// various destructive functions ("procedures") on arrays
public class Exercise4b
{
  // replaces in a all elements at positions 
  // from p (inclusive) to q (exclusive) by x
  public static void fill(char[] a, int p, int q, char x)
  {
    int n = a.length;
    for (int i = p; i < q; i++)
    {
      a[i] = x;
    }
  }

  // reverts a
  public static void revert(int[] a)
  {
    int i = 0;
    int j = a.length-1;
    while (i < j)
    {
      int t = a[i];
      a[i] = a[j];
      a[j] = t;
      i = i+1;
      j = j-1;
    }
  }

  // subtracts from a (an array of natural) numbers the corresponding 
  // elements of b (also an array of natural numbers) but sets
  // the result to 0, if it would become negative
  // the return value indicates, if this has happened
  public static boolean subtract(int[] a, int[] b)
  {
    int n = a.length;
    boolean truncated = false;
    for (int i=0; i<n; i++)
    {
      if (a[i] >= b[i])
        a[i] = a[i]-b[i];
      else
      {
        a[i] = 0;
        truncated = true;
      }
    }
    return truncated;
  }
}