I do not understand error "object not found" inside the function - r

Question

I have roughly this function:
plot_pca_models <- function(models, id) {
library(lattice)
splom(models, groups=id)
}
and I'm calling it like this:
plot_pca_models(data.pca, log$id)
wich results in this error:
Error in eval(expr, envir, enclos) : object 'id' not found
when I call it without the wrapping function:
splom(data.pca, groups=log$id)
it raises this error:
Error in log$id : object of type 'special' is not subsettable
but when I do this:
id <- log$id
splom(models, groups=id)
it behaves as expected.
Please can anybody explain why it behaves like this and how to correct it? Thanks.
btw:
I'm aware of similar questions here, eg:
Help understand the error in a function I defined in R
Object not found error with ddply inside a function
Object disappears from namespace in function
but none of them helped me.
edit:
As requested, there is full "plot_pca_models" function:
plot_pca_models <- function(data, id, sel=c(1:4), comp=1) {
# 'data' ... princomp objects
# 'id' ... list of samples id (classes)
# 'sel' ... list of models to compare
# 'comp' ... which pca component to compare
library(lattice)
models <- c()
models.size <- 1:length(data)
for(model in models.size) {
models <- c(models, list(data[[model]]$scores[,comp]))
}
names(models) <- 1:length(data)
models <- do.call(cbind, models[sel])
splom(models, groups=id)
}
edit2:
I've managed to make the problem reproducible.
require(lattice)
my.data <- data.frame(pca1 = rnorm(100), pca2 = rnorm(100), pca3 = rnorm(100))
my.id <- data.frame(id = sample(letters[1:4], 100, replace = TRUE))
plot_pca_models2 <- function(x, ajdi) {
splom(x, group = ajdi)
}
plot_pca_models2(x = my.data, ajdi = my.id$id)
which produce the same error like above.

Answer 1

The problem is that splom evaluates its groups argument in a nonstandard way.A quick fix is to rewrite your function so that it constructs the call with the appropriate syntax:
f <- function(data, id)
eval(substitute(splom(data, groups=.id), list(.id=id)))
# test it
ir <- iris[-5]
sp <- iris[, 5]
f(ir, sp)

Answer 2

log is a function in base R. Good practice is to not name objects after functions...it can create confusion. Type log$test into a clean R session and you'll see what's happening:
object of type 'special' is not subsettable

Answer 3

Here's a modification of Hong Oi's answer. First I would recommend to include id in the main data frame, i.e
my.data <- data.frame(pca1 = rnorm(100), pca2 = rnorm(100), pca3 = rnorm(100), id = sample(letters[1:4], 100, replace = TRUE))
.. and then
plot_pca_models2 <- function(x, ajdi) {
Call <- bquote(splom(x, group = x[[.(ajdi)]]))
eval(Call)
}
plot_pca_models2(x = my.data, ajdi = "id")
The cause of the confusion is the following line in lattice:::splom.formula:
groups <- eval(substitute(groups), data, environment(formula))
... whose only point is to be able to specify groups without quotation marks, that is,
# instead of
splom(DATA, groups="ID")
# you can now be much shorter, thanks to eval and substitute:
splom(DATA, groups=ID)
But of course, this makes using splom (and other functions e.g. substitute which use "nonstandard evaluation") harder to use from within other functions, and is against the philosophy that is "mostly" followed in the rest of R.