ex2_6.erl

%%==============================================================================
%% Univ. Kent Functional Programing with Erlang
%%   Week 2: Part 6 - Defining function over lists
%%
%% Description:
%%
%% Author: Brian L. Shaver
%% Date  : 2017-03-04
%%==============================================================================
-module(ex2_6).
-include_lib("eunit/include/eunit.hrl").
-export([product/1,maximum/1,productTR/1,maximumTR/1]).

%-------------------------------------------------------------------------------
% recursive product of list
%   the product of [] is defined as 1, so that it is a suitable product 
%   for a basic recursive approach.
product([]) ->
    1;
product([X|Xs]) ->
    X * product(Xs).

product_test() ->
    ?assertEqual(1,product([])),
    ?assertEqual(0,product([0])),
    ?assertEqual(12,product([1,2,6,1])),
    ?assertEqual(30.0,product([2,10,1.5])).

% tail recursive approach to product of list
productTR(L) ->
    productTR(L,1).

productTR([],P) ->
    P;
productTR([X|Xs],P) ->
    productTR(Xs,X*P).

productTR_test() ->
    ?assertEqual(1,productTR([])),
    ?assertEqual(0,productTR([0])),
    ?assertEqual(12,productTR([1,2,6,1])),
    ?assertEqual(30.0,productTR([2,10,1.5])).

%-------------------------------------------------------------------------------
% list maximum
maximum([X]) ->
    X;
maximum([X|Xs]) ->
    max(X,maximum(Xs)).

maximum_test() ->
    ?assertEqual(3,maximum([1,2,3])),
    ?assertEqual(9,maximum([9,2,1])),
    ?assertEqual(2,maximum([2])).

% tail recursive approach to maximum of a list
maximumTR(L) ->
    maximumTR(L,1).

maximumTR([X],M) ->
    max(X,M);
maximumTR([X|Xs],M) ->
    maximumTR(Xs,max(X,M)).

maximumTR_test() ->
    ?assertEqual(3,maximumTR([1,2,3])),
    ?assertEqual(9,maximumTR([9,2,1])),
    ?assertEqual(2,maximumTR([2])).