Mixed Merge in R - Subscript solution?

Question

Note: I changed the example from when I first posted. My first example was too simplified to capture the real problem.

I have two data frames

Answer 1

There are several ways to do this (it is R, after all) but I think the most clear is creating an index. We need a function that creates a sequential index (starting at one and ending with the number of observations).

seq_len(3) 
> [1] 1 2 3

But we need to calculate this index within each grouping variable (state). For this we can use R's ave function. It takes a numeric as the first argument, then the grouping factors, and finally the function to be applied in each group.

s1$index <- with(s1,ave(value1,state,FUN=seq_len))
s2$index <- with(s2,ave(value2,state,FUN=seq_len))

(Note the use of with, which tells R to search for the variables within the environment/dataframe. This is better practice than using s1$value1, s2$value2, etc.)

Now we can simply merge (join) the two data frames (by the variables present in the both data frames: state and index).

merge(s1,s2)

which gives

   state index value1 value2
1    IA     1      1      6
2    IA     2      2      7
3    IA     3      3      8
4    IL     1      4      3
5    IL     2      5      4
6    IL     3      6      5

For this to work, there should be the same number of observations by state in each of the data frames.

[Edit: commented the code for clarity.] [Edit: Used seq_len instead of creating a new function as suggested by hadley.]