Related
Lets say I have:
[ X || X<- L, some_expensive_boolean(X), some_expensive_boolean2(X)]
If, for any X in L, some_expensive_boolean(X) is false, is some_expensive_boolean2(X) executed?
TL;DR: No, some_expensive_boolean2/1 is not called.
There a couple of ways you can verify this.
1. Having functions print something as they are called.
-module(lc).
-export([lc/1]).
lc(L) ->
[X || X <- L, f(X), g(X)].
f(X = 2) ->
erlang:display({f, 2}),
false;
f(X) ->
erlang:display({f, X}),
true.
g(X) ->
erlang:display({g, X}),
true.
Then on the Erlang shell:
1> lc:lc(lists:seq(1, 4)).
{f,1}
{g,1}
{f,2} %% g is not called here
{f,3}
{g,3}
{f,4}
{g,4}
[1,3,4]
2. Check the generated Core Erlang code.
Compiling the module with the +to_core option will produce a lc.core file with the Core Erlang code, which looks a little bit like Erlang but has its own syntax but very similar semantics.
erlc +to_core lc.erl
The code generated is quite verbose so I won't paste it here, but the gist is that there are two nested case expressions, one calling f/1 with the clause that matches on true containing the other case that calls g/1.
The answer is no. It is short-circuited.
1> [ X || X <-[1,2], begin io:format("Test 1: ~p~n", [X]), X rem 2 =:= 0 end, io:format("Test 2: ~p~n", [X]) =:= ok ].
Test 1: 1
Test 1: 2
Test 2: 2
[2]
Short-circuits based on the following:
-module(shortcircuit).
-export([test/0]).
test() ->
L = [1, 2, 3],
[ X || X <- L, some_expensive_boolean(X), some_expensive_boolean2(X)].
some_expensive_boolean(X) ->
io:format("In some_expensive_boolean: ~p~n", [X]),
false.
some_expensive_boolean2(X) ->
io:format("In some_expensive_boolean2: ~p~n", [X]),
true.
Execute:
1> shortcircuit:test().
In some_expensive_boolean: 1
In some_expensive_boolean: 2
In some_expensive_boolean: 3
[]
2>
Let's create an example:
$ cat test.erl
-module(test).
-export([show/0]).
show() ->
[ X || X <- [1,2,3,4,5], bigger(X), smaller(X)].
bigger(X) ->
io:format("bigger ~p~n", [X]),
X > 2.
smaller(X) ->
io:format("smaller ~p~n", [X]),
X < 4.
and test it:
14> c(test).
{ok,test}
15> test:show().
bigger 1
bigger 2
bigger 3
smaller 3
bigger 4
smaller 4
bigger 5
smaller 5
[3]
So the answer is: NO.
I would like to use the below Erlang code to get the highest integer in a list of integers but for some reason always end up getting the last integer in the list. Any help?
Solution example -> test:max([2,8,5,6]). should return 8 but with this code it returns 6.
-spec max(L) -> M when
L::[integer()],
M::integer().
max([H | T]) ->
F = fun(L, Acc) -> max([L]) end,
lists:foldl(F, H, T).
Your function F should return the max of L and Acc. You can use the builtin max/2 function for that:
...
F = fun(L, Acc) -> max(L, Acc) end.
...
Test:
1> F = fun(L, Acc) -> max(L, Acc) end.
#Fun<erl_eval.12.52032458>
2> [H | T] = [2, 8, 5, 6].
[2,8,5,6]
3> lists:foldl(F, H, T).
8
What you return in your function F will be the new value of Acc, and eventually the value lists:foldl/3 will return.
What you may want to do is do comparison inside F and check if Acc is greater than the current value. You don't need to recurse max/1 since you're iterating the list in lists:foldl/3 anyway.
Let me know if you need the actual code right away, but I would recommend figuring it out yourself. It's more fun for you that way.
Is there an equivalent expression for the increment/decrement operator e.g. counter++?
I also wonder how to properly do this?
-module(whileloop).
