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?

That is a shorthand of `lapply(conjunctionAttributeSet, function(x) x[, "sctid"])` — Darren Tsai, Sep 15 '20 at 14:26
Argument `X` is `conjunctionAttributeSet`, the function `FUN` is `"["` and then you have further arguments that the function needs and that are passed to it. — Rui Barradas, Sep 15 '20 at 14:26
See here: https://stackoverflow.com/questions/19260951/using-square-bracket-as-a-function-for-lapply-in-r — Bas, Sep 15 '20 at 14:27
@DarrenTsai If the *two* commas after `"["` are right, then `x` should be bideimensional: `x[, "sctid"]`. — Rui Barradas, Sep 15 '20 at 14:27

Gregor Thomas · Accepted Answer · 2020-09-15T14:47:50.840

3

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"]
}

edited Sep 15 '20 at 14:47

answered Sep 15 '20 at 14:25

Gregor Thomas

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With two commas isn't it `conjunctionAttributeSet[[i]][, "sctid"]`? – Rui Barradas Sep 15 '20 at 14:29
Thanks, missed the comma! – Gregor Thomas Sep 15 '20 at 14:49

lapply function arguments in R

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