R：在apply系列函数中使用MoreArgs参数

Question

I have a complicated function that takes a bunch of graphical objects and ggplot maps and combines them into gtables. For purpose of this question, I have represented that function as a simple linear function, MyFun1 or MyFun2, which differ only by the presence of a ... argument.

The function creating the gtables have two two arguments that vary and a bunch of other arguments that are constant in a given run. I am trying to run that function with mapply, using the MoreArgs argument to supply the constant (for each run -- changed in a bigger loop) arguments. However, I got an error saying the passed values were surplusage, though they were used in the body.

mArgs  <- list(y1=1, y2=2)
MyFun1  <- function(x, y){x*y1 + y*y2}
MyFun2 <- function(x, y, ...){x*y1 + y*y2}

R1 <- function(A, B){
   out <- mapply(MyFun, x=A, y=B, MoreArgs = mArgs)
} 
R1(1:3, 4:6)
 Error in (function (x, y)  : unused arguments (y1 = 1, y2 = 2)

Traceback:

3. (function (x, y) 
{
    x * y1 + y * y2
})(x = dots[[1L]][[1L]], y = dots[[2L]][[1L]], y1 = 1, y2 = 2) 
2. mapply(MyFun, x = A, y = B, MoreArgs = mArgs) 
1. R1(1:3, 4:6) 

I thought perhaps the problem was that MyFun1 lacked the requisite formals to pass the additional arguments into the body, so I tried adding a ... argument. But now MyFun could not find the arguments in the MoreArgs list.

R2 <- function(A, B){
   out <- mapply(MyFun2, x=A, y=B, MoreArgs = mArgs)
}
R2(1:3, 4:6)
 Error in (function (x, y, ...)  : object 'y1' not found 

Traceback:

3. (function (x, y, ...) 
{
    x * y1 + y * y2
})(x = dots[[1L]][[1L]], y = dots[[2L]][[1L]], y1 = 1, y2 = 2) 
2. mapply(MyFun2, x = A, y = B, MoreArgs = mArgs) 
1. R2(1:3, 4:6) 

Then I thought, perhaps mArgs in the global environment was not in the scope of MyFun2, though it it seems it should, to my limmited understanding. So I added it at as an argument to each of the calling functions. This did not change the result.

R3 <- function(A, B, mArgs){
   out <- mapply(function(x,y, ...){x*y1 + y*y2}, x=A, y=B,
                 MoreArgs = mArgs)
}
R3(1:3, 4:6, mArgs)
 Error in (function (x, y, ...)  : object 'y1' not found

Traceback:

3. (function (x, y, ...) 
{
    x * y1 + y * y2
})(x = dots[[1L]][[1L]], y = dots[[2L]][[1L]], y1 = 1, y2 = 2) 
2. mapply(function(x, y, ...) {
    x * y1 + y * y2
}, x = A, y = B, MoreArgs = mArgs) 
1. R3(1:3, 4:6, mArgs) 

I tried replacing mArgs with the list itself in each of these functions, which in no case changed the result.

I am baffled. Help!

Answer 1

To solve your problem, you can do:

MyFun2 <- function(x, y, y1, y2){x*y1 + y*y2}

To answer why you method didn't work, see R-lang on dot-dot-dot :

2.1.9 Dot-dot-dot

The '...' object type is stored as a type of pairlist. The components of '...' can be accessed in the usual pairlist manner from C code, but is not easily accessed as an object in interpreted code. The object can be captured as a list, so for example in table one sees

 args <- list(...) 

....

 for (a in args) { 

If a function has '...' as a formal argument then any actual arguments that do not match a formal argument are matched with '...'.

so in your have to extract y1 and y2 manually from the ... :

MyFun2 <- function(x, y, ...){
    additional.args <- list(...)
    y1 <- additional.args[[1]] 
    y2 <- additional.args[[2]]
    x*y1 + y*y2
}

MyFun2(1, 2, 1, 2)
# [1] 5

about moreArgs, it is for arguments that will not be vectorized over. see following example:

mapply(function(x, y){
    print('----------------')
    print(x)
    print(y)
    x + y
}, x = 1:3, y = 1:3)

# [1] "----------------"
# [1] 1
# [1] 1
# [1] "----------------"
# [1] 2
# [1] 2
# [1] "----------------"
# [1] 3
# [1] 3
# [1] 2 4 6

mapply(function(x, y){
    print('----------------')
    print(x)
    print(y)
    x + y
}, x = 1:3, MoreArgs = list(y = 1:3))

# [1] "----------------"
# [1] 1
# [1] 1 2 3
# [1] "----------------"
# [1] 2
# [1] 1 2 3
# [1] "----------------"
# [1] 3
# [1] 1 2 3

#      [,1] [,2] [,3]
# [1,]    2    3    4
# [2,]    3    4    5
# [3,]    4    5    6