[Rd] How to build a list with missing values? What is missing, anyway?
peter.meilstrup at gmail.com
Thu Oct 4 04:37:24 CEST 2012
This is tangentially related to Hadley's question.
Suppose I'm building a function programmatically; I have assembled an
expression for the body and I know the names of the arguments it wants
Suppose I have some convenience function such that writing
make_function(alist(a=, b=), quote(a+b), environment())
is equivalent to writing
So how do I make the list that goes in the first argument? Suppose I
start with a character vector of argument names. What I need is a list
with names and quoted values, but the values are "empty".
Say I have argnames <- c("a", "b", "c").
>From that I want to construct the equivalent of alist(a=, b=, c=).
"missing" seems different from NULL; if I do
arglist <- rep(list(NULL), length(argnames))
names(arglist) <- argnames
I get a function where NULL is the default value for each argument,
rather than a function with no default values.
It seems I can assign a "missing value" to a variable by
the_missing_value <- formals(function(x) NULL)[]
the_missing_value = quote(expr=)
However this backfires when I try to use it:
> arglist <- rep(list(the_missing_value), length(argnames))
Error in list(the_missing_value) : 'the_missing_value' is missing
> arglist <- rep(list(NULL), 3)
> arglist <- the_missing_value
Error: argument "the_missing_value" is missing, with no default
Besides which, roxygen2 (and other package tools) choked on
encountering a missing value assigned to a name in my package.
So, what's going on with missing? Missing and '...' are the two pieces
of R's semantics I'm most fuzzy on.
(Somewhere along this experimentation I also managed to assign a value
to the empty name, and could not figure out how to remove it...)
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