R: passing expression to an inner function
Further delving into the mysteries of R evaluation...This is closely related to my previous question ( How to write an R function that evaluates an expression within a data-frame ). Let's say I want to write a function topfn
that takes a data-frame and an expression involving column-names of that data-frame. I want to pass both these arguments on to another function fn
that actually evaluates the expression within the "environment" of the data-frame. And I want both fn
and topfn
to work correctly when passed a data-frame and an expression
My first attempt, as suggested in the answer to the above question, is to define:
fn <- function(dfr, expr) {
mf <- match.call()
eval( mf$expr, envir = dfr )
}
And define topfn
like this:
topfn <- function(df, ex) {
mf <- match.call()
fn(df, mf$ex)
}
Now if I have a data-frame
df <- data.frame( a = 1:5, b = 1:5 )
the inner function fn
works fine:
> fn(df,a)
[1] 1 2 3 4 5
But the topfn
does not work:
> topfn(df,a)
mf$ex
To fix this I first check the class of topfn(df,a)
,
> class(topfn(df,a))
[1] "call"
This gives me an idea for an ugly hack to re-define fn
as follows:
fn <- function(dfr, expr) {
mf <- match.call()
res <- eval(mf$expr, envir = dfr)
if(class(res) == 'call')
eval(expr, envir = dfr) else
res
}
And now both functions work:
> fn(df,a)
[开发者_如何学运维1] 1 2 3 4 5
> topfn(df,a)
[1] 1 2 3 4 5
As I said, this looks like an ugly hack. Is there a better way (or more standard idiom) to get these working? I've consulted Lumley's curiously-named Standard NonStandard Evaluation Rules document http://developer.r-project.org/nonstandard-eval.pdf but wasn't particularly enlightened after reading it. Also helpful would be any pointers to source-code of functions I can look at for examples.
This is most easily avoided by passing strings into topfn
instead of expressions.
topfn <- function(df, ex_txt)
{
fn(df, ex_txt)
}
fn <- function(dfr, expr_txt)
{
eval(parse(text = expr_txt), dfr)
}
df <- data.frame(a = 1:5, b = 1:5 )
fn(df, "a")
fn(df, "2 * a + b")
topfn(df, "a")
topfn(df, "2 * a + b")
EDIT:
You could let the user pass expressions in, but use strings underneath for your convenience.
Change topfn
to
topfn <- function(df, ex)
{
ex_txt <- deparse(substitute(ex))
fn(df, ex_txt)
}
topfn(df, a)
topfn(df, 2 * a + b)
ANOTHER EDIT:
This seems to work:
topfn <- function(df, ex)
{
eval(substitute(fn(df, ex)))
}
fn <- function(dfr, expr)
{
eval(substitute(expr), dfr)
}
fn(df, a)
fn(df, 2 * a + b)
topfn(df, a)
topfn(df, 2 * a + b)
You can use three dots to gather arguments and pass them to another function, is that what you mean?
ftop=function(...) f(...)
f=function(a,b) a[b]
a=data.frame(b=10)
ftop(a,"b")
f(a,"b")
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