Created
May 9, 2020 09:51
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-module(lists2). | |
-export([join/2,concat/1,member/2,sort/1,sort/2,perms/1]). | |
-export([join_test/0,concat_test/0,member_test/0,sort_test/0,perms_test/0]). | |
%% Join %%% | |
join(L1, L2) when is_list(L1), is_list(L2) -> | |
join_internal(lists:reverse(L1), L2). | |
join_internal(L1, []) -> lists:reverse(L1); | |
join_internal(L1, [H|T]) -> | |
join_internal([H|L1], T). | |
join_test() -> | |
[] ++ [] = join([],[]), | |
[3] ++ [] = join([3], []), | |
[] ++ [3] = join([], [3]), | |
[] ++ [1,2,3] = join([], [1,2,3]), | |
[1,2,3] ++ [] = join([1,2,3], []), | |
[2] ++ [3] = join([2],[3]), | |
[1,2] ++ [3,4] = join([1,2], [3,4]), | |
[1,[2],[3,4],5] = join([1,[2]], [[3,4],5]), | |
passed. | |
%% Concat %%% | |
concat(L) when is_list(L)-> | |
concat_internal(L, []). | |
concat_internal([], Ac) -> Ac; | |
concat_internal([H|T], Ac) -> | |
case is_list(H) of | |
true -> concat_internal(T, concat_internal(H, Ac)); | |
false -> concat_internal(T, join(Ac, [H])) | |
end. | |
concat_test() -> | |
[] = concat([[],[]]), | |
"goodbye" = concat(["goo", "d", "", "by", "e"]), | |
[1,2,3,4,5] = concat([[1], [2,3], 4, [5]]), | |
[1,2,3,4,5] = concat([[1,2],3,4,5]), | |
[1,2,3,4,5] = concat([1,[[2,[3]],[4]],5]), | |
passed. | |
%% Member %%% | |
member(_, []) -> false; | |
member(A, [H|T]) -> | |
case A == H of | |
true -> true; | |
false -> member(A, T) | |
end. | |
member_test() -> | |
true = member(2, [2,0,0,1]), | |
false = member(20, [2,0,0,1]), | |
true = member([2], [1,[2]]), | |
% [3,4] is not a member of the given list, but a "member of a member" of that list | |
false = member([3,4], [1,[2,[3,4]]]), | |
passed. | |
%% Sort %%% | |
sort(L) when is_list(L) -> | |
sort(L, merge). | |
sort(L, Method) when is_list(L) -> | |
case Method of | |
merge -> merge_sort(L); | |
quick -> quick_sort(L); | |
insert -> insert_sort(L); | |
_ -> bad_method | |
end. | |
%% Merge sort %%% | |
merge_sort([]) -> []; | |
merge_sort([H|[]]) -> [H]; | |
merge_sort(L) when is_list(L) -> | |
{L1, L2} = lists:split(length(L) div 2, L), | |
merge_internal(merge_sort(L1), merge_sort(L2), []). | |
merge_internal([], L2, Ac) -> join(Ac, L2); | |
merge_internal(L1, [], Ac) -> join(Ac, L1); | |
merge_internal([H1|T1] = _L1, [H2|_T2] = L2, Ac) when H1<H2 -> | |
merge_internal(T1, L2, join(Ac, [H1])); | |
merge_internal([_H1|_T1] = L1, [H2|T2] = _L2, Ac) -> | |
merge_internal(T2, L1, join(Ac, [H2])). | |
%% Quicksort %%% | |
quick_sort([]) -> []; | |
quick_sort([H|T]) -> | |
T2 = join([H], quick_sort([X || X <- T, X >= H])), | |
join(quick_sort([X || X <- T, X < H]), T2). | |
%% Insertion sort %%% | |
insert_sort([]) -> []; | |
insert_sort(L) -> insert_sort(L, []). | |
insert_sort([], L) -> L; | |
insert_sort([H|T], L) -> | |
insert_sort(T, insert(H,L)). | |
insert(A, []) -> [A]; | |
insert(A, [H|_] = L) when A =< H -> [A|L]; | |
insert(A, [H|T] = _) -> [H | insert(A, T)]. | |
sort_test() -> | |
[] = sort([]), | |
[1] = sort([1]), | |
[1,2,3] = sort([2,3,1]), | |
[1,2,3,4,5] = sort([5,1,3,2,4]), | |
[1,2,3] = sort([2,3,1], merge), | |
[] = sort([], quick), | |
[1] = sort([1], quick), | |
[1,2,3] = sort([2,3,1], quick), | |
[1,2,3,4,5] = sort([5,1,3,2,4], quick), | |
[1,2,3] = sort([2,3,1], merge), | |
[] = sort([], insert), | |
[1] = sort([1], insert), | |
[1,2,3] = sort([2,3,1], insert), | |
[1,2,3,4,5] = sort([5,1,3,2,4], insert), | |
bad_method = sort([], doh), | |
passed. | |
%% Permutations %%% | |
perms([]) -> [[]]; | |
perms(L) -> | |
[[A|B] || A <- L, B <- perms(L--[A])]. | |
perms_test() -> | |
[[]] = perms([]), | |
[[1]] = perms([1]), | |
[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]] = perms([1,2,3]), | |
passed. |
That solution for perms is so clean. I had the base case and then started to flounder at once. Just going round in circles and making no progress.
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That is an awesome review. Thank you so much! :)