- Number of unique values in map - 3 Updates
- Project is making me go mad! Spirolaterals and SDL2. - 1 Update
- Solving the maximum gap problem in linear time - 8 Updates
| Paul <pepstein5@gmail.com>: Aug 14 02:57AM -0700 Please could someone tell me the simplest way to count the number of unique values in a container when the values are not necessarily consecutive? For example, std::unordered_map<int, int> example; example[5] = 3; example[1] = 7; example[0] = 3; If we apply the desired function to example, we would get 2 because there are two distinct values -- 3 and 7. The way I know how to do this simply is to insert or copy into a std::set and apply size() to the set but that seems inefficient and inelegant. Thank you, Paul |
| c.milanesi.bg@gmail.com: Aug 14 06:18AM -0700 Paul wrote: > If we apply the desired function to example, we would get 2 because there are two distinct values -- 3 and 7. > The way I know how to do this simply is to insert or copy into a std::set > and apply size() to the set but that seems inefficient and inelegant. I think it is very elegant, and rather efficient. If you use an unordered_set, it is probably more efficient. Which solution is more elegant than this one? std::unordered_set<int> s; for (auto& pair: example) s.insert(pair.second); -- Carlo Milanesi |
| Barry Schwarz <schwarzb@dqel.com>: Aug 14 11:06AM -0700 On Fri, 14 Aug 2015 02:57:31 -0700 (PDT), Paul <pepstein5@gmail.com> wrote: >Please could someone tell me the simplest way to count the number of unique values in a container when the values are not necessarily consecutive? >For example, >std::unordered_map<int, int> example; Would the issue in question be any different if example had type std::map<int,int> instead of std::unordered_map? -- Remove del for email |
| Kate Sendall <kate.alicia.sendall@googlemail.com>: Aug 14 06:25AM -0700 Hello all, I failed a project for my on-line course about 2 months ago, and just received a letter with a new brief (simpler than the first, which was kind of the Board). It's still in my amateur opinion a pretty tricky brief. I only just started learning C++ and I still struggle with basic concepts of programming, unfortunately. Basically, I'm asking for your help - I DON'T WANT CODE FROM YOU. But I need some serious pointers (ha, pointers. Get it??). My tutors are on leave until the 16th, and my deadline is fast approaching. I need to get something DOWN that WORKS. Otherwise I will have to postpone my second year with them so that I can retake the programming class for a year... which might not be too bad, in all honesty. It would give me time to learn the language and get super good at it. But there's a price tag to postponing. :( The brief is pretty straightforward: I am required to write a simple 'Turtle' graphics program that can do three major things. First, is allow the user to change its parameters so that the spirolateral colour, size, length of segments and/or iterations can change. Second, is to have several preset spirolaterals that the user can choose. Finally, is the ability to save the resulting spirolateral as an image file. I'm using codeblocks, and have imported all my libraries and stuff for SDL2. I'm using the Lazyfoo website to guide me on how to do the basics, such as opening a window, splitting it into viewports etc so I can have my user input sliders and whatnot on the right hand side. I know the rules behind spirolaterals, but I'm not sure how to implement it into a c++ program with SDL2, haha. I also have NO idea how to produce lines from dots or points. The SDL documentation is hard to follow, and the descriptions on the website are pretty vague, and not written for beginners like myself. As much as I hate to admit it, programming isn't coming naturally to me - I am really finding it hard to understand and follow basic ideas. I passed my mathematics exam with flying colours - almost a 'first' - I'm not stupid. But programming is catching me out. I always have so many questions, and I can't find any answers. Thanks for taking the time to read and hopefully write back to me! :) Kate L. |
| JiiPee <no@notvalid.com>: Aug 14 01:22AM +0100 On 13/08/2015 22:43, Paul wrote: > } > return {min, max}; > } I think you can just use std::minmax_element : http://en.cppreference.com/w/cpp/algorithm/minmax_element |
| Melzzzzz <mel@zzzzz.com>: Aug 14 02:36AM +0200 On Fri, 14 Aug 2015 01:22:09 +0100 > I think you can just use > std::minmax_element : > http://en.cppreference.com/w/cpp/algorithm/minmax_element He uses int's. That means in his version compiler has opportunity to use (v)pminsd/(v)pmaxsd instructions to find minimum and maximum because he is not interested in index rather just values. I wouldn't be surprised that on SSE4.1/AVX2 machine his version would perform at least twice as fast as iterator version. |
| Melzzzzz <mel@zzzzz.com>: Aug 14 02:38AM +0200 On Fri, 14 Aug 2015 02:36:39 +0200 > because he is not interested in index rather just values. > I wouldn't be surprised that on SSE4.1/AVX2 machine his version would > perform at least twice as fast as iterator version. But he has to change index type not to be unsigned rather same type as vec.size() returns. I discovered that that prevents this type of optimization(s). |
| JiiPee <no@notvalid.com>: Aug 14 02:18AM +0100 On 14/08/2015 01:38, Melzzzzz wrote: >> He uses int's. That means in his version compiler has opportunity to >> use (v)pminsd/(v)pmaxsd instructions to find minimum and maximum >> because he is not interested in index rather just values. but I guess it depends on how massively he uses that function. If there is only minor benefit then might as well use minmax_element. They say optimization should be done last and only if clear benefit, right? so depends on how heavily he uses it... > But he has to change index type not to be unsigned rather same type as > vec.size() returns. I discovered that that prevents this type of > optimization(s). better maybe: for(const auto a : vec) { if(a < min) min = a; else if (a > max) max = a; } I guess the compiler here would chose the best type, right? auto knows what is the best type.... |
| Melzzzzz <mel@zzzzz.com>: Aug 14 05:10AM +0200 On Fri, 14 Aug 2015 02:18:37 +0100 > there is only minor benefit then might as well use > minmax_element. They say optimization should be done last and only if > clear benefit, right? so depends on how heavily he uses it... According to my testing (i7 4790) with gcc it is faster 1.5 times then minmax_element and with clang 1.9 times. > } > I guess the compiler here would chose the best type, right? auto > knows what is the best type.... Yes, this is version I tested. |
| Melzzzzz <mel@zzzzz.com>: Aug 14 05:21AM +0200 On Fri, 14 Aug 2015 05:10:42 +0200 > > >>> http://en.cppreference.com/w/cpp/algorithm/minmax_element > According to my testing (i7 4790) with gcc it is faster 1.5 times then > minmax_element and with clang 1.9 times. That is with larger than cache, vector. But if in L1 cache speed is about same . I think that compilers produce same optimization in both cases but somehow minmax_element gets worse access times on large vector. |
| Victor Bazarov <v.bazarov@comcast.invalid>: Aug 13 11:39PM -0400 On 8/13/2015 11:21 PM, Melzzzzz wrote: > But if in L1 cache speed is about same . I think that compilers produce > same optimization in both cases but somehow minmax_element gets worse > access times on large vector. But that's not what the OP is after, methinks. Paul needs the largest difference between two adjacent elements of the same vector if sorted. Doesn't minmax give the opposite ends of the sorted vector? V -- I do not respond to top-posted replies, please don't ask |
| Melzzzzz <mel@zzzzz.com>: Aug 14 05:52AM +0200 On Thu, 13 Aug 2015 23:39:59 -0400 > But that's not what the OP is after, methinks. Paul needs the > largest difference between two adjacent elements of the same vector > if sorted. Doesn't minmax give the opposite ends of the sorted vector? Yes, but this is just first step in algorithm. That is, finding min/max values. |
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