Using neural network for classification in matlab

Question

I'm working on optical character recognition problem. I've successfully extracted features which is a [1X32] matrix (I've extracted 32 features from each segmented character). I've the complete training data set (the images of every individual character), but I'm breaking my head on creating Input & Target data set matrices. So please tell me about those matrices, the testing data, & in what format will I get output from neural network.

1)There are 258 different patterns (characters), so, should there be 258 class labels ?

My input matrix size is No. of rows = 32 (features) No. of cols = 258*4=1032 (No of characters*No of instances for each character)

2) what should be the size of my target matrix ? Just draw a dummy target matrix for my case.

Answer 1

Did you checked the Neural Network Toolbox of MATLAB already ( http://www.mathworks.co.uk/help/nnet/examples/crab-classification.html?prodcode=NN&language=en ) ? There you can find some examples how to work with neural networks.

Regarding your two specific questions:

1) Typically if you want to differentiate between N different characters you will need that amount of class labels. So in your case yes you should have 258 class labels. The output of a classification problem using neural networks is typically a binary output where one goes for the identified class and 0 for the remain classes. It can happen however, if you use a sigmoid function as the last activation function that neither output node is exactly 0 or 1, and in this case you can for example take the maximum of all output nodes, to get the highest or more probable class for a certain input.

2) The target matrix should be a binary matrix where 1 goes for the correct class and 0 for all the others classes for each input. So in your case it should be 258*1032 matrix. Again I recommend you to check the link given above.

Good luck.