I'm trying to make sense of how to sort one data.frame based on multiple columns in another. This question does this with vectors. Can someone suggest a way to do the equivalent with data.frames?
Here's some sample data.
x1 <- data.frame(a=1:5, b=letters[1:5], c=rnorm(5))
x2 <- data.frame(a=c(4,4,2), b=c("d", "d", "b"), d=rnorm(3))
So I want to sort x2 by the first two columns of x1. My actual data is much more complicated, but this replicates the idea...