I programmed a a neural network class in python using numpy. I had some trouble getting it to work properly, in my case I wanted to train it with back propagation to approximate XOR, but then it started working, then it didn't, and overall it only seems to work sometimes. Just to be clear, what I mean is that at each step in back propagation I record the network's rms error and in the end I plot it, sometimes the error goes to zero and sometimes it gets hung up at about .85.

This is my code(I guess that makes this a python thread now, but I thought this might be a problem to do with neural networks in general.) I don't know how readable it is but if you have numpy and matplotlib then you can run it a few times and see what I mean.

import numpy
from matplotlib import pyplot

def sigmoid(x):
    return numpy.tanh(x)

def dsigmoid(y):
    return 1.0 - y**2

class ANN:
    def __init__(self, dims):
        self.activations = numpy.zeros((len(dims), max(dims)))
        self.biases = numpy.random.random((len(dims), max(dims)))
        self.weights = numpy.random.random((len(dims), max(dims), max(dims)))
        self.dims = dims
        self.num_layers = len(dims)

    def run(self, inputs):
        if len(inputs) != self.dims[0]:
            raise ValueError, "Network expected " + repr(self.dims[0]) + " inputs but " + repr(len(inputs)) + " were given"

        self.activations[0,:self.dims[0]] = inputs
        for a in xrange(1, self.num_layers):
            for b in xrange(self.dims[a]):
                self.activations[a,b] = sigmoid(self.biases[a,b] + sum(self.weights[a,b,:self.dims[a-1]]*self.activations[a-1,:self.dims[a-1]]))

        return self.activations[-1,:self.dims[-1]]

    def back_propogation(self, inputs, outputs, iterations = 1, learning_rate = .5, momentum = .1):
        if len(inputs) != len(outputs):
            raise ValueError, repr(len(inputs)) + " inputs were given but " + repr(len(outputs)) + " outputs were given"
        for a in xrange(len(inputs)):
            if len(inputs[a]) != self.dims[0]:
                raise ValueError, "Network expected " + repr(self.dims[0]) + " inputs but input " + repr(a) + " gives " + repr(len(inputs[a])) + " inputs"
            elif len(outputs[a]) != self.dims[-1]:
                raise ValueError, "Network returns " + repr(self.dims[0]) + " outputs but output " + repr(a) + " expects " + repr(len(inputs[a])) + " outputs"

        mse = numpy.zeros(iterations)
        deltas = self.biases*0
        inertias = self.weights*0
        bias_inertias = self.biases*0
        for a in xrange(iterations):
            for b in xrange(len(inputs)):
                self.run(inputs[b])
                error = outputs[b] - self.activations[-1,:self.dims[-1]]
                mse[a] += sum(error**2)/len(error)
                deltas[-1,:self.dims[-1]] = dsigmoid(self.activations[-1,:self.dims[-1]]) * error

                for c in xrange(len(deltas) - 2, 0, -1):
                    error = numpy.zeros(self.dims[c])
                    for d in xrange(0, self.dims[c]):
                        error[d] = sum(deltas[c+1,:self.dims[c+1]]*self.weights[c+1,:,d])
                    deltas[c,:self.dims[c]] = dsigmoid(self.activations[c,:self.dims[c]]) * error

                for c in xrange(len(deltas) - 1, 0, -1):
                    for d in xrange(self.dims[c]):
                        change = deltas[c,d]*self.activations[c-1,:self.dims[c-1]]
                        #print change
                        self.weights[c,d,:self.dims[c-1]] += learning_rate*change + momentum*inertias[c,d]
                        self.biases[c,d] += learning_rate*deltas[c,d] + momentum*bias_inertias[c,d]
                        inertias[c,d] = change
                        bias_inertias[c,d] = deltas[c,d]
                
        return mse

a = ANN((2, 2, 1))

inputs = numpy.array([[0, 0], [0, 1], [1, 0], [1, 1]])
outputs = numpy.array([[0], [1], [1], [0]])

error = a.back_propogation(inputs, outputs, 1000)

print a.run(inputs[0])
print a.run(inputs[1])
print a.run(inputs[2])
print a.run(inputs[3])

pyplot.plot(error)
pyplot.show()

Try adjusting your learning rate and momentum to smaller values. Sometimes, with back propagation, it is possible to skip over a minima or maxima. The momentum and learning rate are meant to help avoid localized minima or maxima, but if they are too large for the "nature" of your data you may never reach "equilibrium".

I tried that and it didn't seem to help, in fact if I plot the error instead of the rms error the network seems to oscillate about zero error when it works properly, and when it doesn't work properly it asymtotically goes to a value other than 0.

Attachments correct.png 20.03 KB incorrect.png 18.13 KB
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