A new R trick … for me at least
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What were going to be talking about today are dynamic argument lists for functions. Specifically, how to unpack and prepare them in R using ...
, list()
, and do.call()
Biased by Matlab and varargin
Initially, I based my use of ...
in R on my experience with Matlab’s varargin
. Using varargin
, Matlab functions can have a signature of:
function f(varargin) % do stuff here
Functions that use varargin
are responsible for processing its contents, which is easy since it is simply a cell array. Thus, it can be “unpacked” and modified using cell array methods.
function f(varargin) arg1 = varargin{1} arg2 = varargin{2} return(arg1*arg2)
At call, arguments captured by varargin
can be specified as an expanded cell array:
args = {foo, bar} f(args{:})
As a matter of fact, functions that do not use
varargin
can also be called this way since Matlab effectively interprets an expanded cell array as a comma-separated list
This comes in handy when you have a mixture of required and optional arguments for a function.
f(arg, opts{:})
Back to R …
I used to think ...
was analogous to varargin
since:
- it captures all function arguments not explicitly defined by the call signature
- the number of arguments it captures can vary
However, unlike varargin
:
...
is a special R language expression/object- it needs to be converted to a list to access the arguments (names and/or values) that it captures
The former point is strength and quirk of R, as it allows for arguments encapsulated in ...
to be passed on to additional functions:
f = function(x, ...) { y = g(x, ...) return(y) }
The latter point above (unpacking ...
) is actually easy to do:
f = function(x, ...) { args = list(...) # contains a=1, b=2 return(args$a * args$b) }
Where confusion arises for many is that ...
is essentially immutable (cannot be changed). While conceptually a list()
, you can’t modify it directly using list accessors:
f = function(x, ...) { ...[[1]] = 3 # this produces an error, as would ...$var and ...[1] y = g(x, ...) return(y) }
So, what if I wanted to unpack arguments in ...
, check/change their values, and repackage it for another function call? Since ...
is immutable the code below would throw an error.
f = function(x, ...) { args = list(...) # unpack, contains a='foo' args$a = bar ... = args # ERROR! y = g(x, ...) return(y) }
Also, there isn’t a way (that I’ve found yet) to unroll a list()
object in R into a comma-separated list like you can with a cell array in Matlab.
# this totally doesn't work args = list(a=1, b='foo') result = f(args[*]) # making up syntax here. would be nice, no?
As it turns out, ...
doesn’t even come into play here. In fact, you need to use a rather deep R concept – calls.
Whenever a function is used in R, a call
is produced, which is an unprocessed expression that is then interpreted by the underlying engine. Why the delay? Only the creators/developers of R can fully detail why, but it does allow for some neat effects – e.g. the automatic labeling of plots.
To package a programmatically generated argument list one uses the do.call()
function:
result = do.call('fun', list(arg1, arg2, etc, etc))
where the first argument is the name of the function to call, and the second argument is a list of arguments to pass along. For all intents and purposes, the R statement above is equivalent to the Matlab statement below.
results = fun(args{:}) % where args = {arg1, arg2, etc, etc}
Thus, process to unpack ...
, check/modify an argument, and repack for another function call becomes:
f = function(x, ...) { args = list(...) # unpack, contains a='foo' args$a = bar # change argument "a" y = do.call(g, c(x, args)) # repack arguments for call to g() return(y) }
I must credit this epiphany to the following StackOverflow question and answer: http://stackoverflow.com/questions/3414078/unpacking-argument-lists-for-ellipsis-in-r
Written with StackEdit.
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