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Thread: allocating bidimensional matrix

  1. #16
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    Re: allocating bidimensional matrix

    Quote Originally Posted by Paul McKenzie View Post
    What I meant by "what were your reasons for this requirement" is to see why you needed to create a multidimensional array using 'C' methods instead of just using vector<string>.

    Regards,

    Paul McKenzie
    sorry... misunderstood... well is because i didn't know the power of the std::vector... now i'm studing it

    what a wonderful forum!!! light speed replies and very useful tips!! thank you again!!

  2. #17
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    Re: allocating bidimensional matrix

    sorry but what about reallocating matrix?
    for example...
    a matrix of an HWND array can be declared like that?
    Code:
    typedef std::vector<HWND>        HWNDArray;
    typedef std::vector<HWNDArray>   HWNDMatrix;
    if it's right... for adding a line i do

    Code:
    HWNDMatrix hCheckBox;
    
    hCheckBox.reserve(hCheckBox.size() + 1);
    
    HWND foo = CreateWindowEx ( 0L, "BUTTON", "", BS_AUTOCHECKBOX | WS_VISIBLE | WS_CHILD | BS_CENTER, left, top, right, bottom, hWnd, NULL, GetModuleHandle(NULL), 0);
    
    hCheckBox[hCheckBox.size()].push_back(foo);
    anyway when i try to access to an element it crash...

  3. #18
    Join Date
    Apr 1999
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    Re: allocating bidimensional matrix

    The error is simple -- the vector has size 0. You can't access an element of a vector that is out of bounds.
    Code:
    hCheckBox.reserve(hCheckBox.size() + 1);
    As a quiz, you tell us what this line does. One clue -- it does not give you hCheckBox.size() elements. Given that you do not have those elements, the next line is an illegal memory access.
    Code:
    hCheckBox[hCheckBox.size()].push_back(foo);
    There is no such element as hCheckBox[hCheckBox.size()]. That's why you get an error.

    There are 4 general ways to add elements to a vector.

    1) On construction of the vector.
    2) Using vector::push_back()
    3) Using vector::insert()
    4) using vector::resize().

    Look at item 4. Note that the name of the function is resize(), not reserve().

    Regards,

    Paul McKenzie

  4. #19
    Join Date
    Jul 2005
    Posts
    266

    Re: allocating bidimensional matrix

    Well just for fun I decided to fix your reallocating issue and give you a few pointers ( hopefully valid ) First of all you have no need of the static variable and no need to save the length of strings because what you are reallocating is just an array of pointers to the malloced strings, you dont need to reallocate the strings just the array of pointers so all you need to save is count of elements inside which you already use the n parameter for :P Second mistake you have was the dereferencing of the list pointer , *list[n] which in fact dereferences list[n] but you actually need to dereference list and get its n-th element so a pair of brackets can be very useful here (*list)[n]. So the final code should look like this:
    Code:
    #include <iostream>
    using namespace std;
    
    
    void add(char ***list, int n, char *person)
    {
    *list=(char**)realloc(*list,(n+1)*sizeof(char**));
    (*list)[n] = (char*) malloc (strlen(person) + 1);
    strcpy((*list)[n], person);
    }
    
    int main()
    {
    char **list = NULL;
    int n = 0;
    
    add(&list, 0, "jack");
    printf("list[0]: &#37;s\n", list[0]);
    
    n++;
    
    add(&list, 1, "jeff");
    printf("list[1]: %s\n", list[1]);
    free(list[0]);
    free(list[1]);
    free(list);
    return 0;
    }
    Hope I helped
    Last edited by kolkoo; August 3rd, 2009 at 09:52 AM.

  5. #20
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    Apr 1999
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    27,446

    Re: allocating bidimensional matrix

    Quote Originally Posted by kolkoo View Post
    Well just for fun I decided to fix your reallocating issue
    This code has a huge amount of memory leaks. Where do you deallocate the memory?

    The solutions using C++ containers both work and have no memory leaks.

    Regards,

    Paul McKenzie

  6. #21
    Join Date
    Jul 2005
    Posts
    266

    Re: allocating bidimensional matrix

    There we go fixed in previous post for your viewing pleasure
    I just fixed it because he wanted to see how it would look while using realloc and his original snippet didnt bother with freeing memory, and 'huge' is quite an overstatement there.

  7. #22
    Lindley is offline Elite Member Power Poster
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    Re: allocating bidimensional matrix

    Just for fun: realloc() can be used in place of free() by specifying a new size of 0.

  8. #23
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    Jul 2009
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    Re: allocating bidimensional matrix

    Quote Originally Posted by Lindley View Post
    Just for fun: realloc() can be used in place of free() by specifying a new size of 0.
    cool XD well I learned a lot of things from this discussion and I think I don't want to see more than ONE star before a variable for a while XD
    personally I hate working with dyn mem because I ever feel there's some leak somewhere...
    the realloc for the string has been made because at the beginning I wanted to make an insert possible but (unfortunately / fortunately) std::vector.insert() took all the fun away XD

    about my last post I solved declaring a support single var and "push_back"ed it into a support vector and then "push_back"ed the vector in the matrix...
    std::vector make me feel much quiet about memory :P

    thank you all again!

  9. #24
    Lindley is offline Elite Member Power Poster
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    Location
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    Re: allocating bidimensional matrix

    When you need a matrix of dynamic size which won't *change* size often, you should consider using boost::multi_array.

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