Question

Assume that I have several dataframes in a workspace in R and I want a list of the names of the columns in all the dataframes.

I thought the following would work. But it does not. Try it in your own workspace.

sapply(ls(),names) 

Why does it not work? ls() creates a list of all the dataframes and then names function should be applied to each dataframe. That is my simple question for now.

Coming next: I want to determine all the columns that have a name with the letters “date” in them so that I can the apply the following function to each of those columns no matter what dataframe they are in.

as.Date(dataframe$dateofenrollment,origin="1899-12-30")

Answer 1

It doesn’t work because ls() returns the names of the objects in our workspace, not the objects themselves.

You probably want something like the following:

lapply(ls(), function(x) if(is.data.frame(o <- get(x))) names(o))

This will have NULL elements for any objects that are not data frames, but presumably you can work around that.