Global array - size

This is a discussion on Global array - size within the C Programming forums, part of the General Programming Boards category; How can I declare a global array whose size I calculate in a function later on (based on # of ...

  1. #1
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    Global array - size

    How can I declare a global array whose size I calculate in a function later on (based on # of rows & columns of terminal ).

    Code:
    // global vars
    int row, col;
    char box[row][col];  // causes error
    
    ...
    
    void getsize(){
    isatty(STDIN_FILENO);
    struct winsize sz;
    ioctl(STDIN_FILENO,TIOCGWINSZ,&sz);
    row=sz.ws_row;
    col=sz.ws_col;
    }
    Error I am getting: variably modified 'box' at file scope

  2. #2
    "I Win!" by U. Lose vart's Avatar
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    you need to use malloc
    To be or not to be == true

  3. #3
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    How can I do this?

    Or could someone please point me to a good tutorial.

    Thanks

  4. #4
    C++ Witch laserlight's Avatar
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    The tutorial on pointers has something to say about malloc.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
    Cogito Ergo Sum
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    I wasn't sure myself, based on that tutorial I'd say malloc a size to a created pointer and then use that pointer to point to the rows and columns

    int *ptrRow = malloc (sizeof(int));
    int *ptrCol = malloc (sizeof(int));
    =========================================
    Everytime you segfault, you murder some part of the world

  6. #6
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    Not right.

    A 2D array allocation needs one malloc, then a loop to malloc memory for the other dimension:
    Code:
    int **ptr;
    int i;
    ptr = malloc(sizeof(*ptr) * rows);
    if (!ptr) allocerror();
    for(i = 0; i < rows; i++)
    {
       ptr[i] = malloc(sizeof(int) * cols);
       if (!ptr[i]) allocerror();
    }
    --
    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.

  7. #7
    Cogito Ergo Sum
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    Quote Originally Posted by matsp View Post
    Not right.

    A 2D array allocation needs one malloc, then a loop to malloc memory for the other dimension:
    Code:
    int **ptr;
    int i;
    ptr = malloc(sizeof(*ptr) * rows);
    if (!ptr) allocerror();
    for(i = 0; i < rows; i++)
    {
       ptr[i] = malloc(sizeof(int) * cols);
       if (!ptr[i]) allocerror();
    }
    --
    Mats
    What's wrong with doing it my way? Then assigning values to the pointer after row and column have been determined?

    [*ptrRow][*ptrCol]
    =========================================
    Everytime you segfault, you murder some part of the world

  8. #8
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    But you are only allocating a single integer - in which case, 99% of the time, you can just use the ORIGINAL int value, instead of storing it in a pointer.

    --
    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.

  9. #9
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    Cool BIG thanks for the help!
    Last edited by Tupcia; 04-15-2008 at 03:55 PM.

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