On 28/03/2014, 2:04 AM, Philippe Grosjean wrote:
On 28 Mar 2014, at 00:57, Duncan Murdoch <murdoch.dun...@gmail.com> wrote:
On 27/03/2014, 7:38 PM, Thomas Lumley wrote:
You get what you wanted from
do.call(plot,list(x=quote(x),y=quote(y)))
By the time do.call() gets the arguments it doesn't know how y was
originally computed, just what values it has.
This works, but it doesn't make sense to me. The arguments end up with the
expressions x and y, but why are they evaluated in the right place? Quote
doesn't bind an environment to its expressions, does it? How does plot() know
where to evaluate them?
Duncan Murdoch
I am puzzled here… Isn't it the same when you write plot(x = x, y = y)? You don't tell
the environment for your 'x' and 'y' expressions, isn't it? And they are defined from the
context the function call and definition. You do the same with the env= argument of
do.call() which is equal to parent.frame() by default. As far as I understand, it is
behaving similarly. I have tried a few "risky" situations:
In that case the environment is the evaluation environment for the
plot() call.
I'm in a rush right now, so I haven't analyzed all of these examples,
but I have one more comment down below.
1) S3 dispatch:
## do.call() with quote() inside S3 methods
## Trying to be silly by defining x and y inside the generic too to make sure
do.call() isn't confused
myPlot <- function (x, y, ...) {
x <- 101:110
y <- rep(1, 10)
UseMethod("myPlot")
}
## A method that uses plot() directly
myPlot.anObj1 <- function (x, y, ...)
plot(x = x, y = y)
x <- structure(1:10, class = "anObj1")
y <- (x - 5.5)^2
myPlot(x = x, y = y) # right
## A method that uses do.call() with quote()
myPlot.anObj2 <- function (x, y, ...)
do.call(plot, list(x = quote(x), y = quote(y)))
x <- structure(1:10, class = "anObj2")
myPlot(x = x, y = y) # same result
## A dispatch to the default method
myPlot.default <- function (x, y, ...)
do.call(plot, list(x = quote(x), y = quote(y)))
x <- 1:10
myPlot(x = x, y = y) # same result
This seems to give the same thing.
2) A really crazy and tortuous closure where I try to confuse do.call() with
lexical scoping and all the stuff:
## This one uses plot() directly
makePlotFun1 <- function () {
x <- 1:10
y <- (x - 5.5)^2
Y <- y
## If I use y directly as default value for argument y= in the next
instruction
## I got a "promise already under evaluation" error!
function (x, y = Y) plot(x = x, y = y)
}
## This one uses do.call(plot, ….)
makePlotFun2 <- function () {
x <- 1:10
y <- (x - 5.5)^2
Y <- y ## Same remark as for makePlotFun1()
function (x, y = Y) {
do.call(plot, list(x = quote(x), y = quote(y)))
}
}
## Different x and y values in .GlobalEnv
x <- 101:110
y <- rep(1, 10)
myPlotFun1 <- makePlotFun1()
myPlotFun1(x = x, y = y)
## and...
myPlotFun1(x = x) # It uses y from the closure. It's fine.
## Now, using do.call()
myPlotFun2 <- makePlotFun2()
myPlotFun2(x = x, y = y)
## and...
myPlotFun2(x = x) # Same result
You are probably right, but I cannot think of a situation where do.call(plot,
list(x = quote(x), y = quote(y)) do not give the same result as list(x = x, y =
y). Have you an example?
Rolf's example is one, e.g.
x <- 1:10
y <- x^2
plot(x,y) # gets nice xlab and ylab
do.call(plot, list(x=x, y=y)) # ugly xlab and ylab
do.call(plot, list(x=quote(x), y=quote(y)) # nice xlab and ylab
Duncan Murdoch
Best,
Philippe
-thomas
On Thu, Mar 27, 2014 at 6:17 PM, Rolf Turner <r.tur...@auckland.ac.nz>wrote:
I was under the impression that
do.call(foo,list(x=x,y=y))
should yield the same result as
foo(x,y).
However if I do
x <- 1:10
y <- (x-5.5)^2
do.call(plot,list(x=x,y=y))
I get the expected plot but with the y-values (surrounded by c()) being
printed (vertically) in the left-hand margin of the plot.
The help for do.call() says:
The behavior of some functions, such as substitute, will not be the
same for functions evaluated using do.call as if they were evaluated
from the interpreter. The precise semantics are currently undefined and
subject to change.
Am I being bitten by an instance of this phenomenon? Seems strange.
I would be grateful for enlightenment.
cheers,
Rolf Turner
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