R: How to carry over colnames attribute using sapply?

The following toy code yields a density plot for each column of the y dataframe. However, sapply does not carry over the column name attributes. I'd like to name each new plot with the column name from which the data comes from. Any help is appreciated!

y <- data.frame(sample(1:50), sample(1:50), sample(1:50))
colnames(y) <- c("col1", "col2", "col3")

toy.func <- function(y) {
  X11()
  plot = plot(density(y), main = colnames(y))
  return(plot)
}

result <- sapply(y, toy.func)

Solution

You are right and it makes sense: y is seen as a list and sapply goes over its element that are vectors, and we cannot assign a name to a vector. So, a somewhat minimal deviation from your approach that achieves what you want would be to use mapply:

toy.func <- function(y, name) {
  X11()
  plot = plot(density(y), main = name)
  return(plot)
}
mapply(toy.func, y, colnames(y))

It applies toy.func by taking one element from y and one from colnames(y) in every step.

Another option would be to go over the column names at the same time providing the data frame

toy.func <- function(name, data) {
  X11()
  plot = plot(density(data[, name]), main = name)
  return(plot)
}
sapply(colnames(y), toy.func, y)

Also note that your function can be simplified to, in this case,

toy.func <- function(name, data) {
  X11()
  plot(density(data[, name]), main = name)
}