Create a variable length `alist()`
r
Solution
OK, I'll bite. I would probably use `list(bquote())` to construct a one element list containing the empty symbol, and `rep` that out to the desired length.
n <- 2
rep(list(bquote()), n)
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As a bonus, here is a collection of 5 ways to create/access the empty symbol that is needed as the content of each list element:
bquote()
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substitute()
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quote(expr= )
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formals(function(x) {})$x
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alist(,)[[1]]
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Problem
In this Answer, an `alist()` is proposed as an easy means of creating a list with empty elements. A use case for this would be constructing a list suitable for a call to `[` arranged via `do.call()`. For example: ``` x <- matrix(1:6, ncol = 2) do.call(`[`, alist(x, , 2)) ## extract column 2 of x [1] 4 5 6 ``` The particular Question prompting the `alist()` Answer required the setting of the empty arguments dynamically on basis of an object `shortdim`. If one knew how many dimensions were present one could do ``` al <- alist( , , ) ## 3 arguments for a 2-d object al[[1]] <- x shortdim <- 1 al[[shortdim + 1]] <- 1:2 ## elements 1 & 2 of dim shortdim, plus all other dims do.call(`[`, al) > do.call(`[`, al) [,1] [,2] [1,] 1 4 [2,] 2 5 > x[1:2, ] ## equivalent too [,1] [,2] [1,] 1 4 [2,] 2 5 ``` A list of dynamic length can be created by `vector()`, eg ``` ll <- vector(mode = "list", length = length(dim(x)) + 1) ``` But an `alist` can't be made in that way ``` > vector(mode = "alist", length = length(dim(x)) + 1) Error in vector(mode = "alist", length = length(dim(x)) + 1) : vector: cannot make a vector of mode 'alist'. ``` Is there a way to create an `alist` of dynamic length that can be filled in later where required?