# Feedforward neural network

I am new in neural network and I am trying to figure out how to build a feed-forward neural network to solve a classification problem defined in the following way (X1, X2 being the features and Y the target):

if X1>0 and X2>0 then Y=1

if X1<0 and X2>0 then Y=0

if X1>1 and X2<0 then Y=1

if X1<1 and X2<0 then Y=0

If someone could at least give me a clue

Thanks

Try simplifying your set of rules to a simpler set of rules (one with fewer rules).

What nodes are you allowed to use? I assume you can use some sort of a threshold node to produce the zeros and ones.

Can you do these problems well with only one feature (positive goes to 1 and negative goes to zero)?

edit

I misread the last rule before thinking it said X1 < 0. It's quite easy to do it in three layers (counting the output layer) as long as you can,

1) construct a node asking if the input is greater than a certain value.

2) construct a node performing "and" on two outputs from the previous layer.

Are these things you can handle? Am I correct in assuming you need to cut out one layer? It's possible but harder to lead you there.

• Thanks for your answer. To have less rules I could say: if x1>1 or (x2>0 and x1>0) then y=1 else y=0 It has to be a two layers feed-forward neural network with threshold (I think by two layers they don't count the input layer). How would you move from 2 to 1 feature? Sep 10 '15 at 8:13
• Edited my answer Sep 10 '15 at 20:46
• You don't have to simplify your rules or use less features (by features I assume you mean inputs into the network?) Just feed it examples where x1 and x2 are randomly generated and calculate the target Y using a straightforward function that doesn't involve a neural network. Dec 16 '15 at 5:12

Ah the old XOR problem. This has been written about at length. See here: https://web.stanford.edu/group/pdplab/pdphandbook/

Ultimately, this can be solved with a fully connected 1 hidden layer network. You need two input nodes to read in your input pattern, a two-node hidden layer, and a single output node. Then train with stochastic gradient descent.

Consider a network with input nodes $$i_0$$ and $$i_1$$, hidden nodes $$h_0$$ and $$h_1$$ and output node $$o$$ and a sigmoid activation function.

One solution would have strong positive weights from both input nodes to $$h_0$$ and weak positive weights from both input nodes to $$h_1$$. $$h_0$$ will have a weak negative bias weight and $$h_1$$ will have a strong negative bias weight. $$h_0$$ will have a strong positive weight to $$o$$ and $$h_1$$ will have a strong negative weight to $$o$$. Finally, $$o$$ will have a weak negative bias.

It might help to draw a diagram of this configuration and walk yourself through how the activation would flow for each of the possible 4 input patterns.