Thread: Multi-dimensional Dynamic Arrays

  1. #1
    Registered User
    Join Date
    May 2004

    Multi-dimensional Dynamic Arrays

    I'm trying to create a function that converts a custom structure - Spline - to a 2-dimension array of doubles. The problem is that although the function works fine and stores the correct values in the 2-dimension array p, it seems to store them localy. That is, if i try to access for example p[0][0] within the funcion, i can do it, however outside the function i just get a memory error, as if i was accessing the outer space (which i probably am). Can anyone help me out here?


    double **knots;
    void SplineToVector(const Spline spline, double **p, int *size)
    	int i, j;
    	*size = spline.sections_list_size + 3;
    	p = new double*[*size];
    	for(i = 0; i < spline.sections_list_size; i++)
    		p[i] = new double[3];
    		for(j = 0; j < 3; j++)
    			p[i][j] = spline.sections_list[i].k_matrix[0][j];
    	for(i = 1; i < 4; i++)
    		p[spline.sections_list_size - 1 + i] = new double[3];
    		for(j = 0; j < 3; j++)
    			p[spline.sections_list_size - 1 + i][j] = spline.sections_list[spline.sections_list_size - 1].k_matrix[i][j];
    	printf("Sample Knot: %f\n", p[0][0];  // NO ERROR HERE
    void main()
    	Spline example;
    	//Other stuff related to Spline
    	SplineToVector(example, knots, n_knots);
    	printf("Sample Knot: %f\n", knots[0][0];  // MEMORY ERROR

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    You're not (or knot) returning a value to main (which is int main by the way)

    Easy way is
    knots = SplineToVector(example, n_knots);

    Where your function declares
    double **p
    locally, and finishes with a
    return p;
    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
    May 2004
    Worked, thnx.

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