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Create, Configure, and Initialize Multilayer Neural Networks

This topic presents part of a typical multilayer network workflow. For more information and other steps, see Multilayer Neural Networks and Backpropagation Training.

After the data has been collected, the next step in training a network is to create the network object. The function feedforwardnet creates a multilayer feedforward network. If this function is invoked with no input arguments, then a default network object is created that has not been configured. The resulting network can then be configured with the configure command.

As an example, the file bodyfat_dataset.mat contains a predefined set of input and target vectors. The input vectors define data regarding physical attributes of people and the target values define percentage body fat of the people. Load the data using the following command:

load bodyfat_dataset

Loading this file creates two variables. The input matrix bodyfatInputs consists of 252 column vectors of 13 physical attribute variables for 252 different people. The target matrix bodyfatTargets consists of the corresponding 252 body fat percentages.

The next step is to create the network. The following call to feedforwardnet creates a two-layer network with 10 neurons in the hidden layer. (During the configuration step, the number of neurons in the output layer is set to one, which is the number of elements in each vector of targets.)

net = feedforwardnet;
net = configure(net, bodyfatInputs, bodyfatTargets);

Optional arguments can be provided to feedforwardnet. For instance, the first argument is an array containing the number of neurons in each hidden layer. (The default setting is 10, which means one hidden layer with 10 neurons. One hidden layer generally produces excellent results, but you may want to try two hidden layers, if the results with one are not adequate. Increasing the number of neurons in the hidden layer increases the power of the network, but requires more computation and is more likely to produce overfitting.) The second argument contains the name of the training function to be used. If no arguments are supplied, the default number of layers is 2, the default number of neurons in the hidden layer is 10, and the default training function is trainlm. The default transfer function for hidden layers is tansig and the default for the output layer is purelin.

The configure command configures the network object and also initializes the weights and biases of the network; therefore the network is ready for training. There are times when you might want to reinitialize the weights, or to perform a custom initialization. Initializing Weights (init) explains the details of the initialization process. You can also skip the configuration step and go directly to training the network. The train command will automatically configure the network and initialize the weights.

Other Related Architectures

While two-layer feedforward networks can potentially learn virtually any input-output relationship, feedforward networks with more layers might learn complex relationships more quickly. For most problems, it is best to start with two layers, and then increase to three layers, if the performance with two layers is not satisfactory.

The function cascadeforwardnet creates cascade-forward networks. These are similar to feedforward networks, but include a weight connection from the input to each layer, and from each layer to the successive layers. For example, a three-layer network has connections from layer 1 to layer 2, layer 2 to layer 3, and layer 1 to layer 3. The three-layer network also has connections from the input to all three layers. The additional connections might improve the speed at which the network learns the desired relationship.

The function patternnet creates a network that is very similar to feedforwardnet, except that it uses the tansig transfer function in the last layer. This network is generally used for pattern recognition. Other networks can learn dynamic or time-series relationships.

Initializing Weights (init)

Before training a feedforward network, you must initialize the weights and biases. The configure command automatically initializes the weights, but you might want to reinitialize them. You do this with the init command. This function takes a network object as input and returns a network object with all weights and biases initialized. Here is how a network is initialized (or reinitialized):

net = init(net);
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