Rocks in the Playground

It’s debatable whether neural networks should feature in an introductory course on machine learning. But it’s hard to avoid at least mentioning them, and many people are attracted to machine learning courses because they have heard so much about deep learning. So, reluctantly, we almost always get into neural nets in our Machine learning for geoscientists classes.

Our approach is to build a neural network from scratch, using only standard Python and NumPy data structures — that is, without using a specialist deep-learning framework. The code is all adapted from Gram Ganssle’s awesome Leading Edge tutorial from 2018. I like it because it lays out the components — the data, the activation function, the cost function, the forward pass, and all the steps involved in backpropagation — then combines them into a working neural network.

Figure 2 from Gram Ganssle’s 2018 tutorial in the Leading Edge. Licensed CC BY.

One drawback of our approach is that it would be quite fiddly to change some aspects of the network. For example, adding regularization, which almost all networks use, or even just adding another layer, are both beyond the scope of the class. So I like to follow it up with getting the students to build the same network using the scikit-learn library’s multilayer perceptron model. Then we build the same network using the PyTorch framework. (And one could do it in TensorFlow too, of course.) All of these libraries make it easier to play with the various options.

Introducing the Rocky Playground

Now we have another tool — one that makes it even easier to change parameters, add layers, use regularization, and so on. The students don’t even have to write any code! I invite you to play with it too — check out the Rocky Playground, an interactive deep neural network you can see inside.

Part of the user interface. Click on the image to visit the site.

This tool is a fork of Google’s well-known Neural Network Playground, as described at the bottom of our tool’s page. We made a few changes:

• Added several new real and synthetic datasets, with descriptions.

• There are more activation functions to try, including ELU and Swish.

• You can change the regularization during training and watch the weights.

• Anyone can upload their own dataset! (These stay on your computer, they are not uploaded anywhere.)

• We added an the expression of the network in Python code.

One of the datasets we added is the same shear-sonic prediction dataset we use in the neural network class. So students can watch the same neural net they built (more or less) learn the task in real time. It’s really very cool.

I’ve written before about different expressions of mathematical ideas — words, symbols, annotations, code, etc. — and this is really just a natural extension of that thought. When people can hear and see the same idea in three — or five, or ten — different ways, it sticks. Or at least has a better chance of sticking.

What do you think? Do this tool help you? Could you use it for teaching? If you have suggestions feel free to drop them here in the comments, or submit an issue to the tool’s repo. We’d love your help to make it even more useful.

Life lessons from a neural network

The latest Geophysical Tutorial came out this week in The Leading Edge. It's by my friend Gram Ganssle, and it's about neural networks. Although the example in the article is not, strictly speaking, a deep net (it only has one hidden layer), it concisely illustrates many of the features of deep learning.

Whilst editing the article, it struck me that some of the features of deep learning are really features of life. Maybe humans can learn a few life lessons from neural networks! 

Seek nonlinearity

Activation functions are one of the most important ingredients in a neural network. They are the reason neural nets are able to learn complex, nonlinear relationships without a gigantic number of parameters.

Life lesson: look for nonlinearities in your life. Go to an event aimed at another profession. Take a new route to work. Buy a random volume at your local bookshop. Pick that ice-cream flavour you've never dared try (durian, anyone?).

Iterate

Neural networks learn by repetition. They start with random guesses about what might work, then they process each data point a hundred, maybe 100,000 times, check the answer, adjust weights, and get a little better each time. 

Life lesson: practice makes perfect. You won't get anything right the first time (if you do, celebrate!). The important thing is that you pay attention, figure out what to change, and tweak it. Then try again.

More data

One of the things we know for sure about neural networks is that they work best when they train on a lot of data. They need to see as much of the problem domain as possible, including the edge cases and the worst cases.

Life lesson: seek data. If you're a geologist, get out into the field and see more rocks. Geophysicists: look at more seismic. Whoever you are, read more. Afterwards, share what you find with others, and listen to what they have learned.

Stretch metaphors

Yes, well, I could probably go on. Convolutional networks teach us to create new things by mixing ideas from different parts of our experience. Long training times for neural nets teach us to be patient, and invest in GPUs. Hidden layers with many units teach us to... er, expect a lot of parameters in our lives...?

Anyway, the point is that life is like a neural net. Or maybe, no less interestingly, neural nets are like life. My impression is that most of the innovations in deep learning have come from people looking at their own interpretive and discriminatory powers and asking, "What do I do here? How do I make these decisions?" — and then trying to approximate that heuristic or thought process in code.

What's the lesson here? I have no idea. Enjoy your weekend!

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Thumbnail image by Flickr user latteda, licensed CC-BY. The Leading Edge cover is copyright of SEG, fair use terms.