How to evaluate a call in a data.frame?

Question

In one package I'm working on, I'm bumping into a problem that looks simple - but I can't figure it out:

A subfunction is given two arguments:

• obj, a data.frame
• foo, a call

For example:

> head(obj)
  cadmium copper lead zinc  elev
1    11.7     85  299 1022 7.909
2     8.6     81  277 1141 6.983
3     6.5     68  199  640 7.800
4     2.6     81  116  257 7.655
5     2.8     48  117  269 7.480
6     3.0     61  137  281 7.791
> foo
log(cadmium)
> class(foo)
[1] "call"

In that example, I want to create a vector x <- log(obj$cadmium). How do I do that? I tried using with() but I don't get the expected result:

> with(obj, foo)
log(cadmium)

foo is a call created by the user by specifying a transformation on a column of the data.frame obj:

my_function(obj, foo = log(cadmium)) { ... }

dput() of data snippet:

obj <- structure(list(cadmium = c(11.7, 8.6, 6.5, 2.6, 2.8, 3), copper = c(85L, 
81L, 68L, 81L, 48L, 61L), lead = c(299L, 277L, 199L, 116L, 117L, 
137L), zinc = c(1022L, 1141L, 640L, 257L, 269L, 281L), elev = c(7.909, 
6.983, 7.8, 7.655, 7.48, 7.791)), .Names = c("cadmium", "copper", 
"lead", "zinc", "elev"), class = "data.frame", row.names = c("1", 
"2", "3", "4", "5", "6"))

Answer 1

You need to evaluate the call, for example using eval():

foo <- call("log", quote(cadmium))
with(obj, eval(foo))

which gives:

> with(obj, eval(foo))
[1] 2.4595888 2.1517622 1.8718022 0.9555114 1.0296194 1.0986123

where obj is the snippet of data you showed.

eval() also has an envir argument indicating the environment within which the expression is evaluated. As such you can do what you want without with() using eval() directly:

> eval(foo, envir = obj)
[1] 2.4595888 2.1517622 1.8718022 0.9555114 1.0296194 1.0986123

Answer 2

Something like

z <- data.frame(a1=1:5,b1=LETTERS[1:5],c1=letters[1:5])
foo <- quote(log(a1))
eval(foo,envir=z)

but beware: once you start using eval you are descending into the lower depths of R. I still don't fully understand the distinctions among calling frames, enclosing frames, etc. etc. ...