Dynamic, multi-dimensional pointers!

This is a discussion on Dynamic, multi-dimensional pointers! within the C Programming forums, part of the General Programming Boards category; Hi! I think I'm beginning to figure these pointy things out: Code: int loop; int x = 5; int y ...

  1. #1
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    Dynamic, multi-dimensional pointers!

    Hi!

    I think I'm beginning to figure these pointy things out:

    Code:
    int loop;
    int x = 5;
    int y = 2;
    
    // create first subscript
    long **group_total;
    group_total = malloc (x * sizeof (long));
    
    if (!group_total)
    {
       printf ("\n\nError!  Not enough memory!\n\n");
    }
    
    //create second subscript
    for (loop = 0; loop < x; loop++)
    {
       group_total[loop] = malloc (y * sizeof (long));
    
       if (!group_total[loop])
       {
          printf ("\n\nError!  Not enough memory!\n\n");
       }
    }
    
    //free pointer
    for (loop = 0; loop < x; loop++)
    {
        free (group_total[loop]);
    }
    I was just wondering:

    1) Is this correct? It compiles and seems to run correctly, but there's no certainty there...

    2) Is this everything? ie: Do I need to free the first subscript also?
    Last edited by Lionmane; 07-05-2005 at 10:37 PM.

  2. #2
    Work in Progress..... Jaken Veina's Avatar
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    1) No. See second question.

    2) Your number of malloc() statements should equal your number of free() statements. So yes, after you free the second (y) subscripts, you should free the first (x). It must be after, however, for if you free the first subscript first, you will have no way to access the second subscripts and free them.
    Code:
    void function(void)
     {
      function();
     }

  3. #3
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    Did I do anything wrong in my declaration?

    mw

  4. #4
    C/C++Newbie Antigloss's Avatar
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    and for the first malloc, the space to be allocated should be of type long *
    long **group_total;
    group_total = malloc (x * sizeof (long *));

  5. #5
    Registered User Codeplug's Avatar
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    >> // create first subscript
    >> long **group_total;
    >> group_total = malloc (x * sizeof (long));
    Close. What you need to create is an array of pointers to long. When you create an array of N elements, you need to allocate N times the size of each element. In this case, that's "x * sizeof(long*)".

    And as Jken Veina pointed out, you need to free that array last.

    gg

    [edit]Antigloss is a quick one

  6. #6
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    How about this:

    Code:
    int loop, loop2;
    int x = 5;
    int y = 2;
    
    // create first subscript
    long **group_total;
    group_total = malloc (x * sizeof (long *));
    
    if (!group_total)
    {
       printf ("\n\nError!  Not enough memory!\n\n");
    }
    
    //create second subscript
    for (loop = 0; loop < x; loop++)
    {
       group_total[loop] = malloc (y * sizeof (long));
    
       if (!group_total[loop])
       {
          printf ("\n\nError!  Not enough memory!\n\n");
       }
    }
    
    //free pointer
    for (loop = 0; loop < x; loop++)
    {
        for (loop2 = 0; loop2 < y; loop2++)
        {
            free (group_total[loop][loop2]);
        }
    
        free (group_total[loop]);
    }
    Last edited by Lionmane; 07-05-2005 at 11:19 PM.

  7. #7
    C/C++Newbie Antigloss's Avatar
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    no. it should be
    Code:
    for (loop = 0; loop < x; loop++)
    {
            free (group_total[loop]);
    
    }
    free (group_total);//free the space allocated by the first malloc

  8. #8
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    Sorry 'bout that. ONE MORE TIME... :-)

    Code:
    int loop;
    int x = 5;
    int y = 2;
    
    // create first subscript
    long **group_total;
    group_total = malloc (x * sizeof (long *));
    
    if (!group_total)
    {
       printf ("\n\nError!  Not enough memory!\n\n");
    }
    
    //create second subscript
    for (loop = 0; loop < x; loop++)
    {
       group_total[loop] = malloc (y * sizeof (long));
    
       if (!group_total[loop])
       {
          printf ("\n\nError!  Not enough memory!\n\n");
       }
    }
    
    //free second malloc
    for (loop = 0; loop < x; loop++)
    {
            free (group_total[loop]);
    }
    
    //free first malloc
    free (group_total);
    Last edited by Lionmane; 07-05-2005 at 11:20 PM.

