Write a hash table in C(github.com) |
Write a hash table in C(github.com) |
- BSD queue.h offers linked lists, queues, etc [1].
- UT-hash offers hash tables [2].
I've also read some people being frustrated with them, probably due to the quirky syntax/API. On the plus side (BSD at least) it's just a header file that you can download, include and start using.
https://attractivechaos.wordpress.com/2008/08/28/comparison-...
(Also provides kbtree.h, ksort.h, kstream.h, kvec.h)
[3] https://attractivechaos.wordpress.com/programs/
Also there are the stb header-only libraries. There is a hash table implementation buried inside stb.h
The standard library does come with a half-baked hash table implementation[1] (hcreate/hdestroy/hsearch) that only allows creation of _one_ global hash table. GNU libc[2] adds hcreate_r/hdestroy_r/hsearch_r that allows multiple tables to be created.
The APIs are strange and antiquated. On the other hand, the hash table implementation described in the article presents a much nicer interface.
[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/hcr...
It looks like the same header, search.h, also includes functions for working with binary trees, where the user passes in the root pointer, and so which support multiple trees:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/ts...
So this committee must have signed off on the hashtable stuff even though they had an example of how to do it properly right next to it!
Signed,
POSIX_ME_HARDER
I think the whole thing would be best as a single page, to be honest. Even if it's long, people have scroll wheels. But it's a stylistic choice.
Also, your double hashing scheme has a bug. You identified that it's problematic if hash_b returns 0, but adding 1 to the result doesn't actually solve anything, because now you have the same problem if hash_b returns num_buckets-1.
Hashtables are one of those data structures one can easily get away with never having to write on their own, yet still use every day. In scripting languages it's impractical to roll your own, & in the more abstracted low level languages there'll usually be a pretty good generic version. Yet there's much room for variation
I had the joy to be out of reach of off-the-shelf hashtables for luwa,
init: https://github.com/serprex/luwa/blob/436c7271120fcd116af30e9...
get/set: https://github.com/serprex/luwa/blob/436c7271120fcd116af30e9...
hash: https://github.com/serprex/luwa/blob/436c7271120fcd116af30e9...
Still need to implement Lua's sequential-portion-as-array logic. I do like the API not having a 'remove' method, opting intead for nil by default. nil value entries get vacuumed out when rehashing. This ends up acting as tombstone. In an earlier iteration I was using 0 as a marker distinct from nil, but an implementation detail is that nil is stored at address 0.. oops
The hard part is allocation memory, keeping track of the structure(array) holding the hashtable, etc.
What else is there? Implementing the hash function/code?
If you don't have a CS degree, you probably don't even know how lists, queues, stacks, etc really work. But the kind of programming that is done at a "high" level, you might not even need know it.
Instead of saying people should implement it JS/Ruby/Python/etc ( all of whom are OOPs ), they should learn C and implement it in C.
Not having to deal with boilerplate and housekeeping lets you focus on learning the topic at hand. Especially in a classroom setting where you're probably focusing on the specific subject instead of real-world dirtiness.
But after learning the basic concept, you do need an actual implementation if you want to do anything.
Also the use of strdup, while a reasonable choice, should also come with caveats about ownership and copying
I stopped at the use of pow for exponentiation in the hash function. Just iteratively multiply your factor and take the modulus on each round.
This would handle utf-8 just fine if characters are cast to unsigned in the hash function and a larger prime factor is used
None of these are real criticisms, I'd just love to see a pointer to more in depth information on how to really write hash functions
I hope that i missed it, but this definitely needs a clear statement that this is a pedagogical exercise and not a production ready implementation
Why not have these anyway? Otherwise people will make their own (probably buggy) equivalents.
I'm actually surprised that people aren't aware of them.
The _r versions look perfectly sensible.
Sure. I'm all for learning ML, Scheme, C/C++, Python, Ruby, Java/C#, etc.
> There's a reason why a lot of CS programs have shuffled from primarily C to Python (some had Java in between).
The reason is called dumbing down the curriculum. My fall freshman year, my college tried to switch from C to Java in order to make it easier for the general population to get into CS and there was a major pushback from the CS student body, alumni and a bunch of the CS professors. We went back to C/C++ my sophomore year.
> Not having to deal with boilerplate and housekeeping lets you focus on learning the topic at hand.
I disagree. Having to deal with the boilerplate and housekeeping helps you learn the topic at hand.
> But after learning the basic concept, you do need an actual implementation if you want to do anything.
I disagree. Giving them "training wheels" will keep them dependent on training wheels. In an idealistic world, people go from Java/Python to C/Assembly, but that's rarely the case.
CS should begin with Math ( boolean algebra/logic/etc ), Assembly and C. Then go on to functional paradigms. Then to OOP. Etc ( in the language ). Of course they should also learn about OS( kernel/filesystems/etc ), networks, computation theory/etc, compilers, etc.
The dumbing down of the curriculum so that music/english majors can pass CS classes doesn't do anyone any favors.
This breaks the HN guideline against flamebait. Please don't post like this here.
Introduction to Programming became a requirement and the drop-out/failure rate for non-CS major students was excessively high so the Dean thought teaching the classes in java rather than c would help out those who were new to programming and help them pass these "newly required" classes.
Personally, I prefer to dig into 1 or 2 programming languages deeply instead of shallowly hop around half a dozen. I seem to retain the deep stuff a lot better. Even when coming back to languages I used heavily years ago (Bash or Perl, for example) I end up having to look up basic things like how loops or functions are defined when I pick them up again.
> The reason is called dumbing down the curriculum.
Do you feel the same way about SICP's choice of using Scheme? One of the innovations of SICP was that CS courses spent too much time explaining the details of a programming language [1]. That was 30 years ago. There are orthogonal discussions on the goals of CS changing over time.
> In an idealistic world, people go from Java/Python to C/Assembly, but that's rarely the case.
Maybe there's no business need? My wife works in video games. That's one industry where programmers often have deep knowledge of hardware and use assembly--but, there's still only a couple of them at each studio. Most people programming are doing game scripting or content creation.
> CS should begin with Math
Your argument seems to be against abstraction. Math is all about abstraction. So is programming. It's been interesting to see programmers tinker with hardware as things like Arduino and Raspberry Pi have become more popular. When writing software they take things for granted, but when interacting with hardware they have to account for things like debounce.
You sound butt-hurt
I'm sorry having standards and expectations is something that offends so many people.
C isn't a "difficult" language. It's something any CS major should have an understanding of.
I can't believe on hackernews, I'd get attacked for saying people should learn C.
Nevermind that most OSes and most languages themselves are written in C. Even python interpreters are written in C.
Since when is saying people should learn C for a CS degree considered being "butthurt"? What's next? Saying people should learn latin/ancient greek for a liberal arts degree is considered being butthurt?
Regardless of how much you may disagree with what someone said or how they said it, attacking them for it (even verbally) is not an appropriate response. It only contributes to worsening the discourse. If you can't find a way to disagree or express an opinion constructively, it's better to refrain from commenting at all.