Question

I have point series in a data.frame with duplication. I would like to plot them using level plot, and use as a Z frequency of x, y (in example how many times for x = 1 there was y = 2). How can I do this? Well it is easy for me to explain myself with SQL syntax:

 SELECT x, y, count(*) from data_frame GROUP BY x, y

🙂

Answer 1

These kinds of summarizing a data frame by groups defined by variables is very common in R. Many of the *pply functions would work. The standard response these days is to use ddply from the plyr package:

ddply(data_frame,.(x,y),summarise,total = NROW(piece))

In general, you should really learn the plyr package, along with all the base *pply functions.

But if you’re more comfortable with SQL, you might consider looking at the sqldf package, which lets you do a lot of these kinds of manipulations of data frames directly with SQL.