Untamed Wilds » erlang

Exercise 3-6: Sorting Lists & 3-7 Using Library Modules

Thomas J. Webb — Thu, 05 Nov 2009 03:19:54 +0000

It looks like there’s one other guy on the internet who’s posting his answers for the exercises in this book. So, I’ll be linking to his so you (and I) can compare our answers. He knows what he’s doing. In his posts, he talks about, e.g., what he’d do in Haskell. I’m not a functional programming guy (lisp gives me the heebie-geebies, my earlier post on evolutionary programming where I used it notwithstanding). So, below are my answers (click to see more if you’re looking at the blog and not this post by itself) and here is the Japanese guy’s answers.

[erlang]
-module(sort).
-export([quicksort/1,mergesort/1]).

less([Head|Tail], Pivot) when Head >= Pivot ->
less(Tail, Pivot);
less([Head|Tail], Pivot) when Head < Pivot ->
[Head|less(Tail, Pivot)];
less([],_) ->
[].

more([Head|Tail], Pivot) when Head >= Pivot ->
[Head|more(Tail, Pivot)];
more([Head|Tail], Pivot) when Head < Pivot ->
more(Tail, Pivot);
more([],_) ->
[].

quicksort([Head|Tail]) ->
quicksort(less(Tail,Head)) ++ [Head] ++ quicksort(more(Tail,Head));
quicksort([]) ->
[].

% For reference, here’s the quicksort from wikipedia’s page on erlang… much better-looking than my crap (but in my defense, I didn’t know how to do this kind of list construction… cool!)
% quicksort([]) -> [];
% quicksort([Pivot|Rest]) ->
% quicksort([Front || Front <- Rest, Front < Pivot])
% ++ [Pivot] ++
% quicksort([Back || Back <- Rest, Back >= Pivot]).

mergesort([]) ->
[];
mergesort([Left]) ->
[Left];
mergesort(List) ->
{Left, Right} = lists:split(length(List) div 2, List),
merge(mergesort(Left), mergesort(Right)).

[/erlang]

[erlang]
-module(db_lists).
-export([new/0, destroy/1, write/3, delete/2, read/2, match/2]).

new() ->
[].

destroy(Db) ->
ok.

write(Key, Element, Db) ->
Db ++ [{Key, Element}].

delete(Key, List) ->
lists:keydelete(Key, 1, List).

read(Key, List) ->
case (lists:keyfind(Key, 1, List)) of
{MatchKey, Element} ->
{ok, Element};
_ ->
{error, instance}
end.

match(Element, [{DbHeadKey, DbHeadElement}|DbTail]) ->
case DbHeadElement of
Element -> [DbHeadKey] ++ match(Element, DbTail);
_ -> match(Element, DbTail)
end;
match(Element, []) ->
[].

[/erlang]

Exercise 3-5: Manipulating Lists

Thomas J. Webb — Tue, 03 Nov 2009 18:19:52 +0000

Next installment in my “Thomas does your homework” series…

[erlang]
-module(list).
-export([filter/2, reverse/1, concatenate/1, flatten/1]).

filter([Head|Tail],Num) when Head =< Num ->
[Head|filter(Tail,Num)];
filter([Head|Tail],Num) when Head > Num ->
filter(Tail,Num);
filter([],_) ->
[].

reverse([Head|Tail]) ->
reverse(Tail) ++ [Head];
reverse([]) ->
[].

concatenate([Head|Tail]) ->
Head ++ concatenate(Tail);
concatenate([]) ->
[].

% Probably possible to make this a LOT more efficient, ne
flatten([Head|Tail]) ->
flatten(Head) ++ flatten(Tail);
flatten([]) ->
[];
flatten(NotArry) ->
[NotArry].
[/erlang]

Exercise 3-4 Database Handling Using Lists

Thomas J. Webb — Tue, 13 Oct 2009 06:07:25 +0000

Here’s my clumsy attempt at the database handling exercise. Man, I must be a nerd, but I actually like these exercises.. I’m also enjoying my biology class. Is there something wrong with me?

[code lang="erl"]-module(db).
-export([new/0, destroy/1, write/3, delete/2, read/2, match/2]).

new() ->
[].

destroy(Db) ->
ok.

write(Key, Element, Db) ->
Db ++ [{Key, Element}].

delete(Key, [{DbHeadKey, DbHeadElement}|DbTail]) ->
case DbHeadKey of
Key -> delete(Key, DbTail);
_ -> [{DbHeadKey, DbHeadElement}] ++ delete(Key, DbTail)
end;
delete(Key, []) ->
[].

read(Key, [{DbHeadKey, DbHeadElement}|DbTail]) ->
case DbHeadKey of
Key -> {ok, DbHeadElement};
_ -> read(Key, DbTail)
end;
read(Key, []) ->
{error, instance}.

match(Element, [{DbHeadKey, DbHeadElement}|DbTail]) ->
case DbHeadElement of
Element -> [DbHeadKey] ++ match(Element, DbTail);
_ -> match(Element, DbTail)
end;
match(Element, []) ->
[].

[/code]

Erlang Exercises – Chapter 3

Thomas J. Webb — Sat, 10 Oct 2009 17:34:02 +0000

I learn by doing much more than by reading or listening, so as I go through O’Reilly’s Erlang Programming, I take the exercises at the end seriously. Generally, when I learn a new programming language, while I read, I’ll make a program of my own choosing, perhaps something that will be useful to me or that I could sell. However, with Erlang, I don’t even know where to begin using it yet. I want to get really, really good at it before I even attempt to make anything with it (I think remembering the mistakes I made with my first Ruby on Rails project caused this shift in attitude.. and let’s not forget the triumphs of reading Mastering Regular Expressions in its entirety).

I will be posting my attempts at the exercises, in case others might find it useful or seasoned Erlang programmers might tell me what I may be doing wrong. I’ll be spacing out my answers to chapter 3 over the week to keep things interesting. Here are the first three exercises from Chapter 3:

[erlang]-module(chapter3).
-export([sum/1, sum/2, even/1, create/1, reverse_create/1]).
-import(erlang).

sum(Num) when Num > 1 ->
Num + sum(Num – 1);
sum(Num) when Num == 1 ->
1.

sum(N,M) when N < M ->
M + sum(N,M-1);
sum(N,M) when N == M ->
N;
sum(N,M) when N > M ->
erlang:throw({‘EXIT’,{bad_argument}}).

reverse_create(N) when N > 1 ->
[N|reverse_create(N-1)];
reverse_create(N) when N == 1 ->
[1].

create(N) when N > 1 ->
create(N-1) ++ [N];
create(N) when N == 1 ->
[1].

even(N) when (N > 0) and (N rem 2 == 0) ->
io:format("Number:~p~n",[N]), even(N-1);
even(N) when (N > 0) and (N rem 2 == 1) ->
even(N-1);
even(N) when N == 0 ->
done.
[/erlang]