Looking at your main function, I see this: I don't know what REP() or RI is, so I have to look it up. Sep 7, 2020. @ yes, consteval is an improvement. But the question holds. This is tough because you intimately know what REP() and RI() does, so this looks really obvious to you. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. How to maximize the monthly 1:1 meeting with my boss? The behavior is undefined if the node handle is empty. how could I initialize unordered_map< vector >? Overvoltage protection with ultra low leakage current for 3.3 V. In the final act, how to drop clues without causing players to feel "cheated" they didn't find them sooner? do need a very large size dynamic map, please use std::map instead! Asking for help, clarification, or responding to other answers. What does std::unordered_map::clear() do? I see no reason for this code (that contains at most a few thousands simple operations) to last this long: So my main question is: Which data structure should I use instead of unordered_map to make this code as efficient as possible? Not the answer you're looking for? Insert with object as key fails to compile? This, not only clean up the code It does one pass of iterating through all the elements and destroying them, then a memset to reset all bucket pointers to null. Difference between machine language and machine code, maybe in the C64 community? The map throws a runtime error for non-existent keys like m2["gamma"], when my objective is to enforce this as a compile-time error. std::unordered_map<Key,T,Hash,KeyEqual,Allocator>:: try_emplace - Reference In other words, change the code from this: The code currently has a number of lines like this: This can easily be simplified using uniform initialization syntax. C++ lacks the possibility to force compile time evaluation which is terrible. Connect and share knowledge within a single location that is structured and easy to search. 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned. Find centralized, trusted content and collaborate around the technologies you use most. For the small fixed-size constexpr Map, the compiler normally has more predictions, and so on. Timing unoptimized debug builds is a waste of time. custom minimalist std::map with constexpr operator [] I am currently working on implementing a map container in C++, which should be able to function as a compile-time constant. c++ - Static constexpr of set - Stack Overflow Take the existing item and modify it. Formulating P vs NP without Turing machines. @0x499602D2 Because there is no such a constructor in the class std::unordered_map. Containers library std::unordered_map Unordered map is an associative container that contains key-value pairs with unique keys. I don't think it worth slowing down compilation with no realistic gain. The most obvious flag should be its dynamic structure. Implementation of find is incredibly slower in unordered map? What is the best way to visualise such data? And it's enough for my problem to use std::array<std::pair<K, V> >. In this table, a key-value pair such as m ["pi"] should evaluate to its corresponding . Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Taking your deuxpownmodprime() function for example, it took me too long to understand what it says. UPD: We found that std::array has a lot of constexpr methods. Asking for help, clarification, or responding to other answers. This page has been accessed 87,655 times. How to maximize the monthly 1:1 meeting with my boss? That would include thinks like make_sorted_array which would allow one to pass elements as parameter and return sorted array object. I would like to initialize my const unordered_map in the class initializer list. How can I use std::maps with user-defined types as key? Making statements based on opinion; back them up with references or personal experience. What should be chosen as country of visit if I take travel insurance for Asian Countries. std::vector does not meet these requirements. About; Products . The code we write is written first for human consumption & only secondarily for the computer to understand. The previous loop can similarly be written like this: This looks like a clever algorithm, but it's not obvious how it works or why. Error using custom operator< with std::less. I was astonished by how GCC This page has been accessed 140,701 times. That misidentifies the problem. Do large language models know what they are talking about? What does GCC's implementation do here? How to get rid of the boundary at the regions merging in the plot? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why there's no constexpr version of std::map allocated on - Reddit So in fact it always takes extra time linear in the number of buckets, even if this is "unnecessary" due to there having been no elements in the first place. What are the implications of constexpr floating-point math? optimizing this piece of code. In main, the variables decomps and mod are actually used as constants, so it would make sense to at least declare them as const and preferably constexpr. Connect and share knowledge within a single location that is structured and easy to search. \begin{array}{|l|r|} Had to rewrite the find_if, because it is not consteval. on Jason goes through the assembly GCC generated! All examples I've found but one are non const, and I'm doing it exactly as the only const example I know, but I get compiler errors. Can `head` read/consume more input lines than it outputs? IMPORTANT: constexpr Map is intended to use for small size lookup, if you Should I sell stocks that are performing well or poorly first? Unused variables are a sign of poor code quality, so eliminating them should be a priority. Asking for help, clarification, or responding to other answers. Upcoming Conference Workshops (Swag, links and more) C++ Best Practices Workshop, Sept 18, 19, Norway: https://ndctechtown.com/workshops/c-best-practice. And if the Standard Library provides things that are Fast Enough, it's not advisable to tie yourself to some external library instead without specific, well quantified reasons. