So far in this book, we’ve introduced you to the use of deep learning to process various types of inputs. We started from simple linear and logistic regression on fixed dimensional feature vectors, and then followed up with a discussion of fully connected deep networks. These models take in arbitrary feature vectors with fixed, predetermined sizes. These models make no assumptions about the type of data encoded into these vectors. On the other hand, convolutional networks place strong assumptions upon the structure of their data. Inputs to convolutional networks have to satisfy a locality assumption that allows for the definition of a local receptive field.

How could we use the networks that we’ve described thus far to process data like sentences? Sentences do have some locality properties (nearby words are typically related), and it is indeed possible to use a one dimensional convolutional network to process sentence data. That said, most practitioners resort to a different type of architecture, the recurrent neural network, in order to handle sequences of data.

Recurrent neural networks (RNNs) are designed natively to allow deep networks to process sequences of data. RNNs assume that incoming data takes the form of a sequence of vectors or tensors. If we transform each word in a sentence into a vector (more on how to do this later), sentences can be fed into RNNs. Similarly, video (considered as a sequence of images) can similarly be processed ...

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