Hello,
Is there a way to define a custom unary operator in R (other than making a
class and 'overloading' the normal unary operators in R)? The
documentation seems to suggest that only custom binary operators are
possible with the ``%abc%``construct but I was wondering whether any one
has done so. None of the RSiteSearch or RSeek queries I posed suggested
that this question had been asked before but if it has, a link to the
relevant discussion thread would be wonderful.
For example in the J language the ``! `` sign is a unary operator for the
factorial function and a binary operator for the choose function so I
would like to be able to define a similar operator like this::
'%!%' <- function(e1, e2) {
if (missing(e2)) factorial(e1)
else choose(e2, e1)
}
The binary version of the operator works fine:
> 2 %!% 4
[1] 6
But the unary one doesn't (which is perfectly understandable since there
is no indication in any documentation I have seen that suggests it
should):::
> %!% 4
Error: unexpected SPECIAL in "%!%"
The source code for the '?' operator almost seems to suggest that a mixed
unary / binary non-primitive R function is possible::
> get('?')
function (e1, e2)
{
e1Expr <- substitute(e1)
if (missing(e2)) {
if (is.call(e1Expr))
return(.helpForCall(e1Expr, parent.frame()))
if (is.name(e1Expr))
e1 <- as.character(e1Expr)
eval(substitute(help(TOPIC), list(TOPIC = e1)))
}
else {
if (is.name(e1Expr))
e1 <- as.character(e1Expr)
e2Expr <- substitute(e2)
if (is.name(e2Expr))
e2 <- as.character(e2Expr)
else if (is.call(e2Expr) && identical(e1, "method"))
return(.helpForCall(e2Expr, parent.frame(), FALSE))
topic <- topicName(e1, e2)
doHelp <- .tryHelp(topic)
if (inherits(doHelp, "try-error")) {
stop(gettextf("no documentation of type '%s' and topic
'%s' (or error in processing help)",
e1, e2), domain = NA)
}
}
}
<environment: namespace:utils>
I was playing around with the idea of the ways one could try build a J
interpreter with R and it seemed like being able to define custom unary /
binary operators like that could be convenient.
Thanks,
Trevor Davis
Federal Reserve Board of Governors
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