-export([call/0, while_loop/2]).
call() ->
while_loop(10,0).
while_loop(Var,Counter) ->
case Var =:= Counter of
false ->
Counter += 1,
whileloop(Var);
end.
edit:
-module(whileloop).
-export([call/0, while_loop/2]).
call() ->
while_loop(10,0).
while_loop(Var, Counter) ->
case Var =:= Counter of
false ->
while_loop(Var,Counter + 1)
end.
The meaning of C += 1 is to modify the value of C. It is a non sense in Erlang since it can only give the following result:
1> C = C+1.
* 1: variable 'C' is unbound
C = 1.
1
3> C = C+1.
** exception error: no match of right hand side value 2
Keep in mind that "A = B" does not mean assigns the value of B to A, but "pattern match" A against B,
if A is unbound then it will bind to A the value of B;
if A =:= B nothing is done, the process continue;
if A =/= B then the process crashes.
So yes, if you want to have a counter, or any information that change, you must use a state variable which is passed as argument of a recursive loop. From this point of view, your last code is correct, but lets follow what happens when you call "call()" in the shell.
first it calls, in the same process - the shell - the function while_loop(10,0).
10 is not equal to 0 so it calls immediately while_loop(10,1).
10 is not equal to 1 so it calls immediately while_loop(10,2).
and so on until it calls while_loop(10,10). Now 10 =:= 10 is true, and this result does not match any clause of the case, so you get an error and the process crash.
As your code does not contain any message receive and simply loop an loop until it crashes, thhe wole process takes only a few micro seconds, so it looks like it fails immediately.
Depending on what you expect, you can imagine several types of counter, here are 2 examples:
-module(counter).
-compile(export_all).
% one counter that help you to count some events
% interface
start_c1(End) when is_integer(End) ->
spawn(?MODULE,counter1,[End]).
start_link_c1(End) when is_integer(End) ->
spawn_link(?MODULE,counter1,[End]).
inc_c1(Pid) when is_pid(Pid) ->
Ref = make_ref(),
Pid ! {inc,self(),Ref},
receive
{Ref,done} -> done;
{Ref,V} -> V
after 1000 ->
{error,no_response}
end.
value_c1(Pid) when is_pid(Pid) ->
Ref = make_ref(),
Pid ! {get_value,self(),Ref},
receive
{Ref,V} -> V
after 1000 ->
{error,no_response}
end.
stop_c1(Pid) when is_pid(Pid) ->
Pid ! stop.
% the counter
counter1(End) -> counter1_loop(End,0).
counter1_loop(End,V) ->
receive
{inc,Pid,Ref} when V =/= done ->
NewV = case V+1 of
End -> done;
Nv -> Nv
end,
Pid ! {Ref,NewV},
counter1_loop(End,NewV);
{inc,Pid,Ref} ->
Pid ! {Ref,done},
counter1_loop(End,done);
{get_value,Pid,Ref} ->
Pid ! {Ref,V},
counter1_loop(End,V);
stop ->
ok
end.
% One kind of timeout that execute something after a while -
% note it exists a similar one in the library
start_after(T,M,F,A) when is_integer(T), is_list(A) ->
Ref = make_ref(),
{Ref,spawn(?MODULE,after_receive,[T,M,F,A,self(),Ref])}.
cancel_after(P) when is_pid(P) ->
P ! cancel.
% the counter
after_receive(T,M,F,A,Pid,Ref) ->
receive
{cancel,Ref} -> Pid ! {after_receive,Ref,cancelled}
after T ->
Pid ! {after_receive,Ref,done},
apply(M,F,A)
end.
and here how to use them:
1> c("../src/counter").
{ok,counter}
2> {Ref,P} = counter:start_after(5000,io,format,["This is the end!" ]).
{#Ref<0.0.0.29>,<0.33.0>}
This is the end!3>
3> {Refa,Pa} = counter:start_after(50000,io,format,["This is the end!" ]).
{#Ref<0.0.0.34>,<0.35.0>}
4> Pa ! {cancel,Refa}.
