Dear list,
Consider the following,
library(grid)
w = unit.c(unit(1, "in"), unit(2, "in"))
w2 = w + unit(1, "mm")
w[2] <- 0
w2[2] <- 0
convertUnit(w, "mm")
#[1] 25.4mm 0mm
convertUnit(w2, "mm")
#Error in grid.Call("L_convert", x, as.integer(whatfrom),
as.integer(whatto), :
# INTEGER() can only be applied to a 'integer', not a 'NULL'
The last line fails, as the naive replacement has destroyed the
structure of w2 instead of having assigned a value of 0 to the second
unit element.
I've also tried,
w = unit.c(unit(1, "in"), unit(2, "in"))
w2 = w + unit(1, "mm")
w2[[2]][2] <- 0
but this time, if the structure is licit, it's the result that's not
as I intended:
convertUnit(w2,"mm")
#[1] 26.4mm 1mm
My limited understanding is that an object of class unit.arithmetic is
waiting until the last moment to actually perform its operation,
stored in a tree-like structure. With this premise, I can't think of a
good way to modify one element of a list of unit elements.
As a workaround, I can only think of the following hack where the
objects are forced to be evaluated,
w = unit.c(unit(1, "in"), unit(2, "in"))
w2 = convertUnit(w + unit(1, "mm"), "mm", valueOnly=TRUE)
w2[2] <- 0
w2 <- unit(w2, "mm")
but it clearly isn't a very desirable route.
What is the recommended way to modify one element of a unit vector?
Digging in grid/R/unit.R , I found the following comment,
# Write "[<-.unit" methods too ??
which probably explains the above. Would it be possible to add such a method,
"[<-.unit.list" <- function(x, index, value, top=TRUE, ...) {
this.length <- length(x)
index <- (1L:this.length)[index]
if (top && any(index > this.length))
stop("Index out of bounds (unit list subsetting)")
cl <- class(x)
result <- unclass(x)
result[(index - 1) %% this.length + 1] <- value
class(result) <- cl
result
}
a = unit.c(unit(1,"mm"),unit(2,"in"))
a[2] <- unit(3,"in")
a
but for unit.arithmetic also?
Regards,
baptiste
sessionInfo()
R version 2.9.2 (2009-08-24)
i386-apple-darwin8.11.1
locale:
en_GB.UTF-8/en_GB.UTF-8/C/C/en_GB.UTF-8/en_GB.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets grid methods
[8] base
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