unpack的一点使用问题

• 现象

lua支持不定数量参数，通过...传送，使用unpack解开。看一段代码

local function arg2(...)
    print('****************')
    print(unpack(arg))
    for k, v in pairs(arg) do
        print(k, v)
    end
    print(arg[1], arg[2], arg[3])
    print('++++++++++++++++')
end
arg2('a', nil, 'c')
arg2(nil, 'b', 'c')
arg2(nil, 'b', nil)

其实我们预期传入三个参数，但是由于某种情况，有的参数值为nil，我们当然希望在unpack后还和传入的值一样。看一下执行结果

我们发现，前面两个和预期一致，第三个unpack却没有得到内容，是个空串，但按下标逐个打印的时候内容仍然是存在的，这样势必会导致函数执行结果不能和传入的参数一致。

• 解释

unpack的说明如下，从指定的范围依次取出，如果不指定则从开始1到长度l，由求长度操作符#决定。

上面的代码中没有给出i和j，使用默认处理。换为指定范围如下：

local function arg2(...)
    print('****************')
    print(unpack(arg, 1, 3))
    for k, v in pairs(arg) do
        print(k, v)
    end
    print(arg[1], arg[2], arg[3])
    print('++++++++++++++++')
end
arg2('a', nil, 'c')
arg2(nil, 'b', 'c')
arg2(nil, 'b', nil)

执行后如下

 这时候就和预期的一致了。不幸的是，我们并不总是知道这个长度的。而未指定时结果不对，猜想和#操作的结果有关，其文档说明是

关键是后面补充的部分，如果第一个元素是nil，则长度为0；如果非nil值之间有了“洞”，则不确定长度了。

• 代码

下面是unpack的代码

static int luaB_unpack (lua_State *L) {
  int i, e, n;
  luaL_checktype(L, 1, LUA_TTABLE);
  i = luaL_optint(L, 2, 1);
  e = luaL_opt(L, luaL_checkint, 3, luaL_getn(L, 1));
  n = e - i + 1;  /* number of elements */
  if (n <= 0) return 0;  /* empty range */
  luaL_checkstack(L, n, "table too big to unpack");
  for (; i<=e; i++)  /* push arg[i...e] */
    lua_rawgeti(L, 1, i);
  return n;
}

 下面是求解#的过程

static int unbound_search (Table *t, unsigned int j) {
  unsigned int i = j;  /* i is zero or a present index */
  j++;
  /* find `i' and `j' such that i is present and j is not */
  while (!ttisnil(luaH_getnum(t, j))) {
    i = j;
    j *= 2;
    if (j > cast(unsigned int, MAX_INT)) {  /* overflow? */
      /* table was built with bad purposes: resort to linear search */
      i = 1;
      while (!ttisnil(luaH_getnum(t, i))) i++;
      return i - 1;
    }
  }
  /* now do a binary search between them */
  while (j - i > 1) {
    unsigned int m = (i+j)/2;
    if (ttisnil(luaH_getnum(t, m))) j = m;
    else i = m;
  }
  return i;
}


/*
** Try to find a boundary in table `t'. A `boundary' is an integer index
** such that t[i] is non-nil and t[i+1] is nil (and 0 if t[1] is nil).
*/
int luaH_getn (Table *t) {
  unsigned int j = t->sizearray;
  if (j > 0 && ttisnil(&t->array[j - 1])) {
    /* there is a boundary in the array part: (binary) search for it */
    unsigned int i = 0;
    while (j - i > 1) {
      unsigned int m = (i+j)/2;
      if (ttisnil(&t->array[m - 1])) j = m;
      else i = m;
    }
    return i;
  }
  /* else must find a boundary in hash part */
  else if (t->node == dummynode)  /* hash part is empty? */
    return j;  /* that is easy... */
  else return unbound_search(t, j);
}