学习TensorFlow之tf.placeholder()

TensorFlow版本号：1.1.0

placeholder的中文意思是占位符，类似于函数参数，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，如果不指定，那么可以传入任意的

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.
```