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. In a lot of cases, the answer to "why is there no ___ in std" is because ____ is a great idea, but nobody has thought of it, or they've thought of it but everyone's been working on higher-priority changes. Is it possible to have an unordered_map with non-const key type? Assuming constant operation cost, are we guaranteed that computational complexity calculated from high level code is "correct"? Search, removal, and insertion operations have logarithmic complexity. Boost::unordered_map with initialization list? It only takes a minute to sign up. Each node stores a pair of a key and a value. Connect and share knowledge within a single location that is structured and easy to search. If a key is not present, a compile-time error should be thrown. In addition, an empty node handle can be produced by a failed call to container member function extract. custom minimalist std::map with constexpr operator[] C++ - Qiita What syntax could be used to implement both an exponentiation operator and XOR? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. I don't know if it's fixed now but older releases (e.g. To learn more, see our tips on writing great answers. @chris, I haven't had any problems using std::array in constexpr functions with any number of elements. @AbsolutelyFreeWeb So download Visual Studio 2015 Community edition? How can we compare expressive power between two Turing-complete languages? Do starting intelligence flaws reduce the starting skill count, Options to insulate basement electric panel. Results of that single change on the sample data provided in the question: $$ This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. Passion for learning and sharing knowledge! The behavior is undefined if the node handle is empty. Otherwise iterators are not affected. \text{original C++} & 2475 \\ By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. - DeiDei Dec 16, 2017 at 14:54 You cannot have a constexpr object of std::unordered_set<.>, it has no constexpr constructor. Difference between machine language and machine code, maybe in the C64 community? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Do not concentrate on such performance optimizations before the program itself is ready. Of course, if you have any other remark on how I use C++, I will welcome your insights. Use std::cin instead: find() and operator[] both do essentially the same thing. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How to resolve the ambiguity in the Boy or Girl paradox? Rehashing occurs only if the new number of elements is greater than max_load_factor() * bucket_count(). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Looking for advice repairing granite stair tiles, Options to insulate basement electric panel. Understanding constexpr functions with templates [C++], Non-Arrhenius temperature dependence of bimolecular reaction rates at very high temperatures. Here is a simple sum function showing the difference between compile time and runtime. Why a kite flying at 1000 feet in "figure-of-eight loops" serves to "multiply the pulling effect of the airflow" on the ship to which it is attached? Remember that std::unordered_map is available with C++11. C++ Standard Library Associative Containers | hacking C++ gcc.gnu.org/bugzilla/show_bug.cgi?id=67922. Node handles are move-only, the copy constructor is not defined. CodeProject, 20 Bay Street, 11th Floor Toronto, Ontario, Canada M5J 2N8 +1 (416) 849-8900 To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This page was last modified on 12 October 2021, at 00:42. C++ Containers library std::unordered_map 1) Checks if there is an element with key equivalent to key in the container. 1-2) Inserts value. Is there any political terminology for the leaders who behave like the agents of a bigger power? Candidate expects 0 arguments, 2 provided (std::pair) In this table, a key-value pair should evaluate to its corresponding value, during the compilation. However I'm yet to find the syntax that the com. The aim here is not only to avoid the necessity of runtime computations, but also to make the code more clear by communicating that the map is a predefined static lookup table, and not something that will change dynamically during runtime. Until this blog last modified date, there is still no constexpr constructor Find centralized, trusted content and collaborate around the technologies you use most. This is a quick sketch of a completely static map, assuming you have C++20 floating-point non-type template parameter support: If you don't have floating-point non-type template parameters yet, you still need a distinct type for each constant, but now it has to actually store a member of type double (and you need an actual tuple instance storing those objects). \hline Library building -- remove limitations, inefficiencies, and irregularities from the abstraction mechanisms. rev2023.7.3.43523. for more details. Python code that does the exact same thing but working faster: Alright, I finally decided to actually review how you use C++ this because I want to practice structuring feedback for code structured like this. I have an unordered map: unordered_map<int, std::pair<Data, int>> m_map. once more, just like when you using a const specifier, but the compiler also Thanks for contributing an answer to Code Review Stack Exchange! Formulating P vs NP without Turing machines, Institutional email for mathematical organization. As much as possible, you want to write code in a way that makes reading it possible without having to jump up and down the page. Other two cases are not applicable as well, so you cannot mark std::unordered_set as constexpr. \text{Python 2.7} & 15 \\ Different ways to initialize an unordered_map in C++: Initialization using assignment and subscript operator benefit from the constexpr Map. Frozen data structures in C++14 - GitHub Pages This work is licensed under CC BY NC ND 4.0. On the other hand you could write map_({{23, 1345}, {43, -8745}}). Can Genesis 2:17 be translated "dying you shall die"? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, So it sounds like you are looking for something like. Learn more about Stack Overflow the company, and our products. Inserts element (s) into the container, if the container doesn't already contain an element with an equivalent key. start using this from now on! \text{C++ with const ref} & 3 \\ c++ - What are constexpr containers in C++17? - Stack Overflow Difference between machine language and machine code, maybe in the C64 community? Can you provide an example that demonstrates multiple overloaded constructors? Did you thought about unordered_maps? Or the syntax has to be different? Does the EMF of a battery change with time? Why can clocks not be compared unless they are meeting? Developers use AI tools, they just dont trust them (Ep. What is the best way to visualise such data? Book about a boy on a colony planet who flees the male-only village he was raised in and meets a girl who arrived in a scout ship. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Assuming constant operation cost, are we guaranteed that computational complexity calculated from high level code is "correct"? performance with the compile speed. Keys are sorted by using the comparison function . You need VS 15 or better (compiler version 19). If rehashing occurs due to the insertion, all iterators are invalidated. Cannot reach godbolt right now, I will test it tomorrow. I want to make a class that has methods like std::map, but it should be sorted at compile time.Which constexpr containers are suited to store keys template<class K> and values template<class V>?. Does this change how I list it on my CV? The definition of reserve(N) for unordered associative containers is that it allocates enough buckets such that the load factor of the table would be less than or equal to the max load factor if there were N elements in the container. 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned, Initializing a static std::map in C++. I was solving a CodeForces problem, and kept getting Time Limit Exceeded, so as usual in these cases, I decided to try to migrate my code from Python to C++ in order to go faster. On the other hand you could write map_({{23, 1345}, {43, -8745}}) - Vlad from Moscow. I'm trying to find a solution to what may be a very trivial problem. Should i refrigerate or freeze unopened canned food items? Can be simplified to this: std::unordered_map<long, long> ot{{0,1}}; Use constexpr where practical. Any suggestions or pointers to enhance the implementation in the context of these issues would be highly appreciated. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. And also, the pointer is only being accessed once. Why would the Bank not withdraw all of the money for the check amount I wrote? Instead of having a separate linked list for each bucket, std::unordered_map implementations often use a single linked list (making iteration faster), with buckets pointing to locations within the single linked list. constexpr is such a stunning idea first introduced in c++11, which lets the Thanks for contributing an answer to Stack Overflow! The compiler may choose not to actually execute it at compile time, for any reason. I want to define a function that emplaces a copy of Data object. absl::node_hash_map, on the other hand, has each bucket pointing to a single value, and collisions are handled with open addressin. Can Genesis 2:17 be translated "dying you shall die"? You should not be using macros like this ever. std::unordered_map<Key,T,Hash,KeyEqual,Allocator>:: In this code, nbm is defined but unused. "#pragma GCC optimize ("O0")" seems to point at a weird, non-standard, way of ensuring a non-optimal unoptimized build. Practice unordered_map is an associated container available in the C++ Standard Template Library (STL) that is used to store key-value pairs. A constexpr variable must satisfy the following requirements: Now, from literal type we can conclude that a literal type might be an an aggregate type, a type with at least one constexpr (possibly template) constructor that is not a copy or move constructor or, since C++17, a closure type. Constant on average, worst case linear in the size of the container. How could the