How to use tensorFlow.placeholder in TensorFlow

We usually use tensorFlow.placeholder to handle train or test samples.

placeholder(
    dtype,
    shape=None,
    name=None
)

Explains:

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).

Here is an example:

import tensorflow as tf

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.

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