Programming Riddle: How might you translate an Excel column name to a number?

Question

I was recently asked in a job interview to resolve a programming puzzle that I thought it would be interesting to share. It\'s about translating Excel column letters to actu

Answer 1

Hah - written it already in our code base - about 3 different times :(

%% @doc Convert an string to a decimal integer
%% @spec b26_to_i(string()) -> integer()

b26_to_i(List) when is_list(List) ->
    b26_to_i(string:to_lower(lists:reverse(List)),0,0).

%% private functions
b26_to_i([], _Power, Value) -> 
    Value;

b26_to_i([H|T],Power,Value)->
    NewValue = case (H > 96) andalso (H < 123) of
                   true ->
                       round((H - 96) * math:pow(26, Power));
                   _    ->
                       exit([H | T] ++ " is not a valid base 26 number")
               end,
    b26_to_i(T, Power + 1, NewValue + Value).

The riddle is that it isn't actually a Base26 representation of a number (we are lying to ourselves in our function name here) because there is no 0 in it.

The sequence is: A, B, C ... Z, AA, AB, AC

and not: A, B, C ...Z, BA, BB, BC

(the language is Erlang, mais oui).