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 would convert roman numerals to numbers? 

(I know I can write it myself but I prefer to use already prepared or preferably standard functions, unfortunatelly cannot find one. Standard library or package function is a prefered solution)

Answer: 
`as.roman()` returns an object of class *roman*, so R recognizes it as such. You can directly turn it back into an Arabic numeral with `as.numeric()`. If you have a string that meets the criteria such that it could be a valid roman numeral, you can coerce it to a class roman object with `as.roman()`, and then coerce it into an Arabic numeral by composing the coercion functions. Consider: 

```
> as.roman(79)
[1] LXXIX
> x <- as.roman(79)
> x
[1] LXXIX
> str(x)
Class 'roman'  int 79
> as.roman("LXXIX")
[1] LXXIX
> as.numeric(as.roman("LXXIX"))
[1] 79

```
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

```