import tensorflow as tf
import numpy as np


input_data = np.random.rand(10, 10, 64)
positions = [0,9,8,2,3,5,1,2,4,6]

input_tensor = tf.placeholder(shape = (None,10,64) , dtype = tf.float32)
positions_tensor = tf.placeholder(shape = (None,) , dtype = tf.int32)


def gather_indexes(input_tensor, positions):
  """Gathers the vectors at the specific positions over a minibatch."""

  batch_size = tf.shape(input_tensor)[0]
  seq_length = tf.shape(input_tensor)[1]
  dim = tf.shape(input_tensor)[2]

  flat_offsets = tf.reshape(tf.range(0, batch_size, dtype=tf.int32) * seq_length, [-1, 1])
  flat_positions = tf.reshape(tf.expand_dims(positions, axis=-1) + flat_offsets, [-1])
  flat_sequence_tensor = tf.reshape(input_tensor,
                                    [batch_size * seq_length, dim])
  output_tensor = tf.gather(flat_sequence_tensor, flat_positions)  ## slices tensor by positions
  return output_tensor


init = tf.global_variables_initializer()

with tf.Session() as sess:

    sess.run(init)
    result = sess.run(gather_indexes(input_tensor, positions_tensor),
                      feed_dict = {input_tensor: input_data,
                                   positions_tensor: positions})

## check result
print(result[0])
print(input_data[0][positions[0]])