I'm a noob so I'll make this short. Is the following legal (C99)? I don't want to store more than the one array at a time. I could make an array that is of maximum needed length and use only part of it, but I'd rather do this.

int main(){
  double drm[15000];
  //do some stuff
  double drm[14000];  
  //do some other stuff
}

• arrays
• c99

No, you can't do that. They need to be in different blocks. You could do:

int main(void)
{
  {
    double drm[15000];
    // do some stuff
  }
  {
    double drm[14000];
    // do some other stuff
  }
}

But why not just call a function? Even if you put them in different blocks, you don't really have any guarantees about what the compiler is going to do vis-a-vis storage.

As arrays are strongly related to pointers in C, you could work with dynamic arrays as well. First, allocate a new double array with 15 000 elements. When you don't need it anymore, delete it. Create a new array with 14 000 elements then.

Don't forget to free your heap after using your arrays. People tend to forget this. Moreover, you should check whether the allocation was successful.

int main(void)
{
    double *drm = 0;
    drm = (double*)malloc(15000 * sizeof(double));
    // do something
    free(drm);
    drm = (double*)malloc(14000 * sizeof(double));
    // do some other stuff
    free(drm); // don't forget me
}

However, I would agree with Carl Norum: use functions instead.

You could use malloc and free (or realloc if you don't mind keeping the data):

#include <stdlib.h>
int main(void)
{
      double *drm = (double *)malloc(sizeof(double)*15000);
      //do stuff
      free(drm);
      drm = (double *)malloc(sizeof(double)*14000);
      //do other stuff
      free(drm);
}