How to index a list with a TensorFlow tensor?

Question

Assume a list with non concatenable objects which needs to be accessed via a look up table. So the list index will be a tensor object but this is not possible.

Answer 1

Tensorflow actually has support for a HashTable. See the documentation for more details.

Here, what you could do is the following:

table = tf.contrib.lookup.HashTable(
    tf.contrib.lookup.KeyValueTensorInitializer(tf_look_up, list), -1)

Then just get the desired input by running

target = table.lookup(index)

Note that -1 is the default value if the key is not found. You may have to add key_dtype and value_dtype to the constructor depending on the configuration of your tensors.

Answer 2

tf.gather is designed for this purpose.

Simply run tf.gather(list, tf_look_up[index]), you'll get what you want.