How do I compute the correlation between two features and their output class? I just read these two quotes in a paper and wondered how I do this.

For each classifier, one selects a subset of the input features
  according to their correlation to the corresponding class.

and

Compute the correlation between each feature and Yi the output for class i.

Source: Dimensionality Reduction Through Classifier Ensembles
Can you tell me how to accomplish that?
 A: Correlation is used as a method for feature selection and is usually calculated between a feature and the output class (filter methods for feature selection). It roughly translates to how much will the change be reflected on the output class for a small change in the current feature. If the change is proportional and very high, then we say that the feature is highly correlated with the output class and is usually a very good idea to keep it around for any end of the pipeline tasks.
That being said, a feature having higher correlation in say corpus XYZ won't necessarily mean that it will have a high correlation in some other corpus say ABC and thus cannot be translated from one dataset to another.
There are many metrics to measure the correlation between a feature and a class label such as mutual information, chi-square, correlation coefficient scores etc. 
numpy.correlate in python does the trick for most of my work.
or if i want to use mutual information then sklearn has mutual info score .
A: This is called correlation filter. If you are using R, You can have a look at FSelector package's linear.correlation and rank.correlation functions. When class labels are categorical, you can use Point-Biserial Correlation, polyserial or polychloric correlation.
The idea is if feature 1 is highly correlated with class label while feature 2 is not highly correlated we would select feature 1 over feature 2.
