Change dollar to print negative sign on the outside; allow percent to work with negative number

[dougmitarotonda]

I am not sure where the authoritative source on this would be, but how about:
http://english.stackexchange.com/questions/124797/how-to-write-negative-currency-in-text

This commit is to fix #38

[dougmitarotonda]

Hmm, sorry for the newbie mistake. Apparently my commits for the changes for percent got bundled into the same pull request. Sorry about that.

Anyway, a505095 and 3120cc6 are to fix #39

[trevorld]

I personally prefer -$100.00 style negative numbers (slightly more common in an academic Economics article) but if you tweaked your dollar_format function you could also support the other commonly negative number style ($100.00) ( more common in accounting and Excel's currency default formatting style: http://www.quepublishing.com/articles/article.aspx?p=2024309&seqNum=9 ).

my_dollar_format <- function (largest_with_cents = 1e+05, negative_parentheses = FALSE) {
    old_dollar <- scales::dollar_format(largest_with_cents = largest_with_cents)
    new_dollar <- function(x) {
        x[!is.na(x)] <- old_dollar(x[!is.na(x)])
        if (negative_parentheses) {
            gsub("\\$-(.*)", "($\\1)", x)
        } else {
            gsub("\\$-", "-$", x)
        }
    }
}
> d <- c(100.02, 25.32, -24.42, NA)
> my_dollar_format()(d)
> [1] "$100.02" "$25.32"  "-$24.42" NA   
> my_dollar_format(negative_parentheses=TRUE)(d)
> [1] "$100.02"  "$25.32"   "($24.42)" NA

[dougmitarotonda]

I like your suggestion to allow for the parentheses. I defer to the package authors on what to do here.

[hadley]

These tests seem a bit repetitive. Could you please reduce and break up into separately named tests?

[hadley]

I'd be happy to review a changes to add a use_parens argument

[hadley]

Hmmmm, this got a bit more complicated now that dollar_format() is more general - you can supply arbitrary prefixes and suffixes now, and while -$100 looks better, -USD 100 certainly doesn't. Maybe it's just better to implement that parens argument and leave $-100 as is?

[trevorld]

Well character currency descriptors like USD are usually supposed to come after the number, i.e. http://www.proz.com/forum/linguistics/283051-currency_usd_does_it_come_before_or_after_the_number.html so you would expect people to mainly use characters like USD as a suffix which gives you -500 USD in either case (assuming prefix is then "") instead of the more ugly -USD 500...

I think most people (including me and the original patch contributor doug) want negative currency values where the negative sign goes in front of the prefix like -$100 or - £100.45. Both of us have actually written custom formatters to let us do so since that is the format necessary to have good looking charts if you are an economist or business person. If for some reason they don't want it reversed then it should be straightforward to write a custom formatter like

my_formatter <- function(prefix, ...) {
   function(x) {
     my_dollar <- dollar_format(prefix=prefix, ...)
     values <- my_dollar(x)
     values <- gsub(paste0("-", prefix), paste0(prefix, "-"), values)
     values
   }
}