Function: nreverse

nreverse is a function defined in fns.c.

Signature

(nreverse SEQ)

Documentation

Reverse order of items in a list, vector or string SEQ.

If SEQ is a list, it should be nil-terminated. This function may destructively modify SEQ to produce the value.

Source Code

// Defined in /usr/src/emacs/src/fns.c
{
  if (NILP (seq))
    return seq;
  else if (STRINGP (seq))
    return Freverse (seq);
  else if (CONSP (seq))
    {
      Lisp_Object prev, tail, next;

      for (prev = Qnil, tail = seq; CONSP (tail); tail = next)
	{
	  next = XCDR (tail);
	  /* If SEQ contains a cycle, attempting to reverse it
	     in-place will inevitably come back to SEQ.  */
	  if (EQ (next, seq))
	    circular_list (seq);
	  Fsetcdr (tail, prev);
	  prev = tail;
	}
      CHECK_LIST_END (tail, seq);
      seq = prev;
    }
  else if (VECTORP (seq))
    {
      ptrdiff_t i, size = ASIZE (seq);

      for (i = 0; i < size / 2; i++)
	{
	  Lisp_Object tem = AREF (seq, i);
	  ASET (seq, i, AREF (seq, size - i - 1));
	  ASET (seq, size - i - 1, tem);
	}
    }
  else if (BOOL_VECTOR_P (seq))
    {
      ptrdiff_t i, size = bool_vector_size (seq);

      for (i = 0; i < size / 2; i++)
	{
	  bool tem = bool_vector_bitref (seq, i);
	  bool_vector_set (seq, i, bool_vector_bitref (seq, size - i - 1));
	  bool_vector_set (seq, size - i - 1, tem);
	}
    }
  else
    wrong_type_argument (Qarrayp, seq);
  return seq;
}