exerciese create list

constructing_lists.erl

-module(constructing_lists).

-export([double/1,evens/1,test/0,median/1,modes/1]).

%double function start here.
double(L) -> double(L,[]).

double([],Acc) -> 
	reverse(Acc,[]);
double([H|T],Acc) -> 
	%Not ussing concat \ ++ , have to call reverse latter.
	double(T,[H,H|Acc]). 

%Reverse function to as we can't use lists:reverse().
reverse([],Acc)-> Acc;
reverse([H|T],Acc) -> reverse(T,[H|Acc]).


%Evens function starts here
evens(L) -> evens(L,[]).

evens([],Acc) -> 
	reverse(Acc,[]); 
evens([H|T],Acc) when H rem 2 == 0 -> 
	%Not ussing concat \ ++ , have to call reverse latter.
	evens(T,[H|Acc]);
evens([_|T],Acc) -> 
	evens(T,Acc).

%Median function starts here:

median(L) -> sort(L,[],L).

%sorts a list
%params: 
%	-List: List to be sorted.
%	-List: Classic accumulator.
%	-List: a previous stated that will be compared with Acc to know that the list is sorted.
sort(_,Acc,Acc) -> calculate_median(reverse(Acc,[])); 
sort([],Acc,_) ->
	sort(reverse(Acc,[]),[],Acc);
sort([H1|[H2|[]]],Acc,Initial) -> 
	sort([],[max(H1,H2),min(H1,H2)|Acc],Initial);
sort([H1|[H2|T]],Acc,Initial) -> 
	sort([max(H1,H2)|T],[min(H1,H2)|Acc],Initial).

%Calculates the median from a sorted list.
calculate_median(L) -> 
	case length(L) rem 2 of
		0 -> (get_pos(L,length(L) div 2) + get_pos(L,(length(L) div 2) + 1))/2;
		1 -> get_pos(L,length(L) div 2)
	end.
%Retruns the value of the list in the possition X.
get_pos([H|_],0) -> H; 
get_pos([_|T],X) -> get_pos(T,X-1). 

%%
%% modes exercise statrts here.
%%
modes(L) -> create_tuple_list(L,[]).

%Devuelve el numero de veces que aparece un numero en una lista:
number_count([],_,Acc) -> Acc;
number_count([X|T],X,Acc) -> number_count(T,X,Acc + 1);
number_count([_|T],X,Acc) -> number_count(T,X,Acc).

%Check if the number is in a list of tuples {Number,Times}
%if is not there adds the tuple {Number,Times} to the Acc.
create_tuple_list([],Acc) -> more_coincidences(Acc,0,[]);
create_tuple_list([H|T],Acc) -> 
	Tuple = {H,number_count(T,H,1)},
	case insert_tuple(Tuple,Acc) of
		true -> create_tuple_list(T,[Tuple|Acc]);
		_ -> create_tuple_list(T,Acc)
	end.

%Checks if the tuple is already in the list.
insert_tuple(_,[]) -> true;
insert_tuple({X,_},[{X,_}|_]) -> false;
insert_tuple(Tuple,[_|T]) -> insert_tuple(Tuple,T). 

%Creates a list with the tuples with more coincidences.
more_coincidences([],_,Acc) -> final_list(Acc,[]);
more_coincidences([{_,Y}|T],MaxY,Acc) when Y < MaxY -> more_coincidences(T,MaxY,Acc);
more_coincidences([{X,Y}|T],Y,Acc) -> more_coincidences(T,Y,[{X,Y}|Acc]);
more_coincidences([{X,Y}|T],_,_) -> more_coincidences(T,Y,[{X,Y}]).

%Creates the final list.
final_list([],Acc) -> Acc;
final_list([{X,_}|T],Acc) -> final_list(T,[X|Acc]).



test() ->
	[1,1,2,2,3,3] = constructing_lists:double([1,2,3]),
	[] = constructing_lists:double([]),
	[1,1] = constructing_lists:double([1]),
	[2,4,6,8] = constructing_lists:evens([1,2,3,4,6,8]),
	[] = constructing_lists:evens([]),
	[] = constructing_lists:evens([1]),
	[2] = constructing_lists:evens([2]),
	[0] = constructing_lists:evens([0]),
	26.5 = constructing_lists:median([4,1,2,6,8,523,45,6576]),
	6 = constructing_lists:median([4,1,2,6,8,523,3,45,6576]),
	3 = constructing_lists:number_count([3,4,3,7,8,6,6,5,6],3,1),
	false = constructing_lists:insert_tuple({3,4},[{3,4}]),
        true = constructing_lists:insert_tuple({3,4},[]),
	[6,3] = constructing_lists:modes([2,3,3,4,3,7,8,6,6,5,6]),
	[1] = constructing_lists:modes([1]),
	[6] = constructing_lists:modes([1,2,3,4,6,7,8,6,8,6]),
	ok.