Quick question about passing 2d arrays

This is a discussion on Quick question about passing 2d arrays within the C++ Programming forums, part of the General Programming Boards category; Suppose I have this code: Code: #include <iostream> void foo(int a[][3]); int main() { int num[3][3] = {{1,2,3},{4,5,6},{7,8,9}}; //size of ...

  1. #1
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    Quick question about passing 2d arrays

    Suppose I have this code:

    Code:
    #include <iostream>
    
    void foo(int a[][3]);
    int main()
    {
        int num[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
       //size of array calculated in here
        foo(num);
        
        
        std::cin.get();
        
        
    }
    void foo(int a[][3])
    {
        std::cout<<a[2][1];
        return;
        
    }
    What if I don't know that the number of columns is going to be 3 for the two dimensional array? I just know in the function prototype that I need to receive a two dimensional array and I have another variable that will be used for the size of the array? So it would look more like this:

    Code:
    #include <iostream>
    
    void foo(int a[][], int size);
    int main()
    {
        int num[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
        //size calculated in here
        foo(num);
        
        
        std::cin.get();
        
        
    }
    void foo(int a[][], int size)
    {
        std::cout<<a[2][1];
        return;
        
    }
    I have a program where the size of the puzzle is determined by another file (crossword puzzle) so the program isn't very generic when everytime I want to do another puzzle I have to hard code what the size of the puzzle will be in the function prototype/function definition when it can be easily calculated from the files. Thanks.

  2. #2
    (?<!re)tired Mario F.'s Avatar
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    That is exactly one of the main advantages of STL containers over arrays.

    http://www.cprogramming.com/tutorial/stl/vector.html
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    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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    Registered User Tonto's Avatar
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  4. #4
    Registered User whiteflags's Avatar
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    It would work better like this

    void foo(int **a, size_t cols, size_t rows);

    that way you don't have any guesswork to do about the size of the array and you're doing things right.

    [edit] double pwnt. Read up!

  5. #5
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    Thanks guys.

  6. #6
    and the hat of wrongness Salem's Avatar
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    > void foo(int **a, size_t cols, size_t rows);
    NO NO NO!!!
    You can't pass a 2D array to a function expecting a pointer to a pointer. Your compiler should warn you, and if it doesn't you need a better one.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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