Question

Slightly embarrassed to ask such a simple question but I’ve wasted an hour now and figure its a 30 second solution. The problem is how to edit an existing object that is provided as an input to a function. I’ve also played with the super-assignment <<- without success.

The example function uses 2 inputs (one for an object and one for its name). I just need a form of this that removes the need for the ‘n’ input.

m <- c(2,5,3,7,1,3,9,3,5)
dim(m) <- c(3,3)
m

f <- function(x, n) { # where 'n' is the object name of 'x'
  x[1,] <- c(1,2,3)
  assign(n, x, envir = .GlobalEnv)
}

f(m, 'm')
m

Thanks in advance.

Answer 1

You don’t need to provide the name as an extra argument; substitute will get that for you. To do things in the scope of the calling function you use eval with parent.frame.

f <- function(x) {
  eval(substitute( x[1,] <- c(1,2,3) ), parent.frame())
}

Then,

m <- c(2,5,3,7,1,3,9,3,5)
> dim(m) <- c(3,3)
> m
     [,1] [,2] [,3]
[1,]    2    7    9
[2,]    5    1    3
[3,]    3    3    5
> f(m)
> m
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    5    1    3
[3,]    3    3    5

That said, modifying the caller’s environment is generally a bad idea and it will usually lead to less confusing/fragile code if you just return the value and re-assign it to m instead. This is generally preferable.:

f <- function (x) {
    x[1,] <- c(1,2,3)
    x
}

m <- f(m)

However, I have occasionally found eval shenanigans to come in handy when I really needed to change an array in place and avoid an array copy.