Passing a pointer to two-dimension array in a function

This is a discussion on Passing a pointer to two-dimension array in a function within the C++ Programming forums, part of the General Programming Boards category; I am having no problem in passing a pointer to a one dimension array in a function in my code. ...

  1. #1
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    Passing a pointer to two-dimension array in a function

    I am having no problem in passing a pointer to a one dimension array in a function in my code. However, when I tried passing a pointer to a two-dimension array im receiving an error. Can somebody help or explain?


    Code using one-dimension array:

    Code:
    #include <iostream>
    
    
    void InitializeArrays(double *array1, double *array2)
    {
    
    }
    
    
    int main()
    {
    
    double Array_A[5], Array_B[5];
    
    InitializeArrays(Array_A, Array_B);
    
    
    return 0;
    
    }
    Code using two-dimension array:

    Code:
    #include <iostream>
    
    
    void InitializeArrays(double *array1, double *array2)
    {
    
    }
    
    
    int main()
    {
    
    double Array_A[5][5], Array_B[5][5];
    
    InitializeArrays(Array_A, Array_B);
    
    
    return 0;
    
    }
    Error encountered: error C2664: 'InitializeArrays' : cannot convert parameter 1 from 'double [5][5]' to 'double *'

  2. #2
    C++ Witch laserlight's Avatar
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    When passed as an argument, a double[5][5] becomes a pointer to a double[5], which is quite different from a pointer to a double. A solution is to change the function to:
    Code:
    void InitializeArrays(double (*array1)[5], double (*array2)[5])
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  3. #3
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    you should probably listen to laserlight, but something like this would work also:

    Code:
    void InitializeArrays(double **array1, double **array2,size_t a1l1,size_t a1l2,size_t a2l1,size_t a2l2)
    {
    
    }

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    Quote Originally Posted by m37h0d View Post
    you should probably listen to laserlight, but something like this would work also:

    Code:
    void InitializeArrays(double **array1, double **array2,size_t a1l1,size_t a1l2,size_t a2l1,size_t a2l2)
    {
    
    }
    No, because you do not have a pointer to a pointer in the first place, you have the address of a two-dimensional array. To make that work, you'd have to create a one-dimensional array of pointers that point to the first element of each row of data.

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    Mats
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    I was having trouble cutting my grass by swinging my lawnmower blade about...it worked much better in the lawnmower.

    Code:
    class Matrix {
        double[5][5];
    public:
        Matrix() { // initialize array here }
    };

  6. #6
    C++まいる!Cをこわせ!
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    Better would be vector of vector and pass by reference.
    Then you wouldn't have to guess what type must be placed in the function declaration.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Quote Originally Posted by matsp View Post
    No, because you do not have a pointer to a pointer in the first place, you have the address of a two-dimensional array. To make that work, you'd have to create a one-dimensional array of pointers that point to the first element of each row of data.

    --
    Mats
    yes, you're correct. that's what i ended up having to do when i was in this sort of position...

    is there any other way to pass a multidimensional array without specifying the lengths as a literals?

  8. #8
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    Quote Originally Posted by m37h0d View Post
    is there any other way to pass a multidimensional array without specifying the lengths as a literals?
    No, because the compiler wouldn't be able to generate the correct code, then.
    But it's possible with classes and objects, say a vector of vector.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  9. #9
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    Quote Originally Posted by Elysia View Post
    No, because the compiler wouldn't be able to generate the correct code, then.
    But it's possible with classes and objects, say a vector of vector.
    It is possible if you degenerate the array to a single dimension array [which is essentially how the compiler goes about solving that problem].

    It is not "possible with objects" - but objects can hold information to describe their own contents, so you can hide the information about the array dimensions inside the class.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  10. #10
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    Quote Originally Posted by matsp View Post
    It is possible if you degenerate the array to a single dimension array [which is essentially how the compiler goes about solving that problem].
    True, but this would also make it a 1D array. But a 2D array can also be represented as a 1D array, so it's possible, but it isn't exactly the same.

    It is not "possible with objects" - but objects can hold information to describe their own contents, so you can hide the information about the array dimensions inside the class.
    That's more to the point I was referring to. Classes can hide things such as their dimensions (such as vector) and enable passing themselves in such a way that they can remain 2D-arrays without passing the actual dimensions.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  11. #11
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    it is curious to me that you can treat a pointer-to-pointer like a two dimensional array (provided it's been properly initialized), but the inverse isn't true.

  12. #12
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    They are of different nature.
    A dynamic 2D array is like a linked list:
    Code:
    A*:
        B*:
            C
            D
        E*:
            F
            G
    But 2D arrays are sequential:
    Code:
    A
    B
    C
    D
    E
    F
    G
    So it wouldn't work.
    In the dynamic array, you'd dereference the first point with the first dimension which in turn would have more pointers to the second dimension.
    But in normal 2D arrays, there are no pointers to dereference.
    Last edited by Elysia; 06-19-2008 at 10:59 AM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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