[R] holding argument(s) fixed within lapply

[Blaser Nello]

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