assignment 2.9

a29.erl

% Marcelo Ruiz Camauër
% assignment 2.9

-module(a29).
-export([double/1,evens/1, median/1]).
-include_lib("eunit/include/eunit.hrl").

%Define an Erlang function double/1 to double the elements of a list of numbers
double([])->[];
double(L)  -> [Value * 2 || Value <- L].

% recusive version:
% double([]) -> [];
% double([H|T]) -> [2*H] || double(T).

% Define a function evens/1 that extracts the even numbers from a list of integers.
evens([])->[];
evens(L)->[Value  || Value <- L, Value rem 2 =:=0].

%the median of a list of numbers: 
%this is the middle element when the list is ordered (if the list is of even length you should 
%average the middle two)


% code written by Vasiliy Rotaru, nice code! did not have time to make my own...:
median([X]) -> X;
median([X, Y]) -> (X + Y) / 2;
median([_, _ | _] = Xs) ->
    Min = lists:min(Xs),
    Max = lists:max(Xs),
    Fn = fun(X) -> (X /= Max) and (X /= Min) end,
    NextXs = lists:filter(Fn, Xs),
    median(NextXs).

%the modes of a list of numbers: 
%this is a list consisting of the numbers that occur most frequently in the list; 
%if there is is just one, this will be a list with one element only

% to do...

%%%%%%%%%%%%%%%% tests %%%%%%%%%%%%%%
% invoke with 'a29:tests().' or "eunit:test(a29)."
double_test_()->
     [?_assert(double([-1,2,3])==[-2,4,6]),
      ?_assert(double([])==[])
     ].

evens_test_()->
     [?_assert([2,4,6]==evens([-1,2,3,4,6])),
     ?_assert([]==evens([]))
    ].