lst.erl

-module(lst).
-export([sum/1, product/1, maximum/1, double/1, events/1, median/1, modes/1]).

%%
%% sum
%%
sum([]) -> 0;
sum([X|XS]) -> X + sum(XS).

%%
%% product
%%
product([]) -> 1;
product([H|TL]) -> H * product(TL).

%%
%% maximum
%%
maximum([X]) -> X;
maximum([H|TL]) -> max(H, maximum(TL)).

%%
%% double
%%
double([]) -> [];
double([H|TL]) -> [H*2 | double(TL)].

%%
%% events
%%
events([]) -> [];
events([H|TL]) when H rem 2 == 0 ->
    [H | events(TL)];
events([_H|TL]) ->
    events(TL).

%%
%% median
%%
median([_|_]=L) ->
    median(lists:sort(L), length(L)).

median(X, L) when L rem 2 == 0 ->
    sum(lists:sublist(X, L div 2, 2)) / 2;
median(X, L) ->
    lists:nth(L div 2 + 1, X).

%%
%% modes
%%
modes([_|_]=L) ->
    A = ocurrencies(L),
    B = lists:reverse(lists:keysort(2, A)),
    keys(B).

%%
%% ocurrencies(L)
%%
%% transform a list of numbers into a list of tuples {X,C} where
%%  X -> one element of the list
%%  C -> number of times that X appears into the list.
%%
%% The list is ordered to count items easier: [a,a,b,c,c,d,d,d] -> [{a,2}, {b,1}, {c,2}, {d,3}]
%%
ocurrencies(A) -> occurrencies(lists:sort(A), 0, []).

occurrencies([X], C, TL) ->
    TL ++ [{X,C+1}];
occurrencies([X,X|Xs], C, TL) ->
    occurrencies([X|Xs], C+1, TL);
occurrencies([X,Y|Xs], C, TL) ->
    occurrencies([Y|Xs], 0, TL ++ [{X, C+1}]).

%%
%% keys(L)
%%
%% transform a list of tuples {X,C} into a list of X.
%%
%% [{a,2}, {b,1}, {c,2}, {d,3}] -> [a, b, c, d]
%%
keys([]) -> [];
keys([{X,_C}|Xs]) -> [X | keys(Xs)].