The Art of R Programming

Merging Data Frames

In the relational database world, one of the most important operations is that of a join, in which two tables can be combined according to the values of a common variable. In R, two data frames can be similarly combined using the merge() function.

The simplest form is as follows:

merge(x,y)

This merges data frames x and y. It assumes that the two data frames have one or more columns with names in common. Here’s an example:

> d1
     kids states
1    Jack     CA
2    Jill     MA
3 Jillian     MA
4    John     HI
> d2
  ages    kids
1   10    Jill
2    7 Lillian
3   12    Jack
> d <- merge(d1,d2)
> d
  kids states ages
1 Jack     CA   12
2 Jill     MA   10

Here, the two data frames have the variable kids in common. R found the rows in which this variable had the same value of kids in both ...