x = tf.reshape(x, shape = [-1, 28, 28, 1])中-1的解释

今天在学习tensorflow深度神经网络的时候发现有这样一行代码：

x = tf.reshape(x, shape = [-1, 28, 28, 1])

其中的参数：-1让我不知所措，其实他就是一种参数的省略，几个例子

z = np.array([[1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12],
          [13, 14, 15, 16]])
z.shape
(4, 4)

z.reshape(-1)
array([ 1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16])