Anonymous function in lapply

Question

I am reading Wickham's Advanced R book. This question is relating to solving Question 5 in chapter 12 - Functionals. The exercise asks us to:

Implement a version of lapply() that supplies FUN with both the name and value of each component.

Now, when I run below code, I get expected answer for one column.

c(class(iris[1]),names(iris[1]))

Output is:

"data.frame"   "Sepal.Length"

Building upon above code, here's what I did:

lapply(iris,function(x){c(class(x),names(x))})

However, I only get the output from class(x) and not from names(x). Why is this the case?

I also tried paste() to see whether it works.

lapply(iris,function(x){paste(class(x),names(x),sep = " ")})

I only get class(x) in the output. I don't see names(x) being returned.

Why is this the case? Also, how do I fix it?

Can someone please help me?

Answer 1

Instead of going over the data frame directly you could switch things around and have lapply go over a vector of the column names,

data(iris)

lapply(colnames(iris), function(x) c(class(iris[[x]]), x))

or over an index for the columns, referencing the data frame.

lapply(1:ncol(iris), function(x) c(class(iris[[x]]), names(iris[x])))

Notice the use of both single and double square brackets.
iris[[n]] references the values of the nth object in the list iris (a data frame is just a particular kind of list), stripping all attributes, making something like mean(iris[[1]]) possible.
iris[n] references the nth object itself, all attributes intact, making something like names(iris[1]) possible.