-module(letters).
-export([convert_case/1]).

% Empty list handler
convert_case( [] )->
    [];

% If the ascii value is between 'a' and 'z', convert to upper case
convert_case( [H|T] ) when H>96, H<123 ->
    [to_upper(H)|convert_case(T)];
% If the ascii value is between 'A' and 'Z', convert to lower case
convert_case( [H|T] ) when H>64, H<91 ->
    [to_lower(H)|convert_case(T)];
% Else, leave it alone
convert_case( [H|T] ) ->
    [H|convert_case(T)].


% Convert to upper case
to_upper( X ) ->
    X-32.
% Convert to lower case
to_lower( X ) ->
    X+32.