C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  delete elements in vector          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message david wolf Guest Posted: Tue Apr 19, 2005 4:02 pm    Post subject: delete elements in vector I want to delete all even numbers in a vector, I am not sure if there's any better way to do it. Following program is how I did it. Look at the part of code beginning from comments: //delete all even numbers. My questions is actullay I have to put pos++ in the body of the loop(also in if, else, statement). Can I somehow put it in for(...;...;...). Or can I use index instead of iterator in order to delete elements from vector? I just wish to know what's the most elegant way to do it. Thanks #include <iostream> #include <vector> main(){ int ar[10]={1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; vector<int> tmpint; //populate vector for(int i=0; i<10; i++){ tmpint.push_back(ar[i]); } //delete all even numbers vector for(pos=tmpint.begin(); pos != tmpint.end(); ){ if (*pos % 2 == 0) tmpint.erase(pos); else pos++; } //display all elements in the vector for(pos=tmpint.begin(); pos != tmpint.end();pos++ ){ cout<<*pos< } } Back to top Victor Bazarov Guest Posted: Tue Apr 19, 2005 4:28 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: I want to delete all even numbers in a vector, I am not sure if there's any better way to do it. Following program is how I did it. Look at the part of code beginning from comments: //delete all even numbers. My questions is actullay I have to put pos++ in the body of the loop(also in if, else, statement). Can I somehow put it in for(...;...;...). Or can I use index instead of iterator in order to delete elements from vector? I just wish to know what's the most elegant way to do it. Elegance is in the eye of the beholder. You can try doing it in one line (well, not exactly, you'll need to define your predicate), but whether it's "elegant" or not is not my place to say. Quote: Thanks #include <iostream #include main(){ int main(){ Quote: int ar[10]={1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; vector std::vector<int> tmpint; Quote: //populate vector for(int i=0; i<10; i++){ tmpint.push_back(ar[i]); } Did you know you can initialise 'tmpint' from 'ar' in the same statement where you declared it? std::vector ?? No need for the loop at all... Quote: //delete all even numbers vector<int>::iterator pos; std::vector<int>::iterator pos; Quote: for(pos=tmpint.begin(); pos != tmpint.end(); ){ if (*pos % 2 == 0) tmpint.erase(pos); else pos++; } //display all elements in the vector for(pos=tmpint.begin(); pos != tmpint.end();pos++ ){ cout<<*pos< std::cout << *pos << std::endl; Quote: } } If you use 'std::remove_if' and a custom predicate, you can do #include #include <vector> #include <algorithm> using namespace std; // your custom predicate template<class T> struct is_even { bool operator()(T t) const { return t % 2 == 0; } }; int main(){ int ar[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; vector<int> tmpint(ar, ar+10); // this is essentially your one-liner. tmpint.erase( remove_if(tmpint.begin(), tmpint.end(), is_even<int>()), tmpint.end()); //display all elements in the vector for (size_t i = 0; i < tmpint.size(); i++) cout << tmpint[i] << ' '; cout << endl; } V Back to top Mark Stijnman Guest Posted: Wed Apr 20, 2005 1:26 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: I want to delete all even numbers in a vector, I am not sure if there's any better way to do it. Following program is how I did it. Look at the part of code beginning from comments: //delete all even numbers. My questions is actullay I have to put pos++ in the body of the loop(also in if, else, statement). Can I somehow put it in for(...;...;...). You probably could, but I don't think it's worth it. Quote: Or can I use index instead of iterator in order to delete elements from vector? I just wish to know what's the most elegant way to do it. Thanks #include <iostream #include main(){ int ar[10]={1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; vector //populate vector for(int i=0; i<10; i++){ tmpint.push_back(ar[i]); } //delete all even numbers vector for(pos=tmpint.begin(); pos != tmpint.end(); ){ if (*pos % 2 == 0) tmpint.erase(pos); else pos++; } //display all elements in the vector for(pos=tmpint.begin(); pos != tmpint.end();pos++ ){ cout<<*pos< } } I'm not convinced this piece of code even does what it is supposed to do - AFAIK the iterator pos is not guaranteed to be valid after calling erase. I think you should use pos = tmpint.erase(pos); instead. This will make pos point to the next, non-erased element after pos, or end() if there aren't any. As far as elegance goes, the method erase works in linear time on vectors: to erase an element, it simply moves all elements behind it left one place (filing up the erased spot). This will create a lot of unnecessary copying if you call erase lots of time in succession on the same vector. So this is not very elegant I'm afraid. It is of course possible to examine/erase/move every element in the vector only once, if you think about it for a bit. It is possible that the remove_if that Victor Bazarov suggested is implemented like that (I haven't checked, but it seems likely) so I second his suggestion that you give that a try. grtz Mark Back to top david wolf Guest Posted: Wed Apr 20, 2005 7:44 pm    Post subject: Re: delete elements in vector I do not agree with grtz Mark. I tried my program, it works fine. I means it is not necessary to use pos = tmpint.erase(pos); simply doing tmpint.erase(pos); will work. Any one giving me some opnions? Back to top Victor Bazarov Guest Posted: Wed Apr 20, 2005 7:54 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: I do not agree with grtz Mark. I tried my program, it works fine. I means it is not necessary to use pos = tmpint.erase(pos); simply doing tmpint.erase(pos); will work. Any one giving me some opnions? Mark is right. Once you erase the 'pos' iterator, the next loop tries to use it by dereferencing it. You simply get awfully lucky that it stays the same on your system. 