Arbitrary length multi-dimensional arrays

This is a discussion on Arbitrary length multi-dimensional arrays within the C Programming forums, part of the General Programming Boards category; Newbie question here. It's been a while since I've done C programming, and I hit a wall last night. Let's ...

  1. #1
    thw
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    Arbitrary length multi-dimensional arrays

    Newbie question here. It's been a while since I've done C programming,
    and I hit a wall last night.

    Let's say I have a three dimensional array, like so:
    Code:
    int p[2][3][3] =
    {{{0,0,0},
    {1,1,1},
    {0,1,0}},
    
    {{0,1,0},
    {1,1,0},
    {0,1,0}}};
    I also have a number of other three dimensional arrays, generated with
    a code generating script. The size of each dimension varies with each
    one. Some are [5][3][3], some are [2][5][5], etc...

    How would I declare a variable that could hold any of these
    3-dimensional arrays? I'm tripping over the pointer syntax.

    I'd like to be able to do something like:
    int ***val = p; //The variable p from the last example.

    I'm pretty sure that ***val is the wrong way to go about it.

    Thanks for your help!

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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  3. #3
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    One way would be dynamic memory allocation. Let's say your program knows how big each dimension is and they're stored in the variables s_dim1, s_dim2, s_dim3. You could do something like this:
    Code:
    {
      int ***p;
      int s_dim1 = 4, s_dim2 = 5, s_dim3 = 2;
      int i, j;
    
      p = malloc(sizeof(*p) * s_dim1);
    
      for(i = 0;i < s_dim1;++i)
      {
        p[i] = malloc(sizeof(**p) * s_dim2);
    
        for(j = 0;j < s_dim2;++j)
          p[i][j] = malloc(sizeof(***p) * s_dim3);
      }
    }
    There isn't any way to just make an array in C without giving it a size, so the dynamic memory allocation is probably the best way to go.

    Disclaimer: I didn't test the above code, but it should work.
    Last edited by itsme86; 06-26-2006 at 11:26 AM.
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  4. #4
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    Here is code that's tested for two dimensional array

    int **array1 = (int **)malloc(nrows * sizeof(int *));
    for(i = 0; i < nrows; i++)
    array1[i] = (int *)malloc(ncolumns * sizeof(int));

    access allocated 2D array like: array1[1][2] = 5;

  5. #5
    and the hat of wrongness Salem's Avatar
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    itsme86's answer is much better.
    It avoids casting the result of malloc, and avoids explicit reference to sizeof(type).

    Though it is possible to allocate all 3 dimensions with just 3 malloc's.
    Here is a recent example for 2 dimensions, just to give you the idea.
    http://forums.devshed.com/showpost.p...22&postcount=5
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