C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  STL::vector problem          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message LinuxGuy Guest Posted: Fri Sep 23, 2005 10:17 pm    Post subject: STL::vector problem Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . any help is welcome Thanks Rahul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Sat Sep 24, 2005 8:20 pm    Post subject: Re: STL::vector problem LinuxGuy wrote: Quote: I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . position = iterator_you_got - your_vector.begin(); V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Sat Sep 24, 2005 8:23 pm    Post subject: Re: STL::vector problem LinuxGuy wrote: Quote: Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . any help is welcome Thanks if "it" is an iterator pointing to an element of vector v, then std::distance(v.begin(), it) gives the position of the element. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carlos Moreno Guest Posted: Sun Sep 25, 2005 1:36 am    Post subject: Re: STL::vector problem LinuxGuy wrote: Quote: Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . Why are you unhappy about being returned an iterator? You can do many things with an iterator. To obtain the offset, simply subtract .begin() from the iterator that you obtained. Carlos -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bo Persson Guest Posted: Sun Sep 25, 2005 1:43 am    Post subject: Re: STL::vector problem "LinuxGuy" <rahul.ruikar (AT) gmail (DOT) com> skrev i meddelandet news:1127453799.089567.131480 (AT) g47g2000cwa (DOT) googlegroups.com... Quote: Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . For a vector, you can get the index as the distance from the start of the vector index = position - vec.begin(); Bo Persson [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Timo Geusch Guest Posted: Sun Sep 25, 2005 1:42 pm    Post subject: Re: STL::vector problem LinuxGuy wrote: Quote: Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . Use std::distance to find out how many elements away from the vector's begin() the found element is. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Sun Sep 25, 2005 1:44 pm    Post subject: Re: STL::vector problem LinuxGuy wrote: Quote: I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. Yes, and an iterator is much better because it will stay an iterator even if you switch to a std::list instead one day. Other than that, you could use the difference to the begin() iterator: iterator it = find(...) unsigned index = it-vec.begin(); Just to mention it, you could use the generic std::distance() which tells you how far two iterators are apart. Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Sun Sep 25, 2005 1:44 pm    Post subject: Re: STL::vector problem "LinuxGuy" <rahul.ruikar (AT) gmail (DOT) com> writes: Quote: I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . An iterator does give you the position of the element in the vector. If you need the index, subtract vector.begin() from the result of find(). E.g.: std::vector<int> v; // fill v std::vector<int>::iterator const pos7(std::find(v.begin(),v.end(),7)); if (pos7==v.end()) std::cout << "not foundn"; else std::cout << "index: " << (pos7-v.begin()) << 'n'; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Sun Sep 25, 2005 1:49 pm    Post subject: Re: STL::vector problem "LinuxGuy" wrote: Quote: I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . If vec is the vector and iter is the iterator, then std::distance(vec.begin(), iter) is the index. Best regards, Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ramashish Guest Posted: Sun Sep 25, 2005 1:50 pm    Post subject: Re: STL::vector problem Quote: Hi, I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . You can use std::find with std::distance to do that, e.g. std::vector<T>::iterator it=std::find(v.begin(), v.end(), element); int pos = std::distance(v.begin(), it); if(pos != v.size()) { // we found the element ... } Ram [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top lancediduck@nyc.rr.com Guest Posted: Sun Sep 25, 2005 1:56 pm    Post subject: Re: STL::vector problem Once you have an iterator from any container, you can find the ordinal position using std::distance i.e. typedef std::vector<foo> foovec; foovec vec_of_foo;//put some foo's in foovec::iterator pos=find(vec_of_foo.begin(),vec_of_foo.end(),somefoo); std::cout<<"somefoo is at position "< For vector or anything with a RandomAccessIterator distance is an O(1) operation. For list and other containers it is O(N); see http://www.roguewave.com/support/docs/sourcepro/stdlibref/distance.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg Herlihy Guest Posted: Sun Sep 25, 2005 6:56 pm    Post subject: Re: STL::vector problem Thomas Tutone wrote: Quote: "LinuxGuy" wrote: I have vector with some elements. now I want to search particular element and find out the position of that element in that vector ( index ). I used find algorithm but it gives Iterator in return. please help me to find out position of any element in that vector . If vec is the vector and iter is the iterator, then std::distance(vec.begin(), iter) is the index. Calling std::distance wll return the index, but is there a better way to calculate it? And if so, why is it better? I would argue that subtraction: iter - vec.begin() is the better method. For one, that is how std::distance calculates the result for vector iterators itself anyway. And since the iterator's container type is a runtime constant, using std::distance is a pointless abstraction. But not necessarily harmless. std::distance is really too generic for non templated code. It can find the distance between iterators no matter the type of their container; subtraction on the other hand is defined only when it is efficient. Using std::distance, changing the vector to a std::list silently turns an efficient operation into an inefficient one. Using subtraction instead, such a change would trigger a compiler error. The error (subtraction not defined for a list's iterators) is preferable since it alerts the programmer who changed the container's type to this new cost. The programmer can then decide whether obtaining the index of a list's iterator is still a worthwhile operation. Greg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Mon Sep 26, 2005 8:22 am    Post subject: Re: STL::vector problem Greg Herlihy wrote: [snip] Quote: Calling std::distance wll return the index, but is there a better way to calculate it? And if so, why is it better? I would argue that subtraction: iter - vec.begin() is the better method. For one, that is how std::distance calculates the result for vector iterators itself anyway. And since the iterator's container type is a runtime constant, using std::distance is a pointless abstraction. Not entirely pointless - it does document what the programmer is doing more clearly, IMO, then the subtraction. And for most compilers, the code generated with optimization turned on should be identical. Quote: But not necessarily harmless. std::distance is really too generic for non templated code. It can find the distance between iterators no matter the type of their container; subtraction on the other hand is defined only when it is efficient. Using std::distance, changing the vector to a std::list silently turns an efficient operation into an inefficient one. Using subtraction instead, such a change would trigger a compiler error. The error (subtraction not defined for a list's iterators) is preferable since it alerts the programmer who changed the container's type to this new cost. The programmer can then decide whether obtaining the index of a list's iterator is still a worthwhile operation. I agree with you in part. But two points in response. First, the OP wanted an index - presumably because he was going to use that index elsewhere in connection with operator[]. If so, then if he changed the container to a list, but continued to use the index, then the compiler would complain when he attempted to refer to a list element by index anyway. A more interesting issue would be raised, I suppose, if he changed the vector to a map, where operator[] is defined. But second and more importantly - if it is important that the index (or distance) be computed in constant rather than linear time, I believe that a compile-time assertion that the iterator must be random access, along with an explanatory comment, more clearly documents the programmer's intent, compared to a subtraction that simply mysteriously fails when the container type is changed (months or years later). Best regards, Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] 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 (Moderated) 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.