Intel 4004 address 640 bytes if it was only 4-bit? Developers use AI tools, they just dont trust them (Ep. Connect and share knowledge within a single location that is structured and easy to search. Because the DEMO Thus size () is usually between 0.4375*bucket_count () and 0.875*bucket_count (). Optionally uses user supplied bucket_count as a minimal number of buckets to create, hash as the hash function, equal as the function to compare keys and alloc as the allocator. Unoptimized builds (the default for most compilers) only care about generating code that is easy to debug and performance is secondary and debug builds are notoriously slow and do lots of things that the optimizer usually just removes. To illustrate, I am aiming to create a map similar to the following: I want m["pi"] to evaluate to 3.14 during compilation, while m["gamma"] should produce a compile-time error. I am currently working on implementing a map container in C++, which should be able to function as a compile-time constant. Why are lights very bright in most passenger trains, especially at night? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Therefore, this function should do nothing in my program, and should take much less than a millisecond. Why did only Pinchas (knew how to) respond? It's really important that you put yourself in the shoes of someone coming in to your code fresh. The results turn out to be 9.6 times speedup from this constexpr Map To learn more, see our tips on writing great answers. It's not the data structure but rather its misuse that is the cause of the performance problem here. They may well be Fast Enough for typical uses, when used properly, as Frank corrected - & which let them beat the original Python, which was the aim (without any stated time limit). std::vector does not meet these requirements. The compiler could do all kind of crazy things to optimize your code, and with The behavior is undefined if both nodes are not empty and std::allocator_traits::propagate_on_container_swap is false and the allocators do not compare equal. Performance-improving alternatives include You should use one of several alternatives to them, such as Google's sparse or dense hash, Intel TBB's maps, khash, uthash and others mentioned at the link. You either hit a compiler bug, your compiler does not support the latest C++ standard. However I'm yet to find the syntax that the compiler (GCC 6.2.0) will accept. Making statements based on opinion; back them up with references or personal experience. I find myself choosing inferior libs choices and trying to find workarounds bc of it :(. add a pair into an unorderedmap with minimum copies directly assigns it to the register, therefore, all the function stack could If you would know standard algorithms, you wouldnt need most of those macros. Then I would have been able to understand it all without having to interrupt my reading, and really, it's not really any harder to write. The max load factor is 87.5%, after which the table doubles in size (making load factor go down by 2x). : consteval Value operator[](const Key& key) const { for (auto i: data) { if (std::get<0>(i)==key) { return std::get<1>(i); } } }, As @useless suggests, if you were to put in a distinct type into, custom minimalist std::map with constexpr operator[]. I want to emphasize again if you have a very large map size, std::map binary The code currently contains equivalent to these lines (after undoing macros and use operator>> instead of horrible scanf): What happens if one of the input numbers is greater than 71? Is Linux swap still needed with Ubuntu 22.04. Connect and share knowledge within a single location that is structured and easy to search. Should I disclose my academic dishonesty on grad applications? In addition, try_emplace treats the key and the arguments to the mapped_type separately, unlike emplace, which requires the arguments to construct a value_type (that is, a std::pair). C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14.The name follows the tradition of naming language versions by the publication year of the specification, though it was formerly named C++0x because it was expected to be published before 2010. Therefore, it's quite possible that clear() for a hash-based container may take additional time linear in the number of buckets, without violating the standard. Node handle (C++17) - cppreference.com Does the EMF of a battery change with time? but for the C++ programmer, I assume you would like to trade the runtime does unordered_map::clear() releases the memory occupied by the elements in c++? 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned, static constexpr member of same type as class being defined, Constexpr construction and static member is not working, Static constexpr members of same type as class defined (additional details), Understanding static constexpr member variables, constexpr: definition and declaration for constexpr members, static constexpr variables in a constexpr function. To my surprise, it takes more than a second to execute this program. What should be chosen as country of visit if I take travel insurance for Asian Countries. Developers use AI tools, they just dont trust them (Ep. I also enjoy template programming, but here my point is the computation of pure data, like matrix. But to be more precise: complexity requirements specify the number of operations on container elements.
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