Version 13 of Neural Networks

A Tcl extension for neural networks processing using the FANN library can be found at tcl-fann.

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(RS would call this A little feed-forward back-propagation learning neural network.)

There came a time when I needed to do a series of simulations of a feed-forward back-propagation (FFBP) neural network . Rather then spend just an hour doing the homework, I instead spent over 10 hours writing a full-fledged network builder and simulator. As you can see from the screenshot above the result is a little tool that allows the user to graphically draw a network and then run simulations upon it.

Features include:

hidden and bias nodes
choice of the sigmoid (or logistic function ) or bipolar squashing function
configurable eta value (learning rate)
learning by way of steepest descent and delta rule

The bipolar function is g(x)=2f(x)-1, where f(x) is the sigmoid function. Conveniently, its derivative is g'(x)=0.5(1+g(x))(1-g(x)). A more general-purpose steepest descent algorithm may be found at Differentiation and steepest-descent, though my FFBP does not use his code.

Usage

Shift-click the canvas to place a node. Shift-drag between nodes to add a weight link. Reposition nodes by dragging them around. Double click a node or weight to change its properties.

The real power of the program is its ability to run a number of trials and to learn after each test datum. To do this requires modifying the code a bit. See the last three functions in ffbp.tcl. The current functions demonstrate how to learn the XOR function; the corresponding network is saved in ffbp.net.

While you are at it, take a look at another kind of neural network, Hopfield Networks.

To do list:

add momentum value
better interface for running trials

Downloads

Version 0.2 - http://tcl.jtang.org/ffbp/ffbp-0.2.tar.gz

This version breaks save file compatibility with previous version!
Remove nodes and weights by double-clicking them, then clicking on 'Remove' button.
Show current weights directly on the canvas.
Adjust weight format using format codes; set string to empty to see raw value.

Version 0.1 - http://tcl.jtang.org/ffbp/ffbp-0.1.tar.gz

Initial hack-ish release.