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| %%% Ninety-Nine Erlang Problems
-module(problems).
-compile(export_all).
%%% P01 Find the last element of a list.
%%% eg
%%% p01([1,2,3,4])
%%% > 4
p01([]) -> [];
p01([H|[]]) -> H;
p01([_|T]) ->
p01(T).
%%% P02 Find the last but one box of a list.
%%% eg
%%% problems:p02([1,2,3,4]).
%%% > [3,4]
p02([X,Y|[]]) ->
[X,Y];
p02([_|Y]) ->
p02(Y);
p02([]) ->
[].
%%% P03 Find the K'th element of a list
%%% eg
%%% problems:p03([1,2,3], 3).
%%% > 3
p03([], _) ->
[];
p03([H|_], 1) ->
H;
p03([_|T], K) ->
K1 = K - 1,
p03(T, K1).
%%% P04 Find the number of elements of a list.
%%% eg
%%% problems:p04([1,2,3,4,5]).
%%% > 5
p04([]) ->
0;
p04([_|T]) ->
1 + p04(T).
%%% P05 Reverse a list.
%%% eg
%%% problems:p05([1,2,3,4]).
%%% > [4,3,2,1]
p05(List) ->
p05(List, []).
p05([H|T], Rev) ->
p05(T, [H|Rev]);
p05([], Rev) ->
Rev.
%%% P06 Find out whether a list is a palindrome.
%%% eg
%%% problems:p06([1,2,3,2,1]).
%%% > true
p06(List) ->
Other = p05(List),
compare(Other, List).
compare([H|T], [H|T1]) ->
compare(T, T1);
compare([H|_], [H1|_]) ->
if H =/= H1 ->
false
end;
compare([],[]) ->
true.
%%% P07 Flatten a nested list structure
%%% eg
%%% problems:p07([a,[b,[c,d],e]]).
%%% > [a,b,c,d,e]
p07([H|T]) ->
if
is_list(H) ->
lists:append(p07(H), p07(T));
true ->
[H|p07(T)]
end;
p07([]) -> [].
%%% p08 Eliminate consecutive duplicates of list elements.
%%% eg
%%% problems:p08([a,a,a,a,b,c,c,a,a,d,e,e,e,e).
%%% > [a,b,c,d,e]
p08([]) ->
[];
p08([H|[]]) ->
[H];
p08([H,H|T]) ->
p08([H|T]);
p08([H1,H2|T]) ->
if
H1 =/= H2 ->
[H1|p08([H2|T])]
end.
%%% p09 Pack consecutive duplicates of list elements into sublists.
%%% eg
%%% problems:p09([a,a,a,a,b,c,c,a,a,d,e,e,e,e]).
%%% > [[a,a,a,a],[b],[c,c],[a,a],[d],[e,e,e,e]]
p09([]) ->
[];
p09([H|[]]) ->
[[H]];
p09([H,H|C]) ->
[Head|Tail] = p09([H|C]),
X = lists:append([H],Head),
[X|Tail];
p09([H,H2|C]) ->
if H =/= H2 ->
[[H]|p09([H2|C])]
end.
%%% p10 Run-length encoding of a list.
%%% eg
%%% problems:p10([a,a,a,a,b,c,c,a,a,d,e,e,e,e]).
%%% > [[a,4],[b,1],[c,2],[a,2],[d,1],[e,4]]
p10(List) ->
M = p09(List),
encode(M).
encode([]) ->
[];
encode([H|T]) ->
if
is_list(H) ->
[H2|_] = H,
[[H2,length(H)]|encode(T)];
true ->
[[H,1]|encode(T)]
end.
%%% p11 Modified run-length encoding
%%% eg
%%% problems:p10([a,a,a,a,b,c,c,a,a,d,e,e,e,e]).
%%% > [[a,4],b,[c,2],[a,2],d,[e,4]]
p11(List) ->
M = p09(List),
encode_mod(M).
encode_mod([]) ->
[];
encode_mod([H|T]) ->
if
is_list(H) ->
[H2|_] = H,
Length = length(H),
case Length of
1 ->
[H2|encode_mod(T)];
_ ->
[[H2,Length]|encode_mod(T)]
end
end.
%%% p12 Decode a run-length encoded list.
%%% eg
%%% problems:p12([[a,4],b,[c,2],[a,2],d,[e,4]]).
%%% > [a,a,a,a,b,c,c,a,a,d,e,e,e,e]
p12([]) ->
[];
p12([H|T]) ->
if
is_list(H) ->
[Element,Number] = H,
lists:append(repeat(Element, Number), p12(T));
true ->
lists:append([H], p12(T))
end.
repeat(_, 0) ->
[];
repeat(H, Number) ->
[H|repeat(H, Number -1)].
%%% p13 Run length encoding of a list
%%% eg
%%% problems:p13([a,a,a,a,b,c,c,a,a,d,e,e,e,e]).
%%% > [[4,a],b,[2,c],[2,a],d,[4,e]]
p13([]) ->
[];
p13(L) ->
{Crumbs, Eaten} = chomp(L, []),
[H|_] = Eaten,
Length = length(Eaten),
case Length of
1 -> [H|p13(Crumbs)];
_ -> [[length(Eaten), H]|p13(Crumbs)]
end.
chomp([], R) ->
{[], R};
chomp([H|T], []) ->
chomp(T, [H]);
chomp([H|T], [HR|Result]) ->
if H =:= HR ->
chomp(T, [H,HR|Result]);
true ->
{[H|T], [HR|Result]}
end.
%%% p14 Duplicate elements of a list.
%%% eg
%%% problems:p14([a,b,c,d]).
%%% > [a,a,b,b,c,c,d,d]
p14([]) ->
[];
p14([H|T]) ->
[H,H|p14(T)].
%%% p15 Replicate the elements of a list a given number of times.
%%% eg
%%% problems:p15([a,b,c],3).
%%% > [a,a,a,b,b,b,c,c,c]
p15([], _) ->
[];
p15([H|T], Number) ->
lists:append(repeat(H, Number), p15(T, Number)).
|