tensorflow - x = tf.placeholder(tf.float32，[None，784])是什么意思？

Question

我知道tf.placeholder的基本用法:

x = tf.placeholder(tf.float32, shape=(1024, 1024))
y = tf.matmul(x, x)

with tf.Session() as sess:
   print(sess.run(y))  # ERROR: will fail because x was not fed.

   rand_array = np.random.rand(1024, 1024)
   print(sess.run(y, feed_dict={x: rand_array}))  # Will succeed.

我知道第二个参数是关于 shape 的。但是，我不知道第一个形状为时是什么意思。例如:[无，784]。

Answer 1

在本教程中:Deep MNIST for Experts

Here we assign it a shape of [None, 784], where 784 is the dimensionality of a single flattened 28 by 28 pixel MNIST image, and None indicates that the first dimension, corresponding to the batch size, can be of any size.