Using get inside lapply, inside a function

问题

this may seem like a overly complicated question, but it has me driving me a little nuts for some time. It is also for curiosity, because I already have a way of doing what I need, so is not that important.

In R, I need a function to return a named list object with all the arguments and the values entered by the user. For this I have made this code (toy example):

foo <- function(a=1, b=5, h='coconut') {
    frm <- formals(foo)
    parms <- frm
    for (i in 1:length(frm))
        parms[[i]] <- get(names(frm)[i])
    return(parms)
}

So when this is asked:

> foo(b=0)

$a
[1] 1

$b
[1] 0

$h
[1] "coconut"

This result is perfect. The thing is, when I try to use lapply to the same goal, so as to be a little more efficient (and elegant), it does not work as I want it to:

foo <- function(a=1, b=5, h='coconut') {
    frm <- formals(foo)
    parms <- lapply(names(frm), get)
    names(parms) <- names(frm)
    return(parms)
}

The problem clearly is with the environment in which get evaluates it's first argument (a character string, the name of the variable). This I know in part from the error message:

> foo(b=0)
Error in FUN(c("a", "b", "h")[[1L]], ...) : object 'a' not found

and also, because when in the .GlobalEnv environment there are objects with the right names, foo returns their values instead:

> a <- 100
> b <- -1
> h <- 'wallnut'
> foo(b=0)
$a
[1] 100

$b
[1] -1

$h
[1] "wallnut"

Obviously, as get by default evaluates in the parent.frame(), it searches for the objects in the .GlobalEnv environment, instead of that of the current function. This is strange, since this does not happen with the first version of the function.

I have tried many options to make the function get to evaluate in the right environment, but could not do it correctly (I've tried pos=-2,0,1,2 and envir=NULL as options).

If anyone happen to know a little more than me about environments, specially in this "strange" cases, I would love to know how to solve this.

Thanks for your time,

Juan

回答1:

Edit of 2013-08-05

Using sapply() instead of lapply(), simplifies this considerably:

foo4 <- function(a=1, b=5, h='coconut') {
    frm <- formals(sys.function())
    sapply(names(frm), get, envir=sys.frame(sys.parent(0)), simplify=FALSE)
}
foo4(b=0, h='mango')

This, though, without sapply() or lapply() might be the more elegant solution:

foo5 <- function(a=1, b=5, h='coconut') {
    modifyList(formals(sys.function()), as.list(match.call())[-1])
}
foo5(b=0, h='mango')

Original post (2011-11-04)

After casting about a bit, this looks to be the best solution.

foo <- function(a=1, b=5, h='coconut') {
    frm <- formals(foo)
    parms <- lapply(names(frm), get, envir=sys.frame(sys.parent(0)))
    names(parms) <- names(frm)
    return(parms)
}
foo(b=0, h='mango')
# $a
# [1] 1

# $b
# [1] 0

# $h
# [1] "mango"

There's some subtle stuff going on here with the way that lapply scopes/evaluates the calls that it constructs. The details are hidden in a call to .Internal(lapply(X, FUN)), but for a taste, compare these two calls:

# With function matched by match.fun, search in sys.parent(0)
foo2 <- function(a=1, h='coconut') {
    lapply(names(formals()), 
           get, envir = sys.parent(0))
}

# With anonymous function, search in sys.parent(2)    
foo3 <- function(a=1, h='coconut') {
    lapply(names(formals()), 
           FUN = function(X) get(X, envir = sys.parent(2)))
}

foo4(a=0, h='mango')
foo5(a=0, h='mango')

回答2:

Just convert the current environment into a list:

foo <- function(a=1, b=5, h='coconut') {
  as.list(environment())
}
foo(a = 0, h = 'mango')

回答3:

This is adapted from @Josh O'Brien's solution above using sapply to automatically assign the correct names to the resulting list (saves one line of code):

foo <- function(a=1, b=5, h='coconut') {
    frm <- formals(foo)
    parms <- sapply(names(frm), get, envir=sys.frame(sys.parent(-1)), simplify=FALSE)
    return(parms)
}

来源：https://stackoverflow.com/questions/8016636/using-get-inside-lapply-inside-a-function

标签

lapply