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POHow to use R's ellipsis feature when writing your own function?
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copied!<p>The R language has a nifty feature for defining functions that can take a variable number of arguments. For example, the function <code>data.frame</code> takes any number of arguments, and each argument becomes the data for a column in the resulting data table. Example usage:</p> <pre><code>> data.frame(letters=c("a", "b", "c"), numbers=c(1,2,3), notes=c("do", "re", "mi")) letters numbers notes 1 a 1 do 2 b 2 re 3 c 3 mi </code></pre> <p>The function's signature includes an ellipsis, like this:</p> <pre><code>function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, stringsAsFactors = default.stringsAsFactors()) { [FUNCTION DEFINITION HERE] } </code></pre> <p>I would like to write a function that does something similar, taking multiple values and consolidating them into a single return value (as well as doing some other processing). In order to do this, I need to figure out how to "unpack" the <code>...</code> from the function's arguments within the function. I don't know how to do this. The relevant line in the function definition of <code>data.frame</code> is <code>object <- as.list(substitute(list(...)))[-1L]</code>, which I can't make any sense of.</p> <p>So how can I convert the ellipsis from the function's signature into, for example, a list?</p> <p>To be more specific, how can I write <code>get_list_from_ellipsis</code> in the code below?</p> <pre><code>my_ellipsis_function(...) { input_list <- get_list_from_ellipsis(...) output_list <- lapply(X=input_list, FUN=do_something_interesting) return(output_list) } my_ellipsis_function(a=1:10,b=11:20,c=21:30) </code></pre> <hr> <h2>Edit</h2> <p>It seems there are two possible ways to do this. They are <code>as.list(substitute(list(...)))[-1L]</code> and <code>list(...)</code>. However, these two do not do exactly the same thing. (For differences, see examples in the answers.) Can anyone tell me what the practical difference between them is, and which one I should use?</p>

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