{cancel,#Ref<0.0.0.34>}
5> flush().
Shell got {after_receive,#Ref<0.0.0.29>,done}
Shell got {after_receive,#Ref<0.0.0.34>,cancelled}
ok
6> P1 = counter:start_c1(5).
<0.52.0>
7> counter:inc_c1(P1).
1
8> counter:inc_c1(P).
{error,no_response}
9> counter:inc_c1(P1).
2
10> counter:inc_c1(P1).
3
11> counter:value_c1(P1).
3
12> counter:inc_c1(P1).
4
13> counter:inc_c1(P1).
done
14> counter:value_c1(P1).
done
15> counter:inc_c1(P1).
done
16> counter:stop_c1(P1).
stop
17> counter:inc_c1(P1).
{error,no_response}
18>
Just recursively call while_loop with the Counter argument incremented by one:
while_loop(Var, Counter + 1)
Your edited version doesn't have a clause when Var =:= Counter and thus crashes. And you'd better use pattern matching in function clauses.
-module(whileloop).
-export([call/0, while_loop/2]).
call() ->
while_loop(10,0).
while_loop(Var, Var) ->
ok;
while_loop(Var, Counter) ->
while_loop(Var, Counter + 1).
And of course you'd need to do something inside the loop. You can use lambdas for that:
-module(whileloop).
-export([call/0, while_loop/3]).
call() ->
while_loop(10, 0, fun(Counter) -> io:format("Counter: ~p~n", [Counter]) end).
while_loop(Var, Var, _) ->
ok;
while_loop(Var, Counter, Fun) ->
Fun(Counter),
while_loop(Var, Counter + 1, Fun).
I have a list of values "Z0010", "Z0011", "Z0012", "Z0013" "Z0014", "Z0015", "Z0016", "Z0017", "Z0018", "Z0019"
I want to develop a function that takes a value in parameter
and I want to do a test in my function if the value passed as a parameter is equal to a value in the list in this cases it will show "existe" if not it displays "not existe"
I try with :
test(Token)->
case get_user_formid_by_token(Token) of
{ok, H} ->
FormId=string:substr(H, 2, length(H)),
Form=string:to_integer(FormId),
case verify (0019,1,Form) of
{ok}->io:format("existe");
{notok}->io:format("not existe")
end;
{error, notfound}-> io:format("qq")
end.
verify(VariableLength,VariableIncrement,FormId)->
lists:map(fun(I) -> if I =:= FormId ->
{ok};
I =/= FormId ->
{notok}
end
end,
lists:seq(0010, VariableLength, VariableIncrement)).
but when I execute this code it displays :
1> model:test("21137900").
** exception error: no case clause matching [{notok},
{notok},
{notok},
{notok},
{notok},
{notok},
{notok},
{notok},
{notok},
{notok}]
in function model:test/1
I try now with this solution :
get_user_formid_by_token(Token) ->
Q = qlc:q([{X#person.formid} || X <- mnesia:table(person),
X#person.token =:= Token]),
case do(Q) of
[{H}] ->
{ok, H};
[] ->
{error, notfound}
end.
test(Token)->
case get_user_formid_by_token(Token) of
{ok, H} ->
io:format("~s~n",[H]),
FormId=string:substr(H, 5, length(H)),
io:format("~s~n",[FormId]),
Form=string:to_integer(FormId),
io:format("~p~n",[Form]),
lists:member(Form, lists:seq(313, 320, 1));
{error, notfound}-> io:format("qq")
end.
but when I test I have this message in the console:
1> model:test("21137900").
Z000313
313
{313,[]}
false
the result should be true and not false
I think that Form=string:to_integer(FormId), it not return in this case 313
and another thing I want to add in my code
for example if H equal "Z000010" FormId=string:substr(H, 2, length(H)),
it return "000010"
Now I want to eliminate the first zero before the first integer not null so extarct 0000
before 1
lists:map/2 takes one list and creates a new list with the same number of values, so your list of 10 values is transformed into a list of 10 {ok} or {notok} tuples.
