C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Vector of vector question          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message BCC Guest Posted: Tue Jan 13, 2004 1:05 am    Post subject: Vector of vector question If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? Do I have to loop through table1 and initialize each vector of doubles using new? And in cleaning up, manually delete each of these vectors of doubles? Thanks, B Back to top Cy Edmunds Guest Posted: Tue Jan 13, 2004 1:27 am    Post subject: Re: Vector of vector question "BCC" <bryan (AT) akanta (DOT) com> wrote Quote: If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? No. They are default initialized. You have an empty vector of empty vectors. Quote: Do I have to loop through table1 and initialize each vector of doubles using new? No. Please. Quote: And in cleaning up, manually delete each of these vectors of doubles? No again. std::vector is a well designed class that doesn't require a lot of handholding. Quote: Thanks, B -- Cy http://home.rochester.rr.com/cyhome/ Back to top Donovan Rebbechi Guest Posted: Tue Jan 13, 2004 1:36 am    Post subject: Re: Vector of vector question In article <gvHMb.9111$wP1.6082 (AT) newssvr27 (DOT) news.prodigy.com>, BCC wrote: Quote: If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? They're empty. table1.size() will produce 0 and table1[0].size results in undefned behaviour (since there is no first element yet) Quote: Do I have to loop through table1 and initialize each vector of doubles using new? You don't use new, the vector class manages its own storage. You use vector member functions. If you want the vectors to have some entries, you need to do something like this: typedef std::vector<double>::size_type dvecsize; dvecsize m = 10, n = 5; std::vector< std::vector table1 (m,n); std::cout << table1.size() << std::endl; // 10 std::cout << table1[0].size() << std::endl; // 5 Quote: And in cleaning up, manually delete each of these vectors of doubles? No, the destructor of the vector takes care of deallocating storage. That's the main point of having a vector. Cheers, -- Donovan Rebbechi http://pegasus.rutgers.edu/~elflord/ Back to top Daniel T. Guest Posted: Tue Jan 13, 2004 1:37 am    Post subject: Re: Vector of vector question "BCC" <bryan (AT) akanta (DOT) com> wrote: Quote: If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? Do I have to loop through table1 and initialize each vector of doubles using new? And in cleaning up, manually delete each of these vectors of doubles? Any doubles created by the vector will be initialized to 0.0, they don't need to be 'new'ed nor 'delete'ed. Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. See the FAQ for a sample implementation. Back to top Jon Bell Guest Posted: Tue Jan 13, 2004 1:40 am    Post subject: Re: Vector of vector question In article <gvHMb.9111$wP1.6082 (AT) newssvr27 (DOT) news.prodigy.com>, BCC <bryan (AT) akanta (DOT) com> wrote: Quote: If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? In fact, at this point, you have *no* vector<double>s at all. The "outer" vector that is supposed to contain vector<double>s has zero size. No memory has been allocated at all for storing vector<double>s. Quote: Do I have to loop through table1 and initialize each vector of doubles using new? Assuming you know how big the table is supposed to be (numRows x numCols) at run time, before you declare the table, the easiest way is to make the table the appropriate size when you declare it: std::vector<std::vector table1 (numRows, std::vector<double>(numCols)); Then fill the table using the usual table1[row][col] notation. Quote: And in cleaning up, manually delete each of these vectors of doubles? No, std::vector's destructor will take care of any cleanup that is necessary, in this case. If you had declared a vector of pointers, then you would need to either delete the pointers individually or make sure other pointers are pointing to the objects being pointed to, before the vector goes out of scope. But you still wouldn't have to worry about deleting the vector itself, because you didn't use new to create it. -- Jon Bell <jtbellm4h (AT) presby (DOT) edu> Presbyterian College Dept. of Physics and Computer Science Clinton, South Carolina USA Back to top David Fisher Guest Posted: Tue Jan 13, 2004 2:00 am    Post subject: Re: Vector of vector question "Daniel T." <postmaster (AT) eathlink (DOT) net> wrote: Quote: Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. See the FAQ for a sample implementation. The reference is http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.17 if you didn't already have it. David F Back to top Ron Natalie Guest Posted: Tue Jan 13, 2004 3:32 am    Post subject: Re: Vector of vector question "BCC" <bryan (AT) akanta (DOT) com> wrote Quote: If I create a vector of vectors of double: std::vector< std::vector table1; Are my vectors of doubles uninitialized? Do I have to loop through table1 and initialize each vector of doubles using new? There are no elements to initialize, you've