Marc -- thanks, very interesting.
I was in fact tinkering at a very simple default arguments assignment
to a generic command-line R script header:
#!/bin/sh
# graph a fertility run
tail --lines=+4 "$0" | R --vanilla --slave --args $*; exit
args <- commandArgs()[-(1:4)]
# the krivostroi library
source("/w/ct/r/kriv/krivostroi.r")
# NB: vector assignment in R? defaults?
file <- args[1]
maxruns <- if (is.na(args[2])) 1 else args[2]
prefix <- if (is.na(args[3])) file.basename(file) else args[3]
-- turns out, args[N] here are NA if not supplied on the command-
line. I'd like to see a nicer way to do it still, without repeating
args[N] twice for each assignment though.
Another thing from this snippet, unrelated to assignment, is that was
the only way to get R script to be packed in a single file runnable
from the command line in a stand alone way. Yet when I want to source
() it, R obviously chokes on the shell command tail. My previous
solution was to have a pair of files, script.r/script.sh for each R
script, where .sh would look like
echo "argv <- c('$1','$2'); source('main.r')" | R --vanilla --slave
Wonder if there's a way to have a single file which is a stand-alone
command script, and can be source()'d in R.
Cheers,
Alexy
On Nov 20, 2007, at 4:03 AM, Marc Schwartz wrote:
>
> On Tue, 2007-11-20 at 03:32 +0300, Alexy Khrabrov wrote:
>> What's the idiom of assigning a default value to a variable if it's
>> not set? In Ruby one can say
>>
>> v ||= default
>>
>> -- that's an or-assign, which triggers the assignment only if v is
>> not set already. Is there an R shorthand?
>>
>> Cheers,
>> Alexy
>
> If 'v' is not set, then it does not exist, hence you can use exists
> () to
> check for it. However, you need to [potentially] distinguish where the
> variable might be located. Keep in mind that R uses lexical scoping,
> hence the exists() function has other arguments to define where to
> look.
>
> A simple example:
>
>> v
> Error: object "v" not found
>
> if (!exists("v")) v <- "Not Set"
>
>> v
> [1] "Not Set"
>
> v <- "Set"
>
> if (!exists("v")) v <- "Not Set"
>
>> v
> [1] "Set"
>
>
> See ?exists for more information.
>
> That being said, just as an example of extending R, you could do the
> following, which is to create a new function %||=% (think %in% or %*%)
> which can then take two arguments, one preceding it and one following
> it, and then basically do the same thing as above. Again here, scoping
> is critical.
>
>
> "%||=%" <- function(x, y)
> {
> Var <- deparse(substitute(x))
> if (!exists(Var))
> assign(Var, y, parent.frame())
> }
>
>> v
> Error: object "v" not found
>
> v %||=% "Not Set"
>
>> v
> [1] "Not Set"
>
> v <- "Set"
>
> v %||=% "Not Set"
>
>> v
> [1] "Set"
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