This is a continuation of the previous video.
Note: We should mention the very useful function
subset() which allows us to extract subsets from data frames based on multiple logical tests. For example
subset(cars, hp<150 & cyl<8). It is worth noting that the dataframe created by
subset() will contain all factor levels of the original dataframe, even if some are not used -
droplevels() can be used to remove unused levels.