Find top deciles from dataframe by group

Question

I am attempting to create new variables using a function and lapply rather than working right in the data with loops. I used to use Stata and would have solved this

Answer 1

Stick to your Stata instincts and use a single data set:

require(data.table)
DT <- data.table(data)

DT[,r:=rank(v2)/.N,by=v1]

You can see the result by typing DT.

---

From here, you can group the within-v1 rank, r, if you want to. Following Stata idioms...

DT[,g:={
  x = rep(0,.N)
  x[r>.8] = 20
  x[r>.9] = 10
  x
}]

This is like gen and then two replace ... if statements. Again, you can see the result with DT.

---

Finally, you can subset with

DT[g>0]

which gives

   custID v1 v2     r  g
1:      1  A 30 1.000 10
2:      2  A 29 0.900 20
3:      1  B 20 0.975 10
4:      2  B 19 0.875 20
5:      6  B 20 0.975 10
6:      7  B 19 0.875 20

These steps can also be chained together:

DT[,r:=rank(v2)/.N,by=v1][,g:={x = rep(0,.N);x[r>.8] = 20;x[r>.9] = 10;x}][g>0]

---

(Thanks to @ExperimenteR:)

To rearrange for the desired output in the OP, with values of v1 in columns, use dcast:

dcast(
  DT[,r:=rank(v2)/.N,by=v1][,g:={x = rep(0,.N);x[r>.8] = 20;x[r>.9] = 10;x}][g>0], 
  custID~v1)

Currently, dcast requires the latest version of data.table, available (I think) from Github.