[R] holding argument(s) fixed within lapply

[Benjamin Tyner]

Thank you Blaser; this is helpful.

On 03/13/2013 12:22 PM, Blaser Nello wrote:
> One way is to use the do.call function. For example:
>
> ret2 <- lapply(X=mylist2, 
>        FUN=do.call, 
>        what=function(...) f2(y=Y, ...))
>
> Best, 
> Nello
>
> -----Original Message-----
> Date: Tue, 12 Mar 2013 22:37:52 -0400
> From: Benjamin Tyner <btyner at gmail.com>
> To: r-help at r-project.org
> Subject: Re: [R] holding argument(s) fixed within lapply
> Message-ID: <513FE680.2070404 at gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Apologies; resending in plain text...
>
> Given a function with several arguments, I would like to perform an
> lapply (or equivalent) while holding one or more arguments fixed to some
> common value, and I would like to do it in as elegant a fashion as
> possible, without resorting to wrapping a separate wrapper for the
> function if possible. Moreover I would also like it to work in cases
> where one or more arguments to the original function has a default
> binding.
>
>     # Here is an example; the original function
>     f <- function(w, y, z){ w + y + z }
>
>     # common value I would like y to take
>     Y <- 5
>
>     # I have a list of arguments for the lapply()
>     mylist <- list(one = list(w = 1, z = 2),
>                    two = list(w = 3, z = 4)
>                    )
>
>     # one way to do it involves a custom wrapper; I do not like this
> method
>     ret <- lapply(FUN = function(x,...) f(w = x$w, z = x$z, ...),
>                   X   = mylist,
>                   y   = Y
>                   )
>
>     # another way
>     ret <- lapply(FUN  = with.default,
>                   X    = mylist,
>                   expr = f(w, y = Y, z)
>                   )
>
>     # yet another way
>     ret <- lapply(FUN  = eval,
>                   X    = mylist,
>                   expr = substitute(f(w, y = Y, z))
>                   )
>
>     # now, the part I'm stuck on is for a version of f where z has a
> default binding
>     f2 <- function(w, y, z = 0){ w + y + z }
>
>     # the same as mylist, but now z is optional
>     mylist2 <- list(one = list(w = 1),
>                     two = list(w = 3, z = 4)
>                     )
>
>     # undesired result (first element has length 0)
>     ret2 <- lapply(FUN = function(x,...) f2(w = x$w, z = x$z, ....),
>                    X   = mylist2,
>                    y   = Y
>                    )
>
>     # errors out ('z' not found)
>     ret2 <- lapply(FUN  = with.default,
>                    X    = mylist2,
>                    expr = f2(w, y = Y, z)
>                    )
>
>     # errors out again
>     ret2 <- lapply(FUN  = eval,
>                    X    = mylist2,
>                    expr = substitute(f2(w, y = Y, z))
>                    )
>
>     # not quite...
>     ret2 <- lapply(FUN = gtools::defmacro(y = Y, expr = f2(w, y = Y,
> z)),
>                    X   = mylist2
>                    )
>
> It seems like there are many ways to skin this cat; open to any and all
> guidance others care to offer.
>
> Regards,
> Ben
>