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I’m new to machine learning and am a bit confused about how to create a neural network (multilayer perceptron) with a specific design.

I would like to create a neural network with 40 features/neurones in input layer, 2 hidden layers each with 200 neurones and sigmoid activation function then one neurone in output layer to predict a scalar.

Should I use NetChain and then have the layers:

LinearLayer[{40,200}],ElementwiseLayer[LogisticSigmoid],LinearLayer[{200,200}], ElementwiseLayer[LogisticSigmoid],LinearLayer[{200,1}]

In order to produce the network I have in mind?

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net = NetChain[{LinearLayer[200], ElementwiseLayer[LogisticSigmoid], 
   LinearLayer[200], ElementwiseLayer[LogisticSigmoid], 
   LinearLayer[1]}, "Input" -> 40, "Output" -> NetDecoder["Scalar"]]

Information[net, "SummaryGraphic"]

Mathematica graphics

This can also be produced more concisely:

net = NetChain[{200, LogisticSigmoid, 200, LogisticSigmoid, 1}, 
  "Input" -> 40, "Output" -> NetDecoder["Scalar"]]
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  • $\begingroup$ Thank you. Could you explain please why we need a 3rd linear layer and activation layer? Why after the two hidden layers can we not go directly to an output neuron/layer? $\endgroup$
    – Gabi23
    Aug 19, 2021 at 17:09
  • $\begingroup$ I have edited the question and removed the last activation layer. You don't need it unless you want to force your output to the $[0,1]$ range. You do, however, need the last layer (imagine it as being the "output" layer) so that you get from 400 nodes to a scalar output. $\endgroup$
    – Domen
    Aug 19, 2021 at 17:15
  • $\begingroup$ Thank you for great explanation. $\endgroup$
    – Gabi23
    Aug 19, 2021 at 17:16
  • $\begingroup$ When I use this net code, I get an error when I go to train it - output is not a scalar (which I want it to be) but a 9000-element array. My trainingdata set has 9000 elements which are of the form {-4.007, 0.3698, -1., -0.1528, -0.5851, 0.4517, -0.1968, -0.1535, 0.076, 0.2107, -0.32301, -0.3082, 1., 0.2733, 0.5368, -0.31, 0.01269, 0.298, -0.3772, -0.3255, 0.223, 0.2298, -0.683, 0.0712, -0.87, 0.4012, -0.324, 0.3523, 0.8496, 0.084, -0.007, 0.6469, -0.222, 0.8281, -0.4817, 0.282618, 0.0196, -0.1807, -0.246, -0.1187} -> 0.267 . What could this be? $\endgroup$
    – Gabi23
    Aug 19, 2021 at 21:08
  • $\begingroup$ As you can see in the SummaryGraphic, the output is supposed to be a vector with size 1. So your data should look like {x1, x2, ..., x40} -> {y1}. However, the other solution is to explicitly specify that you want the output to be a scalar: NetChain[{200, LogisticSigmoid, 200, LogisticSigmoid, 1}, "Input" -> 1, "Output" -> NetDecoder["Scalar"]]. I hope either of these two solve your problem. $\endgroup$
    – Domen
    Aug 19, 2021 at 21:16

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