Thread: Declaring sizeless 2 dimensional array

  1. #1
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    Apr 2012

    Declaring sizeless 2 dimensional array

    Im a newbie in C. But i know fortran very well. I try things i do in fortran in c. I'm trying to declare a 2 dimensional matrix globally without specific sizes. Later im planing to allocate it according to input. I can do this

    float a[]; or this a[10] or this a[10][10]

    But i cant do this

    float a[][] or this a[2][] or this a[][3]

    Im trying to do something like

    real :: a(:,: ) (in fortran)

    Can you show me how to declare and allocate according to input?

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    If you don't know the size until runtime, you do this
    double **array;
    int row, col;
    // initialise row/col somehow
    array = malloc( row * sizeof(*array);
    for ( r = 0 ; r < row ; r++ ) {
      array[r] = malloc( col * sizeof(*array[r]);
    // do stuff
    // when you're done
    for ( r = 0 ; r < row ; r++ ) {
    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.

  3. #3
    Registered User
    Join Date
    Mar 2011
    the alternative that has a lot less complexity to it, and less management of pointers is to allocate a single block of data and compute the array index yourself.
    Question 6.16

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