'pos' is invalidated by the call to erase(), and dereferencing it invokes undefined behaviour. You ought to use pos = tmpint.erase(pos); V Back to top david wolf Guest Posted: Thu May 05, 2005 5:42 pm    Post subject: Re: delete elements in vector I mean on Linux (red hat) system, I am using gcc. Whenever I did tmpint.erase(pos); pos automatically points to the element after the deleted one. Can you tell me a case/os that this will not work? ------------------------------- Victor said ------------- 'pos' is invalidated by the call to erase(), and dereferencing it invokes undefined behaviour ------------------------------------------------------------------------------ Also, victor said above things. Could victor tell me where is it specified to work this way? Back to top Victor Bazarov Guest Posted: Thu May 05, 2005 6:07 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: I mean on Linux (red hat) system, I am using gcc. Whenever I did tmpint.erase(pos); pos automatically points to the element after the deleted one. Can you tell me a case/os that this will not work? It will not work if the container you're using is not 'vector'. Quote: ------------------------------- Victor said ------------- 'pos' is invalidated by the call to erase(), and dereferencing it invokes undefined behaviour ------------------------------------------------------------------------------ Also, victor said above things. Could victor tell me where is it specified to work this way? I take it back. It is not invalidated, dereferencing it may not cause undefined behaviour. Undefined behaviour only occurs if 'pos' was the last element in the vector. However, for other containers it is true, and if the element you're erasing is the last one, it is true. So, in a _general_case_ it is true, then. V Back to top Default User Guest Posted: Thu May 05, 2005 6:43 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: I do not agree with grtz Mark. I tried my program, it works fine. I means it is not necessary to use pos = tmpint.erase(pos); Please quote enough of the previous message to provide context for your replies. To do so using the Google interface, click on "show options" and use the Reply shown in the expanded header. Brian Back to top david wolf Guest Posted: Wed May 11, 2005 7:46 pm    Post subject: Re: delete elements in vector Victor Bazarov wrote: Quote: david wolf wrote: I mean on Linux (red hat) system, I am using gcc. Whenever I did tmpint.erase(pos); pos automatically points to the element after the deleted one. Can you tell me a case/os that this will not work? It will not work if the container you're using is not 'vector'. ------------------------------- Victor said ------------- 'pos' is invalidated by the call to erase(), and dereferencing it invokes undefined behaviour ------------------------------------------------------------------------------ Also, victor said above things. Could victor tell me where is it specified to work this way? I take it back. It is not invalidated, dereferencing it may not cause undefined behaviour. Undefined behaviour only occurs if 'pos' was the last element in the vector. However, for other containers it is true, and if the element you're erasing is the last one, it is true. So, in a _general_case_ it is true, then. V I tried and found out that, even 'pos' was the last element in the vector. If we invoke: tmpint.erase(pos); pos is still defined after this erase method. After execute erase(), pos will be point to the end of the container which means pos=tmpint.end(). So at least for vector, we do not have use pos = tmpint.erase(pos), I think. Can you help confirm this? David Back to top Victor Bazarov Guest Posted: Wed May 11, 2005 8:04 pm    Post subject: Re: delete elements in vector david wolf wrote: Quote: [...] I tried and found out that, even 'pos' was the last element in the vector. If we invoke: tmpint.erase(pos); pos is still defined after this erase method. And how *did* you find that out? Quote: After execute erase(), pos will be point to the end of the container which means pos=tmpint.end(). So at least for vector, we do not have use pos = tmpint.erase(pos), I think. Can you help confirm this? No. If 'pos == tmpint.end()' yields 'true', then you cannot use 'pos' to do anything except compare its value to the result of calling 'end()' or anything else you usually do with the result of calling 'end()'. The Standard gives no guarantees that "one-past-the-end" iterator would be dereferenceable. It therefore is not necessarily valid. There are no special provisions in the Standard regarding 'vector' and its iterators in that respect. V Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.