spreadsheet_column_utils.rb

# converting back and forth between (excel) spreadsheets column name / alpha and column number / index
# by Rafael Belvederese, inpired by this thread:
# http://stackoverflow.com/questions/22708/how-do-i-find-the-excel-column-name-that-corresponds-to-a-given-integer

def alpha_to_index(alpha)
  alpha.upcase!
  if alpha.length == 1
    return alpha.bytes.to_a[0] - 65
  elsif alpha.length != 0
    return ((alpha_to_index(alpha[0]) + 1) * (26 ** (alpha.length - 1))) + alpha_to_index(alpha[1..-1])
  else
    return nil
  end
end

def index_to_alpha(index)
  q = (index) / 26
  if q > 0
    index_to_alpha(q - 1) + ((index % 26) + 65).chr
  else
    (index + 65).chr
  end
end

index = alpha_to_index('ach')
puts index # 761

alpha = index_to_alpha(index)
puts alpha # ACH

I'm using it with Roo and ExcelCompare in a project.