A hint

You will be working with parts of arrays. The following construction is quite useful (and very common).

The idea is to pass the address of an element of an array and receive it as an array.

First in Fortran:

      program main
      integer  v(10)

      do k = 1, 10
	v(k) = k
      end do

      print*, 'before: ', v
      call sub(v(5))         ! Note v(5)
      print*, 'after:  ', v

      end

      subroutine sub(v)
      integer  v(*)          ! Note, array

      do k = 1, 3
	v(k) = 0
      end do

      end

When we run it we get the printout:
 

% a.out
 before:   1  2  3  4  5  6  7  8  9  10
 after:    1  2  3  4  0  0  0  8  9  10   Note the zeros

Now the same thing in C:

#include <stdio.h>

void sub(int []);

int main()
{
  int v[10], k;

  for(k = 0; k < 10; k++)
    v[k] = k + 1;


  printf("before: ");
  for(k = 0; k < 10; k++)
    printf("%d ", v[k]);

  sub(&v[4]);              /* Note & */

  printf("\nafter:  ");
  for(k = 0; k < 10; k++)
    printf("%d ", v[k]);
  printf("\n");

  return 0;

}

void sub(int v[])
{
  int k;

  for(k = 0; k < 3; k++)
    v[k] = 0;
}

This is the run:

% a.out
before: 1 2 3 4 5 6 7 8 9 10 
after:  1 2 3 4 0 0 0 8 9 10