学习TensorFlow之tf.placeholder()

TensorFlow版本号：1.1.0

placeholder的中文意思是占位符，相似于函数参数，运行时必须传入值。

def placeholder(dtype, shape=None, name=None):
  """Inserts a placeholder for a tensor that will be always fed.

  **Important**: This tensor will produce an error if evaluated. Its value must
  be fed using the `feed_dict` optional argument to `Session.run()`,
  `Tensor.eval()`, or `Operation.run()`.

  Args:
    dtype: The type of elements in the tensor to be fed.
    shape: The shape of the tensor to be fed (optional). If the shape is not
      specified, you can feed a tensor of any shape.
    name: A name for the operation (optional).

  Returns:
    A `Tensor` that may be used as a handle for feeding a value, but not
    evaluated directly.
  """

上述为官方文档中的说明，讲述的很详细。

dtype是指数据类型，shape是指tensor的维度，若是不指定，那么能够传入任意的

For example:

  ```python
  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.
  ```