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Types of RNN

The main reason that the recurrent nets are more exciting is that they allow us to operate over sequences of vectors: Sequence in the input, the output, or in the most general case, both. A few examples may this more concrete:

Types of RNN

Each rectangle in the above image represents vectors, and arrows represent functions. Input vectors are Red, output vectors are blue, and green holds RNN's state.


This is also called Plain Neural networks. It deals with a fixed size of the input to the fixed size of output, where they are independent of previous information/output.

Example: Image classification.


It deals with a fixed size of information as input that gives a sequence of data as output.

Example: Image Captioning takes the image as input and outputs a sentence of words.


It takes a sequence of information as input and outputs a fixed size of the output.

Example: sentiment analysis where any sentence is classified as expressing the positive or negative sentiment.


It takes a Sequence of information as input and processes the recurrently outputs as a Sequence of data.

Example: Machine Translation, where the RNN reads any sentence in English and then outputs the sentence in French.

Bidirectional Many-to-Many:

Synced sequence input and output. Notice that in every case are no pre-specified constraints on the lengths sequences because the recurrent transformation (green) is fixed and can be applied as many times as we like.

Example: Video classification where we wish to label every frame of the video.

Advantages of Recurrent Neural Network

  • RNN can model a sequence of data so that each sample can be assumed to be dependent on previous ones.
  • A recurrent neural network is even used with convolutional layers to extend the active pixel neighborhood.

Disadvantages of Recurrent Neural Network

  • Gradient vanishing and exploding problems.
  • Training an RNN is a complicated task.
  • It could not process very long sequences if it were using tanh or relu like an activation function.

Next TopicCNN vs RNN

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