/
QUESTION
I'm attempting to implement a subclass of QQuickAsyncImageProvider
in Pyside6, referring to the official example. I'm already aware of the potential hurdle that, unlike in the C++ example code, with PySide it's not possible to inherit from both QRunnable
and QObject
. In order to send a signal from my QRunnable
I'm instead using an intermediary QObject
, as suggested here. That part works fine.
Here's my attempt so far at using this arrangement along with my own subclass of QQuickAsyncImageProvider
:
ANSWER
Answered 2022-Feb-22 at 13:19I reported this in case it was a bug, and one of the maintainers realized that the issue is that the AsyncImageResponse
gets immediately deleted. The workaround for this is to hold a reference to it in the AsyncImageProvider
:
QUESTION
I wondered if there is an equivalent to the browser()
statement available in RStudio for debugging purposes for Julia (I am using the Juno IDE at the moment).
The R
function browser()
halts execution and invokes an environment browser
when it is called. So, in principle, we can put browser()
anywhere in our code to stop in this particular line and see what's stored in the environment at that moment, which is terrific for debugging purposes.
For instance, the code below will stop when i>3
. Hence, that's exactly what we will see in the environment browser
available in RStudio, where we will observe that i=4
at that moment in the code.
ANSWER
Answered 2022-Feb-11 at 08:31Have a look at Debugger.jl. Specifically the Place breakpoints in source code section:
It is sometimes more convenient to choose in the source code when to break. This is done for instance in Matlab/Octave with keyboard, and in R with browser(). You can use the @bp macro to do this
Your R example translated to Julia:
QUESTION
Looking into UTF8 decoding performance, I noticed the performance of protobuf's UnsafeProcessor::decodeUtf8
is better than String(byte[] bytes, int offset, int length, Charset charset)
for the following non ascii string: "Quizdeltagerne spiste jordbær med flØde, mens cirkusklovnen"
.
I tried to figure out why, so I copied the relevant code in String
and replaced the array accesses with unsafe array accesses, same as UnsafeProcessor::decodeUtf8
.
Here are the JMH benchmark results:
ANSWER
Answered 2022-Jan-12 at 09:52To measure the branch you are interested in and particularly the scenario when while
loop becomes hot, I've used the following benchmark:
QUESTION
I have just browsed the Linux kernel source tree and read the file tools/include/nolibc/nolibc.h.
I saw the syscall
in this file uses %r8
, %r9
and %r10
in the clobber list.
Also there is a comment that says:
rcx and r8..r11 may be clobbered, others are preserved.
As far as I know, syscall
only clobbers %rax
, %rcx
and %r11
(and memory).
Is there a real example of syscall
that clobbers %r8
, %r9
and %r10
?
ANSWER
Answered 2022-Jan-05 at 10:28Only 32-bit system calls (e.g. via int 0x80
) in 64-bit mode step on those registers, along with R11. (What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?).
syscall
properly saves/restores all regs including R8, R9, and R10, so user-space using it can assume they keep their values, except the RAX return value. (The kernel's syscall entry point even saves RCX and R11, but at that point they've already been overwritten by the syscall
instruction itself with the original RIP and before-masking RFLAGS value.)
Those, with R11, are the non-legacy registers that are call-clobbered in the function-calling convention, so compiler-generated code for C functions inside the kernel naturally preserves R12-R15, even if an asm entry point didn't save them.
Currently the 64-bit int 0x80
entry point just pushes 0
for the call-clobbered R8-R11 registers in the process-state struct that it will restore from before returning to user space, instead of the original register values.
Historically, the int 0x80
entry point from 32-bit user-space didn't save/restore those registers at all. So their values were whatever compiler-generated kernel code left sitting around. This was thought to be innocent because 32-bit mode can't read those registers, until it was realized that user-space can far-jump to 64-bit mode, using the same CS value that the kernel uses for normal 64-bit user-space processes, selecting that system-wide GDT entry. So there was an actual info leak of kernel data, which was fixed by zeroing those registers.
IDK whether there used to be or still is a separate entry point from 64-bit user-space vs. 32-bit, or how they differ in struct pt_regs
layout. The historical situation where int 0x80
leaked r8..r11 wouldn't have made sense for 64-bit user-space; that leak would have been obvious. So if they're unified now, they must not have been in the past.
QUESTION
In Excel VBA with the SQLite ODBC Driver, my simple SELECT query run against a single table retrieves 'long' integers (10 or more decimal places) incorrectly. How can these values be retrieved correctly, without truncation or whatever garbling is going on?
(PLEASE NOTE: the database structure/field definitions can't be modified — the database belongs to an open source application, Anki, and changing the structure would break the software.)
The particular table I'm querying contains (at least) several fields that can contain longer integer values (10 or more decimal places). The primary key ("id") contains Unix timestamp (datetime) values, with milliseconds, so the integer in the primary key field always occupies 13 decimal places.
Here is the table definition:
...ANSWER
Answered 2022-Jan-02 at 17:15As commented, essentially the issue derives from how to include the BigInt
parameter in connection string. While MSDN documentation appears to differ from implementation, key/value pairs should avoid whitespaces:
QUESTION
I know Apple provides tensorflow-metal on their conda -c apple
channel.
How do I see the rest of the packages in that channel?
The "answers" I found online address finding a package to install, like conda search *tensorflow*
but this is not what I want.
Instead, I would like to see what packages Apple has in their -c apple
channel that I can install: in short "browse apple repository", a concept that I'm surprised is so difficult to achieve under conda...
p.s. I have tried conda search -c apple
but it only lists all the packages from conda-forge
, even after I did a conda config --add channels apple
.
ANSWER
Answered 2021-Dec-07 at 03:39You can browse all packages in an Anaconda Cloud channel by visiting:
QUESTION
I'm trying to create a single file asp.net core 5 web app. Goal is to have a single .exe file, run the Kestrel server by executing this exe file and load the page in the browser.
I created an ASP.NET Core 5 template app in VS 2019. Then using cli I run this command:
...ANSWER
Answered 2021-Nov-29 at 21:46I tried to reproduce your problem on new asp net core empty project and it works fine.
Perhaps Startup is missing some configuration.
Here's what I did.
csproj
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