occurences.erl

-module( occurences ).
-export( [ occurences/1 ] ).

filter( _F, [] ) ->
    [];
filter( F, [ X | Xs ] ) ->
    case F( X ) of
        true -> [ X | filter( F, Xs ) ];
        _ -> filter( F, Xs )
    end.

% counts the number of occurrences of each value in a list, and returns a list of tuples,
% one for each value, containing the value and its occurences.
% 
% > occurences( [ 1, 2, 3, 2, 1 ] )
% [{1,2},{2,2},{3,1}]

occurences( Xs ) ->
    occurences( Xs, [] ).

occurences( [], R ) ->
    R;
occurences( [ X | Xs ], R ) ->
    Curr = filter( fun( Y ) ->
        case Y of
            { X, _ } -> true;
            _ -> false
        end
    end, R ),
    New = case Curr of
        [] -> { X, 1 };
        [ { X, N } ] -> { X, N + 1 }
    end,
    NoX = filter( fun( Y ) ->
        case Y of
            { X, _ } -> false;
            _ -> true
        end
    end, R ),
    occurences( Xs, [ New | NoX ] ).