Implicit variable reference in R

Question

How can I evaluate c[,2] through a call to z?

a <- c(1,2,3)
b <- c(4,5,6)
c <- cbind(a,b)
z <- "c[,2]"

eval(z) is not working.

What are you trying to do? Parsing arbitrary strings is unlikely to be the right answer. — hadley, Jun 22 '11 at 15:57
Related: [R eval expression](http://stackoverflow.com/q/1743698/2157640) — Palec, May 21 '15 at 06:12

score 5 · Accepted Answer · edited Jun 22 '11 at 08:00

5

It may be below:

eval(parse(text=z))

edited Jun 22 '11 at 08:00

Marek

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answered Jun 22 '11 at 01:38

Triad sou.

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`require(fortunes); fortune(106)` – Gavin Simpson Jun 22 '11 at 08:30

score 3 · Answer 2 · answered Jun 22 '11 at 16:44

If you really need to dynamically assemble a function call and then evaluate it, do.call is typically much better (and more efficient). It's a bit hard to pass the missing parameter though, but TRUE also works in this case:

z <- c[TRUE,2]

is equivalent to:

z <- do.call('[', list(c, TRUE, 2))

But here's a hack to get the missing symbol, which can then be used:

m <- quote(f(,))[[2]] # The elusive missing symbol
z <- do.call('[', alist(c, m, 2))

Implicit variable reference in R

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