# Multilayer perceptron does not converge

I have been coding my own multi layer perceptron in MATLAB and it compiles without error. My training data features, x, has values from 1 to 360, and the training data output, y, has the value of $$\sin(x)$$.

The thing is my MLP only decreases the cost for the first few iterations and will get stuck at 0.5. I have tried including momentum, but it does not help and increasing the layers or increasing the neurons does not help at all. I am not sure why this is happening.

The summary of my code is:

1. I normalize my input data either using min-max or zscore

2. Initialize random weights and bias within the range of -1 to 1

for i = 1:length(nodesateachlayer)-1
weights{i} = 2*rand(nodesateachlayer(i),nodesateachlayer(i+1))-1;
bias{i} = 2*rand(nodesateachlayer(i+1),1)-1;
end

3. Then, I do a forward pass where the input is multiplied by weights and added with the bias and then activated by a transfer function (sigmoid)

for i = 2:length(nodesateachlayer)
stored{i} = nactivate(bsxfun(@plus,(weights{i-1}'*stored{i-1}),bias{i-1}),activation);
end

4. Then calculate the error then do a backward pass

dedp = 1/length(normy)*error;
for i = length(stored)-1:-1:1
dpds = derivative(stored{i+1},activation);
deds = dpds'.*dedp;
dedw = stored{i}*deds;
dedb = ones(1,rowno)*deds;
dedp = (weights{i}*deds')';
weights{i}=weights{i}-rate.*dedw;
bias{i}=bsxfun(@minus,bias{i},rate.*dedb');
end

5. I have the cost plotted out at every iteration to see the descent

I assume there is something wrong with the code so where could the error possibly lie in it?

• Have you tried making any predictions on hold-out data? Which value range do you use? Sep 12, 2018 at 10:03
• I think something has to be wrong inside your backprop implementation. The code you uploaded is quite a lot and I doubt many people here will read it. Can you outline the important parts of your implementation and explain? E.g. how you calculate your gradients and update the weights Sep 12, 2018 at 10:07
• @n1k31t4 I did not. Because the cost always descends until around 0.5 and stop descending Sep 12, 2018 at 13:33
• @André thanks for the suggestions. Yes i am also thinking there's something wrong that's why I re-coded the entire MLP. At first I thought is the cost which I define wrongly since it suppose to be singular value and in my code, it's an array. After modification, the same thing still happen. and I have no idea where the hell went wrong Sep 12, 2018 at 13:35
• What's the value you set for nodesateachlayer? It seems you didn't share the code to run your functions. Sep 12, 2018 at 15:14

My view on your question, is that tiny networks seldom work. The above method uses a Neural Network to learn the function $$y=\sin(x)$$. Although this problems seems simple, it cannot be expected to be solved by a really tiny network (the above model uses a 5-layer MLP with hidden size [5,6,7], which is small).

Even if back-propagation is implemented correctly, would the model learn anything? No. I suppose Tensorflow implemented back-propagation correctly, here is the result using Tensorflow: You see, it learns almost nothing. In fact, the MSE loss is very close to 0.5 as stated above.

My suggestion is to try a 3 layer MLP with hidden size 256. Here is the result: You can see it's much better. MSE<0.1 now.

------------------code---------------------

x_ =np.atleast_2d(np.arange(0,360,1)).T
y_ = np.atleast_2d(np.sin(x_/180*np.pi))
g = tf.Graph()
with g.as_default():
with tf.variable_scope("mlp"):
input_x = tf.placeholder(shape=[None, 1], dtype=tf.float32)
input_y = tf.placeholder(shape=[None,1], dtype=tf.float32)
layer1 = tf.layers.dense(inputs=input_x, units=256, activation=tf.nn.sigmoid)
#layer2 = tf.layers.dense(inputs=input_x, units=6, activation=tf.nn.sigmoid)
#layer3 = tf.layers.dense(inputs=input_x, units=7, activation=tf.nn.sigmoid)
output_y = tf.layers.dense(inputs=layer1, units=1) # inputs=layer1
loss = tf.losses.mean_squared_error(input_y, output_y)