how could i get both the final hidden state and sequence in a LSTM layer when using a bidirectional wrapper

Question

i have followed the steps in https://machinelearningmastery.com/return-sequences-and-return-states-for-lstms-in-keras/ But when it comes to the Bidirectional lstm, i tried this

lstm, state_h, state_c = Bidirectional(LSTM(128, return_sequences=True, return_state= True))(input)

but it won't work.

is there some approach to get both the final hidden state and sequence in a LSTM layer when using a bidirectional wrapper

Answer 1

The call Bidirectional(LSTM(128, return_sequences=True, return_state=True))(input) returns 5 tensors:

1. The entire sequence of hidden states, by default it'll be the concatenation of forward and backward states.
2. The last hidden state h for the forward LSTM
3. The last cell state c for the forward LSTM
4. The last hidden state h for the backward LSTM
5. The last cell state c for the backward LSTM

The line you've posted would raise an error since you want to unpack the returned value into just three variables (lstm, state_h, state_c).

To correct it, simply unpack the returned value into 5 variables. If you want to merge the states, you can concatenate the forward and backward states with Concatenate layers.

lstm, forward_h, forward_c, backward_h, backward_c = Bidirectional(LSTM(128, return_sequences=True, return_state=True))(input)
state_h = Concatenate()([forward_h, backward_h])
state_c = Concatenate()([forward_c, backward_c])