Convert Excel column letter codes to numbers (inverse of `openxlsx::int2col`)

excelColsToNums.R

# Convert Excel column letter codes to numbers
# Very similar to openxlsx::convertFromExcelRef() (see https://rdrr.io/cran/openxlsx/man/convertFromExcelRef.html).
# The inverse function to openxlsx::int2col() (see https://rdrr.io/cran/openxlsx/man/int2col.html).
#
# excelColsToInt(c('a','F','x','aa','az','aaa','a','Az','aZ','AZ'))
# #=>  int [1:10]   1   6   24  27   52   703   1   52   52   52
#
# x <- c('a','F','x','aa','az','aaa','a','Az','aZ','AZ')
# n <- excelColsToInt(x)
# x2 <- openxlsx::int2col(n)
# identical(toupper(x),x2)
# #=>  TRUE

excelColsToInt <- function(charvec) {
  charvec. <- toupper(charvec)
  lettersToNums <- function(x) {
    n <- which(LETTERS %in% x)
    if (length(n)==0)
      stop('`',x,'` is not a letter',call.=FALSE)
    n
  }
  chvToNums <- function(x)
    rev(tryCatch(sapply(x, lettersToNums, USE.NAMES=FALSE),
                 error = function(e)
                   stop('in `',x,'`\n',geterrmessage(),
                        call.=FALSE)))
  nums <- lapply(strsplit(charvec.,""), chvToNums)
  expon <- function(x) seq_along(x)-1
  colnum <- function(x) sum(x*26^expon(x))
  as.integer(sapply(nums, colnum))
}