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

The idiomatic way to do this kind of thing in R would be to use a combination of split and lapply. You're halfway there with your use of lapply; you just need to use split as well.

lapply(split(data, data$v1), function(df) {
    cutoff <- quantile(df$v2, c(0.8, 0.9))
    top_pct <- ifelse(df$v2 > cutoff[2], 10, ifelse(df$v2 > cutoff[1], 20, NA))
    na.omit(data.frame(id=df$custID, top_pct))
})

Finding quantiles is done with quantile.