lapply function arguments in R

Question

I have been converting an R script into python script and I stumbled upon this strange way of calling lapply() function in R. It's a github project (SnoLyze) that I am converting into python.

lapply(conjunctionAttributeSet, "[", , "sctid")

Let's suppose the first argument is a list with values, named conjunctionAttributeSet. What about these others? Shouldn't this function take two arguments, X and FUN? This code is working fine too! How? What am I missing?

Answer 1

The function being applied is [ , with extra blank argument , and an extra argument "sctid" . This is saying "extract the "sctid" column from each element of conjunctionAttributeSet .

The code results = lapply(conjunctionAttributeSet, "[", , "sctid") is equivalent to

results = list()

for(i in seq_along(conjunctionAttributeSet)) {
  results[[i]] = conjunctionAttributeSet[[i]][, "sctid"]
}