Quantiles by factor levels in R

Question

I have a data frame and I\'m trying to create a new variable in the data frame that has the quantiles of a continuous variable var1, for each level of a factor

Answer 1

Use ave on your dat data frame. Full example with your simulated data and qfun function:

# some data
set.seed(472)
dat <- data.frame(var1 = rnorm(50, 10, 3)^2,
              strata = factor(sample(LETTERS[1:5], size = 50, replace = TRUE))
              )

# function to get quantiles
qfun <- function(x, q = 5) {
    quantile <- cut(x, breaks = quantile(x, probs = 0:q/q), 
        include.lowest = TRUE, labels = 1:q)
    quantile
}

And my addition...

dat$q <- ave(dat$var1,dat$strata,FUN=qfun)

Answer 2

I think your issue is that you don't really want to aggregate, but use ave, (or data.table or plyr)

qdat <- transform(dat, qq = ave(var1, strata, FUN = qfun))

#using plyr
library(plyr)

qdat <- ddply(dat, .(strata), mutate, qq = qfun(var1))

#using data.table (my preference)


dat[, qq := qfun(var1), by = strata]

Aggregate usually implies returning an object that is smaller that the original. (inthis case you were getting a data.frame where x was a list of 1 element for each strata.