4.8) Summarizing Data

Use tapply() to find the median Dms value for GRASS plants with each number of Tlrs.
What happens if we name length as the function for the third argument of tapply()?
Use colMeans() to find the means of the columns of swiss. Check its help file and you’ll see some other useful summary functions. But there isn’t one for medians - so use apply() to find the medians of the columns.

At the end of this session, you are a master of square brackets! You should know:

several ways of referring to columns of data frames
how to refer to rows of data frames, including how to get subsets of data that meet specified conditions
some useful functions to summarise data frames; and
a bit about lists and arrays as well.