R function to convert excel column names ('Q','AD', etc) to numbers

xl_col_to_num.R

xl_col_to_num <- function(column) {
  
  # calc value for a letter given its position i
  # eg for'CZ', C's contrib is 26^(1-1)*3 and Z's contrib is 26^(2-1)*26
  calc_pos_val <- function(x,i) 26^(i-1) * which(LETTERS == x)         
  
  # for a single column like 'CZ', calc each positional value and sum the total
  single_col   <- function(x) calc_pos_val %>% mapply(x, rev(seq_along(x)) ) %>% sum()
  
  column %>% 
    toupper %>% 
    strsplit(NULL) %>% # split c('AA','AB') into list(c('A','A'),c('A','B'))
    sapply(single_col) 
    
}

xl_col_to_num('AAA')
xl_col_to_num(letters)
xl_col_to_num(c('X','Y','Z','AA','AB','AZ','BZ','CA','AAA','AAB'))

# another way to 
# xl_col_to_num <- function(column) {
#  
#   parts <- strsplit(column, NULL) %>% unlist %>% rev()
#   num <- 0
#   for(i in seq_along(parts)) {
#     num <- num + 26^(i-1) * which(toupper(letters) == parts[i])
#   }
#   num
# }

#sapply(toupper(letters), xl_col_to_num)
#xl_col_to_num('AAA')