# Matlab-style multiple assignment in R

August 26, 2010
By

(This article was first published on Struggling Through Problems » R, and kindly contributed to R-bloggers)

R again!

You know how in Matlab you can do?

`S, I = sort(M)`

I like that.

R generic functions makes this possible. First, let’s genericize assignment. I feel like regular “=” and “<-” oughta stay nongeneric, so let’s make a new one.

`'%=%' = function(l, r, ...) UseMethod('%=%')`

Now the next step is a bit tricky. We need to group several variables on the left of %=%

`... a, b, ... %=% foo()`

The trick is they have to stay unevaluated. Luckily R uses (what IIRC is called) “pass by name”, so we can do this. Let’s start with a function to take a, b, …

```l = function(...) {

}```

The hard part is grabbing the ‘…’ without evaluating it.

```> substitute(...)
...```

won’t work, nor will alist(…). I have NO IDEA why, but the following expression (stolen from data.frame()) works:

`as.list(substitute(list(...)))[-1L]`

So now we have our function

```l = function(...) {
List = as.list(substitute(list(...)))[-1L]
class(List) = 'lbunch'

List
}```

And we can add a specific implementation for our generic %=%:

```'%=%.lbunch' = function(l, r, ...) {
Names = lapply(l, as.character)
Envir = as.environment(-1)

for (II in 1:length(Names)) {
Name = Names[[II]]
assign(Name, r[[II]], pos=Envir)
}
}```

Which just treats the objects “a”, “b” … as strings, and assigns into the caller’s environment.

That’s what I had first, but it can be made better. With the above implementation:

```> l(a, b) %=% list(3, 4)
Warning message:
In assign(Name, r[[II]], pos = Envir) :
only the first element is used as variable name```

It doesn’t play nice with assignment functions.

It can be modified to call ‘<-’ directly:

```'%=%.lbunch' = function(l, r, ...) {
Envir = as.environment(-1)

for (II in 1:length(l)) {
do.call('<-', list(l[[II]], r[[II]]), envir=Envir)
}
}```

And now all is good.

```> l(attr(a, 'foo'), b) %=% list(3, 4)
> a
 3
attr(,"foo")
 3```

To leave a comment for the author, please follow the link and comment on their blog: Struggling Through Problems » R.

R-bloggers.com offers daily e-mail updates about R news and tutorials on topics such as: Data science, Big Data, R jobs, visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...

If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Tags: