How do I convert a spreadsheet "letternamed" column coordinate to an integer?

Question

In spreadsheets I have cells named like "F14", "BE5" or "ALL1". I have the first part, the column coordinate, in a variable and I want to convert it to a 0-based integer column index.

How do I do it, preferably in an elegant way, in Ruby?

I can do it using a brute-force method: I can imagine loopping through all letters, converting them to ASCII and adding to a result, but I feel there should be something more elegant/straightforward.

Edit: Example: To simplify I do only speak about the column coordinate (letters). Therefore in the first case (F14) I have "F" as the input and I expect the result to be 5. In the second case I have "BE" as input and I expect getting 56, for "ALL" I want to get 999.

Answer 1

Not sure if this is any clearer than the code you already have, but it does have the advantage of handling an arbitrary number of letters:

class String
  def upcase_letters
    self.upcase.split(//)
  end
end

module Enumerable
  def reverse_with_index
    self.map.with_index.to_a.reverse
  end

  def sum
    self.reduce(0, :+)
  end
end

def indexFromColumnName(column_str)
  start = 'A'.ord - 1
  column_str.upcase_letters.map do |c| 
    c.ord - start
  end.reverse_with_index.map do |value, digit_position|
    value * (26 ** digit_position)
  end.sum - 1
end

I've added some methods to String and Enumerable because I thought it made the code more readable, but you could inline these or define them elsewhere if you don't like that sort of thing.