https://twitter.com/theprimeagen/status/1683671315377541121
best way to describe pointers pic.twitter.com/vwpcRFqwtL
— ThePrimeagen (@ThePrimeagen) July 25, 2023
Pointers
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if you use double pointers anywhere outside plain C (not C++) where it's required in some cases then you should be executed
not an opinion, just an objective fact
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thats why i always use references to pointers to references.
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I remember at one point using some reference objects but I threw in the towel when I realized I could just use shared_ptr instead and it wasn't even in a hot loop so the overhead didn't matter.
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C is my bread and butter. Double pointers for 2d variable length arrays.
Putting the
in 
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That’s cool I guess
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you use
int**for 2d arrays? neighbor that doesn't even work since you've erased the size info from both dimensions, unless by "2d" you mean "array of pointers to arrays of pointers" which isn't 2d, it's just an array of arraysJump in the discussion.
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Yeah and a matrix is just a vector of vectors, what's your point?
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it's not the same in software, mathcel
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what are you talking about, it's different because it:
involves an extra array of pointers in memory
requires two pointer dereferences instead of one to get a value
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Unless you are using a boost array, nothing beats pointers for speed when variable size is required.
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the number of scenarios where you need variable size and also
std::vector<>is too slow for you is vanishingly smallnot to mention that I'm talking about double (or triple or more) pointers, your scenario is only a single pointer. there are certainly some uses for raw pointers in C++ (although
std::unique_ptr<>is better in the vast majority of cases and has zero runtime overhead for dereferencing) but frankly they're few and far between these days.Jump in the discussion.
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Depending on the design space, having triple or double pointers are better for representing a 3D space, with your vector field. If you order your data right, it becomes really easy to pass along entire blocks for MPI messages by abusing how pointers work across rows.
Is dogshit slow if you are initializing a buffer during your computation. It initializes all elements where a raw array does not. This is a huge performance hit when you are talking millions of elements. There is a reason high end computational codes avoid std:vector.
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int**is a pointer to a pointer to an int. Not a "2d array". There's no dimension so if you think you're gonnax[4][2]or something it's not gonna work the way you think it will.If you're really looking for a 2d array you're better off having a plain
int*/int[]buffer in a struct with the dimensions and performing lookups via getters or operator overloads. You can also slice up the larger buffer at will.What
int**is actually used for, in reality, is when you want something like anint*output parameter to a function. Even then a reference would be better so the only acceptable use is if you also need to support passingnullptrto "ignore" that output parameter.Jump in the discussion.
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Its funny, I never said the words 2d array,
Out of the box, no, but you can initialize the outer pointer as an array, and then the inner array and end up with an array of arrays that can be accessed as A[i][k]. Which if you setup right it becomes trivial to pass the start of your sections for MPI with
A + r * m nfor MPI or other parallel paradigms, like passing parts of the array array to a GPU.Let me guess, you're a CS, not a CE?
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You'd need one actual buffer with the data then a separate array to the side that contains pointers into that buffer. This extra array of pointers is generally completely unnecessary, at least if your data is rectangular - the only possible requirement for it is if you have a "jagged 2d array", basically where each row has its own length.
What you're describing is adding a whole extra array and indirection because you're too r-slurred to use an operator overload or getter, and apparently don't think compilers optimize getters (hint: they're virtually always inlined unless you define them outside the class/struct def).
Let me guess, you do this shit on some paltry raspberry pi "cluster" and don't actually have a job doing it for real?
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So I guess the answer to my question is "Yes" you are a CS kiddie, based on not actually replying to what I wrote.
You are not going to what to use getter on a GPU, you want to pass the least amount of data to the GPU. For instance of you are running Strassen's Algorithm you can pass the submatricies using linear offsets to MPI, and derefencing the outer pointer after your linear offset lets you pass that submatrix to CUDA. This becomes a lot more important when your Matrix is actually a third order tensor with a vector of your flow field, which keeps it cleaner.
I do have a job doing this, I even helped get one of our GPU codes to work on Summit.
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There's literally nothing stopping you from slicing up a buffer and you don't need a separate array of pointers off to the side to do it.
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C is still useful in some embedded contexts where you don't want to bring along an entire C++ runtime but yeah that's about it.
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And building robots that run on microcontrollers is really fun so everyone should learn c
Putting the
in 
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This
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When I don’t use C I use Go or, if really lazy, Python
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Python is just so cozy
I use it for the vast majority of my personal projects since the data sets I'm working with aren't large enough to need better performance and they aren't complicated enough to require static typing.
