Convert roman numerals to numbers in R

Question

In R, there is a great function as.roman in the very base setup:

as.roman(79)
# [1] LXXIX

Is there an inverse function that wo

Answer 1

From as.roman code you can find .roman2numeric and its code can be seen if you run getAnywhere(".roman2numeric")

The code is:

function (x) 
{
  romans <- c("M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", 
              "IX", "V", "IV", "I")
  numbers <- c(1000L, 900L, 500L, 400L, 100L, 90L, 50L, 40L, 
               10L, 9L, 5L, 4L, 1L)
  out <- integer(length(x))
  ind <- is.na(x)
  out[ind] <- NA
  if (any(!ind)) {
    y <- toupper(x[!ind])
    y <- gsub("CM", "DCCCC", y)
    y <- gsub("CD", "CCCC", y)
    y <- gsub("XC", "LXXXX", y)
    y <- gsub("XL", "XXXX", y)
    y <- gsub("IX", "VIIII", y)
    y <- gsub("IV", "IIII", y)
    ok <- grepl("^M{,3}D?C{,4}L?X{,4}V?I{,4}$", y)
    if (any(!ok)) {
      warning(sprintf(ngettext(sum(!ok), "invalid roman numeral: %s", 
                               "invalid roman numerals: %s"), paste(x[!ind][!ok], 
                                                                    collapse = " ")), domain = NA)
      out[!ind][!ok] <- NA
    }
    if (any(ok)) 
      out[!ind][ok] <- sapply(strsplit(y[ok], ""), function(z) as.integer(sum(numbers[match(z, 
                                                                                            romans)])))
  }
  out
}

You can access to .roman2numeric and convert roman number to decimal numbers the way @rawr suggested in his/her comment.

> utils:::.roman2numeric("III")
[1] 3
> utils:::.roman2numeric("XII")
[1] 12
> utils:::.roman2numeric("LXXIX")
[1] 79