double ***x_matrix

This is a discussion on double ***x_matrix within the C Programming forums, part of the General Programming Boards category; My instructor has given me a project, specifically stating that we are not allowed to modify function prototypes. The thing ...

  1. #1
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    double ***x_matrix

    My instructor has given me a project, specifically stating that we are not allowed to modify function prototypes. The thing is, this double matrix bounces from being a double pointer for allocation, a triple pointer for assigning values, and back to a double pointer for multiplication. (It's for a matrix multiplication thingie)

    As I have it so far, x_matrix[0][0][1] = #, x_matrix[0][1][0] = #, for example. The first [] never changes. The values are assign a'ok, but in the multiplication function I only have access to x_matrix[#][#] , which doesn't do any good.

    double **thecolumn;
    thecolumn = mm_alloc(thesizeofmatrix);
    x_matrix = &thecolumn;

    Is there anyway I can turn x_matrix into a double pointer, seeing as it's first [] is useless?

  2. #2
    CSharpener vart's Avatar
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    double pointer for allocation, a triple pointer for assigning values
    Are you sure it is not other way around?

    Probably - you teacher wants to see something along the lines:
    Code:
    double **matrix = malloc(rows * sizeof *matrix);
    
    if(!matrix)
       return ERROR_OUT_OF_RESOURCES;
    
    
    for(i=0;i<rows;i++)
    {
       matrix[i] = malloc(cols * sizeof(*matrix[i]);
       if(!matrix[i] )
       {
          /* free allocated memory */
          return ERROR_OUT_OF_RESOURCES;
       }
    }
    now you can pass matrix as double** pointer to function
    and access members as matrix[i][j]
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    Thank you for the quick response! I'm pretty sure I wasn't mistaken though, here's the function prototypes:

    double ** mm_alloc( int xysize)

    void mm_read (int *xysize, double ***x_matrix, double ***y_matrix)

    double ** mm_multiply (int xysize, double **x, double **y)


    mm_alloc is called from inside of mm_read. The only way I know how to is assign a pointer from x_matrix to the double pointer returned from mm_alloc. But I'm kinda screwed for mm_multiply.

  4. #4
    CSharpener vart's Avatar
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    mm_alloc is called from inside of mm_read.
    it means you pass the pointer to be initialized inside the function

    Code:
    double **x_matrix;
    double **y_matrix;
    mm_read (&xysize, &x_matrix, &y_matrix);
    /*here should be an error check but strangly enough mm_read does not return error code */
    and inside the mm_read

    Code:
    *x_matrix = mm_alloc( *xysize);
    *y_matrix = mm_alloc( *xysize);
    where the *xysize was previously initialized - probably from the file or user input

    But I'm kinda screwed for mm_multiply.
    Show the code - it should be very straightforward
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    I think I've gotten my mm_alloc fixed up now, that way makes a lot of sense. After that in mm_read, I assign values to the matrices from an array of doubles. What is proper syntax for this?

    *x_matrix[0][0] = alldoubles[z];

    gives a segmentation fault

  6. #6
    CSharpener vart's Avatar
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    (*x_matrix)[0][0] = alldoubles[z];

    but to be easier -
    double** temp_x = *x_matrix;

    and then
    temp_x[0][0] = value;
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    That did it! Thank you vart, I can now access x[1][1] easily from inside mm_multiply. This is my first project in C (used to C++), and the prof wants us to get good with pointers for when we go into Assembly. Wish me luck, I'm going to need it.

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