  9. #9
    I typecast anyway.
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    I haven't put it into my compiler to check it, but it looks like you've got the hang of it so far.

    EDIT: If you check a malloc and find there wasn't enough memory, you should probably do something about it instead of continuing to run through the program.

  10. #10
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    Ha! Yeah, good idea! :-)

    How do I create a 3D array by adding a "z-axis"?

    mw

  11. #11
    and the hat of wrongness Salem's Avatar
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    > How do I create a 3D array by adding a "z-axis"?
    Add another * to your variable - so its long ***group_total;
    Then add another loop to malloc each group_total[x][y]
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  12. #12
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    So would that be:

    Code:
    int loop, loop2;
    int x = 5;
    int y = 2;
    int z = 3;
    
    // create first subscript
    long ***group_total;
    group_total = malloc (x * sizeof (long *));
    
    if (!group_total)
    {
       printf ("\n\nError!  Not enough memory!\n\n");
       exit (1);
    }
    //create second subscript
    for (loop = 0; loop < x; loop++)
    {
       group_total[loop] = malloc (y * sizeof (long *));
    
       if (!group_total[loop])
       {
          printf ("\n\nError!  Not enough memory!\n\n");
          exit (1)
       }
    }
    
    //create third subscript
    for (loop = 0; loop < x; loop++)
    {
       for (loop2 = 0; loop2 < y; loop2++)
       {
          group_total[loop][loop2] = malloc (z * sizeof (long));
    
          if (!group_total[loop][loop2])
         {
             printf ("\n\nError!  Not enough memory!\n\n");
             exit (1)
         }
       }
    }
    
    //free third malloc
    for (loop = 0; loop < x; loop++)
    {
        for (loop2 = 0; loop2 < y; loop2++)
        {
            free (group_total[loop][loop2]);
        }
    }
    
    //free second malloc
    for (loop = 0; loop < x; loop++)
    {
            free (group_total[loop]);
    }
    
    //free first malloc
    free (group_total);
    I changed the second malloc to:
    group_total[loop] = malloc (y * sizeof (long *));

    Is that right?

    mw
    Last edited by Lionmane; 07-06-2005 at 02:53 PM.

  13. #13
    C/C++Newbie Antigloss's Avatar
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    but the first should be malloc(x * sizeof (long **))

  14. #14
    Work in Progress..... Jaken Veina's Avatar
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    And, you could combine the two loops.
    Code:
    int loop, loop2;
    int x = 5;
    int y = 2;
    int z = 3;
    
    // create first subscript
    long ***group_total;
    group_total = malloc (x * sizeof (long **));
    
    if (!group_total)
     {
      printf ("\n\nError!  Not enough memory!\n\n");
      exit (1);
     }
    
    //create second subscript
    for (loop = 0; loop < x; loop++)
     {
      group_total[loop] = malloc (y * sizeof (long *));
    
      if (!group_total[loop])
       {
        printf ("\n\nError!  Not enough memory!\n\n");
        exit (1)
       }
    
      //create third subscript
      for (loop2 = 0; loop2 < y; loop2++)
       {
        group_total[loop][loop2] = malloc (z * sizeof (long));
    
        if (!group_total[loop][loop2])
         {
          printf ("\n\nError!  Not enough memory!\n\n");
          exit (1)
         }
       }
     }
    
    for (loop = 0; loop < x; loop++)
     {
      //free third malloc
      for (loop2 = 0; loop2 < y; loop2++)
        free (group_total[loop][loop2]);
    
      //free second malloc
      free (group_total[loop]);
     }
    
    //free first malloc
    free (group_total);
    Code:
    void function(void)
     {
      function();
     }

  15. #15
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    Thanks guys! Your help has been extremely efficient!

    mw

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