f1 <- function(formula, data) {
mf <- match.call(expand.dots = FALSE)
mf[[1]] <- as.name("model.frame")
eval(mf, parent.frame())
}-roger
Heather Turner wrote:
I'm trying to evaluate a pre-built expression using eval(), e.g.
dataset <- data.frame(y = runif(30, 50,100), x = gl(5, 6))
# one like this mf <- expression(model.frame(y~x)) eval(mf, dataset, parent.frame())
# rather than this eval(expression(model.frame(y~x)), dataset, parent.frame())
In the example above there is no problem, the problem comes when I try to do a similar thing within a function, e.g.
f1 <- function(formula, data) { mt <- terms(formula) mf <- as.expression(as.call(c(as.name("model.frame"), formula = mt))) eval(mf, data, parent.frame()) }
f1(formula = y ~ x, data = dataset)
Error in eval(expr, envir, enclos) : Object "y" not found
I can get round this by building a call to eval using paste, e.g.
f2 <- function(formula, data) { mt <- terms(formula) mf <- as.expression(as.call(c(as.name("model.frame"), formula = mt))) direct <- parse(text = paste("eval(expression(", mf, "), data, parent.frame())")) print(direct) eval(direct) }
f2(formula = y ~ x, data = dataset)
expression(eval(expression(model.frame(formula = y ~ x)), data, parent.frame()))
y x
1 92.23087 1
2 63.43658 1
3 55.24448 1
4 72.75650 1
5 67.58781 1
...
but this seems rather convoluted. Can anyone explain why f1 doesn't work (when f2 does) and/or suggest a neater way of dealing with this?
Thanks
Heather
Mrs H Turner Research Assistant Dept. of Statistics University of Warwick
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