Question

Hi there I am having a bit of trouble with defining an array of structures within a structure.
This is my idea, I need to have a structure called figure which holds the name of the figure, coordinate count and the coordinates (x,y). Each figure can have an arbitrary amount of coordinates.
I also need to be able to dynamically reallocate space for an ever increasing list of coords… Please help point me in the right direction.
thank you,

tyler

typedef struct {
  char fig_name[FIGURE_LEN + 1];
  int coordcount;
  /* here i need to declare an array of coord structures that 
     but i am not sure how to do this properly. I was originally
     going to try something like as follows */
  coords *pointer;
  pointer = malloc(sizeof(coords));
  pointer = coords figcoord[];
  /* however i am quite certain that this would not work */
} figure;

typedef struct {
  double x;
  double y;
} coords;

Answer 1

Kick toward the right direction. Try something like this. I apologize for the lack of error checking for the malloc() calls, but you will get the general idea (I hope):

#include <stdlib.h>

#define FIGURE_LEN  128

typedef struct
{
    double x;
    double y;
} coords;

typedef struct
{
    char fig_name[FIGURE_LEN + 1];
    int coordcount;
    coords *pointer;
} figure;


/* allocate a dynamic allocated figure */
figure* alloc_figure(char* name, int coordcount)
{
    figure *fig = malloc(sizeof(figure));
    fig->coordcount = coordcount;
    fig->pointer = malloc(sizeof(coords) * coordcount);
    strncpy(fig->fig_name, name, FIGURE_LEN);
    fig->fig_name[FIGURE_LEN] = 0;
    return fig;
}

/* release a dynamic allocated figure */
void free_figure(figure** ppfig)
{
    if (!*ppfig)
        return;

    free((*ppfig)->pointer);
    free(*ppfig);
    *ppfig = NULL;
}

int main(int argc, char *argv[])
{
    figure fig;
    fig.coordcount = 10;
    fig.pointer = malloc(10 * sizeof(coords));

    /* access fid.pointer[0..9] here... */
    fig.pointer[0].x = 1.0;
    fig.pointer[0].y = 1.0;

    /* don't  forget to free it when done */
    free(fig.pointer);

    /* dynamic allocation function use */
    figure *fig1 = alloc_figure("fig1", 10);
    figure *fig2 = alloc_figure("fig2", 5);

    fig1->pointer[9].x = 100.00;
    fig2->pointer[0].y = fig1->pointer[9].x;

    /* and use custom free function for releasing them */
    free_figure(&fig1);
    free_figure(&fig2);

    return EXIT_SUCCESS;
}