I found that the order SQL processes commands is crucial to understanding SQL, especially for beginners.

When reading a SQL script from top to bottom, it’s not easy. What’s more, there are certain rules that are not intuitively understandable. Such as why we cannot put aggregation commands in where, but…

Please note this is not a comprehensive article of PCA. I’m just sharing some key thoughts on PCA that might be helpful to others who are confused by this technique.

  1. The basic idea is that not every feature is necessarily useful/important.

Taking the Iris dataset for example. Original dataset has…

A good example to intuitively understand the concept:

Say you have 100 dollars to buy 5 things.

You can buy whatever you want for the first 4 things as long as there’s money left. But for the fifth thing, the amount of money you can spend is restricted at the moment you bought the fourth thing. You cannot choose whatever you want anymore.

In this case, the degree of freedom is 4, because you can freely choose the first 4 things.

Revised on 4/21

I would like to share what I’ve come to understand.

・First, I would like correct my misconception that Xgboost does not add up trees. In fact, it does add trees.

・To add a tree, we need to set a learning rate ( called “eta” in the xgboost…

I’ve tried many ways to pip install but kept running into the following error:

XGBoostLibraryNotFound: Cannot find XGBoost Library in the candidate path, did you install compilers and run build.sh in root path?

Eventually I gave up pip and this worked for me:

In terminal run:

$ git clone — recursive https://github.com/dmlc/xgboost.git

This should downloads a folder called “xgboost”.

Go to /xgboost/python-package

In terminal run:

$ python setup.py install

This should start installing xgboost.

If you run into the following error here again:

XGBoostLibraryNotFound: Cannot find XGBoost Library in the candidate path, did you install compilers and run build.sh in root path?

You need to download xgboost.dll file and put it into /xgboost/python-package/xgboost

Then run the following command again:

$ python setup.py install

This time it should work.

In week 2, after I downloaded the exercise zip file “machine-learning-ex”, I found that I could not unzip it in Mathlab online.

Here is the fix:

After clicking the machine-learning-ex provided in the session, when the window pop up, DO NOT click on SAVE but choose OPEN WITH, then click OK.

You will get a zip local file named as “machine-learning-ex-1.zip”. Don’t be mistaken by the file name. It actually includes all the exercise.

Upload this zip file to matlab online and run unzip, you’ll surprisingly find that it gets unzipped into a folder named “machine-learning-ex”, under which ex1–8 are included.

P.S. I’m a mac user. Not sure if windows users get the same problem.

Happy Studying!

Jodie Qiu

My memos of machine learning algorithms, data pre-prcocessing and statistics. Git: https://github.com/qhqqiu

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