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I would like to solve the following classification problem: Given an input sequence and zero or more initial terms of the true output, predict the next term in the output sequence.

For example, my training set is a large corpus of question and answer pairs. Now I would like to predict the response to "How are you?" one word at a time.

Some reasonable responses might be "I'm fine.", "OK.", "Very well, thanks." etc. So the predicted distribution over possible first words would include the first words in those examples, among others.

But now if I see the first word is "Very", I would like my prediction for the second word to reflect that "OK" is now less likely, because "Very OK" is not a common response.

And I would like to repeat this process of predicting the next word given the input sentence and every word so far in the output sentence.

One approach might be to train a stateful RNN on examples like How are you<END> I am fine<END>. But if I understand correctly this would also learn to predict words in the "How are you" part, which I think might detract from the goal.

Maybe separate layers for the input and partial output that are then merged into a decoder layer?

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I assume, that your data looks somehow like this (a pair of question and answer)

Question,Answer
How are you?,I'm fine.
How are you?,OK.
How are you?,Very well, thanks.

I'd suggest to transform it in three features: Question (remains the same), InputSeq is the prefix of the answer generated in all possible lengths between zero and the number of words minus one.

Finally the NextWord is the expected result.

So for your initial data you ends with the following setup.

Question, InputSeq, NextWord
How are you,,I'm
How are you,I'm,fine
How are you,,OK
How are you,,Very
How are you,Very,well
How are you,Very well,thanks

The model takes Question and InputSeq as features and predicts the NextWord.

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  • $\begingroup$ Thanks. This looks like a promising approach. I thought about this already a bit and I was struggling with how best to tell the network where the input sequence ends and where the prefix sequence begins. For example "How are you today?" Should not be treated the same as "How are you?" followed by a response that starts with "Today". eg "Today I am fine." Is this as simple as having a special token to indicate the end of my input sequence? $\endgroup$
    – Imran
    Aug 27 '18 at 15:53
  • $\begingroup$ Can you also elaborate on what type of model you have in mind? I was struggling to decide whether this approach would make sense with an RNN, because I thought it wasn't standard to give RNNs successively longer prefixes like this. $\endgroup$
    – Imran
    Aug 27 '18 at 15:57
  • $\begingroup$ No, you need not a special token to indicate the end of input sequence, the question and the prefix of the answer are defined as two distinct features. $\endgroup$ Aug 27 '18 at 17:27
  • $\begingroup$ Ok how to have two features of variable length then? What type of model would take these? $\endgroup$
    – Imran
    Aug 27 '18 at 17:32
  • $\begingroup$ Frankly said I have no idea which model should be used for your setup. Intuitlively I'd try simplest model and over-fit to the maximum. How do you expect the answer to the question ´*"What time is it?"* should be generalized? $\endgroup$ Aug 28 '18 at 17:51
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After some more research, I believe this can be solved with a straightforward application of encoder-decoder networks.

In this tutorial, we can simply replace sampled_token_index and sampled_char with actual_token_index and actual_char, computed according. And of course in our case it's actual_word.

To summarize we divide our training set into input/output pairs, where output examples begin with a <START> token and end with <STOP>, and train a sequence to sequence model on these pairs, as described in the tutorial.

Then at inference time we feed the <START> token to the model to predict the next word. Then after we receive the actual next word, we feed the actual (observed) output so far into the model, and so on.

enter image description here The other answers had some interesting ideas, but unfortunately they didn't really address how to deal with variable length inputs and outputs. I believe seq2seq models based on RNNs are the best way to address this.

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How would a high density and low tree depth random forest perform on this? I think it should be very good (if not the best) approach for this task. You can simply use parts of your possible answers also as attributes during the training process.

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  • $\begingroup$ Interesting idea. Can you elaborate on how you want to encode the inputs and feed them to the random forest? $\endgroup$
    – Imran
    Aug 30 '18 at 18:22

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