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This feels less and less common, no?
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Yeah, it really is - I work in embedded and the hardware I'm working with is powerful enough that we can easily use modern C++ and not care. The efficiency gains (whether in execution speed or memory usage) are so small they're not worth the trade-off in developer time.
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It is. People tend to focus on cost improvements with high-end hardware but the same is true of low-end stuff, actually the effect is probably felt even more there. Realistically there's just little reason to have extremely weak microprocessors anymore. You can get something that can run C++ just fine for very very cheap and with low power draw. There are obviously still some specialty applications where you run on a tiny microcontroller with 128 KB RAM (for example) but that's become less and less common.
I just checked and you can buy a Raspberry Pi Zero 2 W for $15. That's a full butt computer that runs Linux with 512 MB RAM, a wifi chip, various connectors (including mini HDMI and video output) for $15. Obviously a major engineering firm would be designing their own boards and stuff and not using this but that's just an example for how cheap low-end microprocessors are these days.
tbh at this point I think most of the embedded systems that are only running C and not at least C++ are doing so for legacy reasons.
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I'm seeing rumblings of Rust being popular for embedded too but clearly that'll take another five years to be really established.
But yeah, I remember the cool HN post of a guy finding a cheap enough chip to run Linux on for his business cards.
https://www.thirtythreeforty.net/posts/2019/12/my-business-card-runs-linux/
Appears to be 2019 even.
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I actually really like Rust and hope it takes off in this space. The community is
garbage but the actual language and ecosystem are good.
I haven't worked in embedded development for many years, but even when I did, the newer hardware we were rolling out was like 20x more powerful than what came before, and cheaper. It was actually kinda annoying because they figured with the substantially more powerful microprocessor, they didn't need any dedicated chips to run the serial buses. Works fine in practice but it means if you have to debug anything on the board, every time you hit a breakpoint (even a conditional breakpoint that misses) it fricks all the serial connections up. Made debugging a bit of a pain.
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Any opinion on the Zephyr RTOS?
Kind of funny that it’s C when we were talking that becoming less and less popular but it seems to have a really good build system.
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Sorry no, it's been many years since I've worked with an RTOS. I had to look up a list of them to find the one my old job used, it was ThreadX. It seemed fine, at least while I was there I didn't uncover any bugs in the RTOS so that's about as good as it gets. All the boards it ran on were custom/proprietary as well so I doubt any of the free ones would work right out the box.
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C is so 1970s. The future is JavaScript, bros.
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I did some triple pointer array for a project in my freshman year C++ class. The teacher said something along the lines of "well I know you didn't have someone do this for you since anyone who knew what they were doing wouldn't have used triple pointers".
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i actually laughed thanks
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Remind me what's the point of pointers ?
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to point at soys
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They're God's greatest
gift to humanity (along with the goto statement)
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To point
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Gay nerd shit
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Explain it to me like I'm r-slurred
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I have a stick
I also have a map that "points" to where the stick is
I also have a map that "points" to that map
ect.
seg fault
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Oh I see now why triple pointers are probably unnecessary
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Like others have mentioned, there is some dark magic that can be done with triple pointers but if you have to use triple pointers to do it you are probably doing it wrong anyway.
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Befriend Jannies. Respect Jannies. Roundhouse kick Janny friendships into existence. Slam dunk some healthy fruit and vegetables into Janny babies' mouths. Love thy Janny. Share some food with a Janny. Launch Jannies into orbit as part of high-paying astronaut jobs. Treat Jannies like human beings. Warn Jannies not to fall into active volcanoes. Share life experiences with Jannies. Watch TV and play video games with Jannies. Invite Jannies to parties. Report Jannies to the Nobel foundation. Karate chop ban evaders in half. Give up your seat for pregnant Jannies. Free Jannies from quicksand. Appreciate Jannies. Observe Janny history month. Eat with Jannies. Judge Jannies by the content of their character. Dance with Jannies with steel-toed boots. Cremate Jannies in the oven, but only if their family didn't want them to be buried. Dignify Jannies. Civil rights for Jannies. Collect Jannies garbage to put in the garbage disposal. Surgically reconstruct Jannies with a ray gun. Help old Jannies cross the road. Admire Jannies. Slice word filter evaders with a katana.
Snapshots:
archive.org
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archive.ph (click to archive)
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