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At this point we need to fill out the 1s in each vector. We can loop over each English-Spanish pair in our training sample using the features dictionaries to add a 1 for the token in question. For example, the dog sentence (["the", "dog", "licked", "me"]) would be split into the following matrix of vectors:

[
  [1, 0, 0, 0], # timestep 0 => "the"
  [0, 1, 0, 0], # timestep 1 => "dog"
  [0, 0, 1, 0], # timestep 2 => "licked"
  [0, 0, 0, 1], # timestep 3 => "me"
]

You’ll notice the vectors have timesteps — we use these to track where in a given document (sentence) we are.

To build out a three-dimensional NumPy matrix of one-hot vectors, we can assign a value of 1 for a given word at a given timestep in a given line:

matrix_name[line, timestep, features_dict[token]] = 1.

Keras will fit — or train — the seq2seq model using these matrices of one-hot vectors:

the encoder input data
the decoder input data
the decoder target data

Hang on a second, why build two matrices of decoder data? Aren’t we just encoding and decoding?

The reason has to do with a technique known as teacher forcing that most seq2seq models employ during training. Here’s the idea: we have a Spanish input token from the previous timestep to help train the model for the current timestep’s target token.

All tokens of the target sentence preceding the current token are passed to the decoder at each timestep.

Instructions

Inside the first nested for loop, assign 1. for the current line, timestep, and token in encoder_input_data.

Inside the second nested for loop, assign 1. for the current line, timestep, and token in decoder_input_data.

Inside the second nested for loop, assign 1. for the current line, the previous timestep (at timestep - 1), and token in decoder_target_data if timestep is greater than 0.

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