week215.erl

-module(week215).
-export([maximum/1, product/1, test/0]).
-include_lib("eunit/include/eunit.hrl").


% Combining list elements: the product of a list
% Using the template from the last session, define an Erlang function to give the product of a list of numbers. The product of an empty list is usually taken to be 1: why?

%% because List is either a term or/and an empty list
%% and zero * anything is zero.

product(L) ->
    product(L, 1).

product([], Result) ->
    Result;
product([X|Xs], Result) ->
    product(Xs, Result * X).

product_direct([]) ->
    1;
product_direct([X|Xs]) ->
    X * product_direct(Xs).



maximum(L) ->
    maximum_(L).

maximum_([]) -> 
    [];
maximum_([X]) ->
    X;
maximum_([X, Y | Zs]) ->
    maximum([max(X, Y)|Zs]).


test() ->
    ?assert(maximum([]) == []),
    ?assert(maximum([1]) == 1),
    ?assert(maximum([1,2]) == 2),

    ?assert(product([]) == 1),
    ?assert(product([1,2]) == 2),

    ?assert(product_direct([]) == 1),
    ?assert(product_direct([1,2]) == 2).