created an empty vector of empty vectors. However, if you were to give it a size arg (or resize it), then absent an explicit value to the constructor or resize call, it will provide default initialized values. Quote: And in cleaning up, manually delete each of these vectors of doubles? No, the vector will take all the elements with them when they go. Back to top David Harmon Guest Posted: Tue Jan 13, 2004 9:48 am    Post subject: Re: Vector of vector question On Tue, 13 Jan 2004 01:37:24 GMT in comp.lang.c++, "Daniel T." <postmaster (AT) eathlink (DOT) net> was alleged to have written: Quote: Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. Well, I can't entirely agree. A vector of vectors is a quick and cheerful way of getting a table sized at run time without having to reinvent the wheel. vector< vector table1(rows, vector<double>(columns)); Back to top Gavin Deane Guest Posted: Tue Jan 13, 2004 1:22 pm    Post subject: Re: Vector of vector question "David Fisher" <nospam (AT) nospam (DOT) nospam.nospam> wrote Quote: "Daniel T." <postmaster (AT) eathlink (DOT) net> wrote: Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. See the FAQ for a sample implementation. The reference is http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.17 if you didn't already have it. David F The next FAQ shows the same thing using a vector of vectors to implement the 2D array class. http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.18 Removes all the need for explicit memory management in the class. And it should be easy to design the class so that it's impossible for the individual vectors-within-a-vector to end up with different sizes. -- hth GJD Back to top Gavin Deane Guest Posted: Tue Jan 13, 2004 11:41 pm    Post subject: Re: Vector of vector question David Harmon <source (AT) netcom (DOT) com> wrote Quote: On Tue, 13 Jan 2004 01:37:24 GMT in comp.lang.c++, "Daniel T." [email]postmaster (AT) eathlink (DOT) net[/email]> was alleged to have written: Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. Well, I can't entirely agree. A vector of vectors is a quick and cheerful way of getting a table sized at run time without having to reinvent the wheel. vector< vector table1(rows, vector<double>(columns)); The potential problem is that careless code could end up altering the sizes of some of the vector<double>s. Depending on your application, you might want the robustness of a class that does not allow this to happen. -- GJD Back to top Daniel T. Guest Posted: Wed Jan 14, 2004 3:25 am    Post subject: Re: Vector of vector question [email]deane_gavin (AT) hotmail (DOT) com[/email] (Gavin Deane) wrote: Quote: "David Fisher" <nospam (AT) nospam (DOT) nospam.nospam> wrote in message "Daniel T." <postmaster (AT) eathlink (DOT) net> wrote: Are you sure you want to use a vector of vectors? I would only do that if I needed a ragged array. If the array represents a table, you would be better off creating a 2D array class. See the FAQ for a sample implementation. The reference is http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.17 if you didn't already have it. David F The next FAQ shows the same thing using a vector of vectors to implement the 2D array class. http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.18 Removes all the need for explicit memory management in the class. And it should be easy to design the class so that it's impossible for the individual vectors-within-a-vector to end up with different sizes. A better choice, of course would be to implement the 2D array using a single vector. This also removes the need for explicit memory management and is easer to design... Back to top RanggaPratama Guest Posted: Wed Feb 04, 2004 12:34 am    Post subject: Re: Vector of vector question Now, how do we initialize the 2d vector? I tried to intialize the 2d vector by doing:(but, it's not working) for(int i = 0; i < row; i++) for(int j = 0; j < col; j++) table1[i].push_back(1); I knew it wouldn't work. I was just trying to do different things. I was thinking of using iterator, but I don't have any idea how to do it with 2d vector. Back to top Clark Cox Guest Posted: Fri Feb 06, 2004 9:24 pm    Post subject: Re: Vector of vector question In article <17db7dfb5ad3326db75c954a1ab97081 (AT) localhost (DOT) talkaboutprogramming.com>, "RanggaPratama" <madrasah2 (AT) hotmail (DOT) com> wrote: Quote: Now, how do we initialize the 2d vector? I tried to intialize the 2d vector by doing:(but, it's not working) for(int i = 0; i < row; i++) for(int j = 0; j < col; j++) table1[i].push_back(1); I knew it wouldn't work. I was just trying to do different things. You're not doing anything to change the size of the outer vector. Since you said "initialize, I assume that the vector is empty to begin with, so table1[i] is undefined. Quote: I was thinking of using iterator, but I don't have any idea how to do it with 2d vector. How about (where T is whatever type the inner vector contains): { table1.clear(); table1.resize(row, std::vector } 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.