On 30/10/2018 4:18 PM, Sebastien Bihorel wrote:
Thanks Duncan for your quick reply.
Ideally, I would want bar1 and bar2 to be independent functions, because they
are huge in actuality and, as the actual foo function grows, I may end up with
10 different bar# functions. So I would like to separate them from foo as much
as possible.
If that's the case, then I think the second solution (passing around
environments) is a really bad idea. Functions should not have side
effects because it makes them harder to understand. Modifying local
variables in some other function is a really dangerous side effect,
especially if both functions are big, because they are already hard to
understand.
If you really have more callers for bar1() than just foo(), it is even
worse.
So I'd suggest having bar1 and bar2 return the new values in a list, and
in foo(), explicitly extract the values you want from the list. Then if
in the future you decide that bar1 should also return a 4th value w, or
you want to rename x to something more meaningful, you don't need to
check your other foo functions to see if x and w are used in the same
way in them. They'll just ignore w if it isn't relevant to them. Your
foo() code becomes something like this:
x <- y <- z <- 0
# here is my scope problem
result <- list(x = x, y = y, z = z) # c == 0 case
if (c==1) result <- bar1()
if (c==2) result <- bar2()
x <- result[["x"]]
y <- result[["y"]]
z <- result[["z"]]
Duncan Murdoch
----- Original Message -----
From: "Duncan Murdoch" <murdoch.dun...@gmail.com>
To: "Sebastien Bihorel" <sebastien.biho...@cognigencorp.com>,
r-help@r-project.org
Sent: Tuesday, October 30, 2018 4:13:05 PM
Subject: Re: [R] Question about function scope
On 30/10/2018 3:56 PM, Sebastien Bihorel wrote:
Hi,
From the R user manual, I have a basic understanding of the scope of function
evaluation but have a harder time understanding how to mess with environments.
My problem can be summarized by the code shown at the bottom:
- the foo function performs some steps including the assignment of default
values to 3 objects: x, y, z
- at some point, I would like to call either the bar1 or bar2 function based
upon the value of the c argument of the foo function. These functions assign
different values to the x, y, z variables.
- then foo should move on and do other cool stuff
Based upon default R scoping, the x, y, and z variables inside the bar1 and
bar2 functions are not in the same environment as the x, y, and z variables
created inside the foo function.
Can I modify the scope of evaluation of bar1 and bar2 so that x, y, and z
created inside the foo function are modified?
PS:
- I know about "<<-" but, in my real code (which I cannot share, sorry), foo is already called within other
functions and x, y, and z variables do not exist in the top-level environment and are not returned by foo. So
"<<-" does not work (per manual: " Only when <<- has been used in a function that was returned as
the value of another function will the special behavior described here occur. ")
I haven't looked up that quote, but it is likely describing a situation
that isn't relevant to you. For you, the important part is that bar1
and bar2 must be created within foo. They don't need to be returned
from it.
So my edit below of your code should do what you want.
foo <- function(a=1, b=2, c=0){
bar1 <- function(){
x <<- 1
y <<- 1
z <<- 1
cat(sprintf('bar1: x=%d, y=%d, z=%d\n', x, y, z))
}
bar2 <- function(){
x <<- 2
y <<- 2
z <<- 2
cat(sprintf('bar2: x=%d, y=%d, z=%d\n', x, y, z))
}
# some setup code
dummy <- a + b
x <- y <- z <- 0
# here is my scope problem
if (c==1) bar1()
if (c==2) bar2()
# some more code
cat(sprintf('foo: x=%d, y=%d, z=%d\n', x, y, z))
}
foo(c=0)
foo(c=1)
foo(c=2)
I get this output:
> foo(c=0)
foo: x=0, y=0, z=0
> foo(c=1)
bar1: x=1, y=1, z=1
foo: x=1, y=1, z=1
> foo(c=2)
bar2: x=2, y=2, z=2
foo: x=2, y=2, z=2
Duncan Murdoch
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