Classifying Handwritten Digits: 1 vs. 5
 

I don't quite understand the first classification method given by the problem: "Linear Regression for classification followed by pocket for improvement". Since the weight returned by linear regression is an analytically optimal result, how can the pocket algorithm improve it?

Re: Classifying Handwritten Digits: 1 vs. 5
 

It is only analytically optimal for regression. It can be suboptimal for classification.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Hi Professor, you said that the weight vector w learnted from the Linear Regression could be suboptimal for classification. However, after run the pocket algorithm with 1,000,000 iteration, the w still not change, which means that the w learnt from the Linear Regression is optimal. Is that true? Maybe I made some mistake.

Re: Classifying Handwritten Digits: 1 vs. 5
 

The pocket algorithm indeed is able to improve the linear regression. Mine decreased the in-sample error from 0.8% to around 0.4%.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Do you set the w learnt from Linear Regression as the initial w for Pocket Algorithm? I did like that, but without any improvement. Maybe some mistakes.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Any one of these three can happen:

1) the linear regression weights are optimal
2) the linear regression weights are not optimal and the PLA/Pocket algorithm can improve the weights.
3) the linear regression weights are not optimal and the PLA/Pocket algorithm cannot improve the weights.

In practice, we will not know which case we are in because actually finding the optimal weights is an NP-hard combinatorial optimization problem.

However, no matter which case we are in, other than some extra CPU cycles, there is no harm done in running the pocket algorithm on the regression weights to see if they can be improved.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Yes, I did. I guess you probably should look at your implementation of the pocket algorithm. I also got no improvement at first. But then I messed around the code a little bit, and it worked.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Does the in-samle-error should use the square error formula, like the one of Linear Regression? Or the in-sample-error of binary function?

Re: Classifying Handwritten Digits: 1 vs. 5
 

Binary classification error.

Re: Classifying Handwritten Digits: 1 vs. 5
 

How to plot the training and the test data, together with the separators learnt by using a 3rd order polynomial transform. Actually, the 3rd order polynomial hypothesis is a unclear formula with the two features. Then, how to plot this polynomial hypothesis in a two dementional axis?

Re: Classifying Handwritten Digits: 1 vs. 5
 

This thread has a response that might help:

http://book.caltech.edu/bookforum/showthread.php?t=2101

Re: Classifying Handwritten Digits: 1 vs. 5
 

Hello Professor Magdon,

I have a slightly different question related to plotting.
When I am asked to "familiarize yourself with the data by giving a plot of two
of the digit images", what do we mean by plotting the data? Are we referring to
generating a digit image from the greyscale value vector? Or are you referring to
somehow plotting the numerical values in the vector, itself? I have a feeling that it is
the former, but I am not familar with image generation from greyscale pixels.

Any guidance you could provide would be greatly appreciated.

Re: Classifying Handwritten Digits: 1 vs. 5
 

If you go to www.amlbook.com, click on 'supporting material' on the right and then scroll down to the `Data' section, you will find some information that can be of use. In particular, there is matlab code for plotting the digit images which takes the matrix of grayscale values and plots an image. This can be of help for developing your own code and utilities.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Thank you for your assistance. That helps me greatly.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Hi Professor, how to plot the separators with the training data if I use Logistic regression for classification using gradient descent. In this way we could compute the probabilities for every point in the figure but how to plot a separator for them?

Re: Classifying Handwritten Digits: 1 vs. 5
 

You can use the weights produced by logistic regression for classification.

Re: Classifying Handwritten Digits: 1 vs. 5
 

Hello. I have a question about the digits assignment. Are we supposed to use two
features separately {1, feature i} or have a input vector consisting of {1,feature 1, feature 2}?

Re: Classifying Handwritten Digits: 1 vs. 5
 

Thanks for your reply. In the logistic regression will the separator still be linear or not? And should we use fixed or variable step size in the logistic regression?

Re: Classifying Handwritten Digits: 1 vs. 5
 

{1,feature 1, feature 2}

Re: Classifying Handwritten Digits: 1 vs. 5
 

The classification function is still which is linear. The weights are obtained using logistic regression.