You probably want lists:member/2 instead.
5> lists:member(0, lists:seq(1, 3, 1)).
false
6> lists:member(3, lists:seq(1, 3, 1)).
true
7> lists:map(fun(X) -> ok end, lists:seq(1, 3, 1)).
[ok,ok,ok]
Have a look at the documentation (http://www.erlang.org/doc/man/string.html#to_integer-1):
to_integer(String) -> {Int, Rest} | {error, Reason}
Types:
String = string()
Int = integer()
Rest = string()
Reason = no_integer | not_a_list
So to_integer returns a tuple containing the number that was consumed from the string and the rest of the string. You can even tell from your test output where it says {313,[]}. In order to get the value of the number bound to your Formvariable, you need to decompose the tuple, which is typically done by pattern matching:
{Form,_Rest}=string:to_integer(FormId)
Now your Form will contain only the number 313.
The string:to_integerfunction will also happily eat the leading zeroes:
1> {Form, _} = string:to_integer("000010"), Form.
10
Part of my program requires me to be able to randomly shuffle list elements. I need a function such that when i give it a list, it will pseudo-randomly re-arrange the elements in the list. A change in arrangement Must be visible at each call with the same list. My implementation seems to work just fine but i feel that its rather long and is increasing my code base and also, i have a feeling that it ain't the best solution for doing this. So i need a much shorter implementation. Here is my implementation:
-module(shuffle).
-export([list/1]).
-define(RAND(X),random:uniform(X)).
-define(TUPLE(Y,Z,E),erlang:make_tuple(Y,Z,E)).
list(L)->
Len = length(L),
Nums = lists:seq(1,Len),
tuple_to_list(?TUPLE(Len,[],shuffle(Nums,L,[]))).
shuffle([],_,Buffer)-> Buffer;
shuffle(Nums,[Head|Items],Buffer)->
{Pos,NewNums} = pick_position(Nums),
shuffle(NewNums,Items,[{Pos,Head}|Buffer]).
pick_position([N])-> {N,[]};
pick_position(Nos)->
T = lists:max(Nos),
pick(Nos,T).
pick(From,Max)->
random:seed(begin
(case random:seed(now()) of
undefined ->
NN = element(3,now()),
{?RAND(NN),?RAND(NN),?RAND(NN)};
Any -> Any
end)
end
),
T2 = random:uniform(Max),
case lists:member(T2,From) of
false -> pick(From,Max);
true -> {T2,From -- [T2]}
end.
On running it in shell:
F:\> erl
Eshell V5.8.4 (abort with ^G)
1> c(shuffle).
{ok,shuffle}
2> shuffle:list([a,b,c,d,e]).
[c,b,a,e,d]
3> shuffle:list([a,b,c,d,e]).
[e,c,b,d,a]
4> shuffle:list([a,b,c,d,e]).
[a,b,c,e,d]
5> shuffle:list([a,b,c,d,e]).
[b,c,a,d,e]
6> shuffle:list([a,b,c,d,e]).
[c,e,d,b,a]
I am motivated by the fact that in the STDLIB there is no such function. Somewhere in my game, i need to shuffle things up and also i need to find the best efficient solution to the problem, not just one that works.
Could some one help build a shorter version of the solution ? probably even more efficient ? Thank you
1> L = lists:seq(1,10).
[1,2,3,4,5,6,7,8,9,10]
Associate a random number R with each element X in L by making a list of tuples {R, X}. Sort this list and unpack the tuples to get a shuffled version of L.
1> [X||{_,X} <- lists:sort([ {random:uniform(), N} || N <- L])].
[1,6,2,10,5,7,9,3,8,4]
2>
Please note that karl's answer is much more concise and simple.
Here's a fairly simple solution, although not necessarily the most efficient:
-module(shuffle).
-export([list/1]).
list([]) -> [];
list([Elem]) -> [Elem];
list(List) -> list(List, length(List), []).
list([], 0, Result) ->
Result;
list(List, Len, Result) ->
{Elem, Rest} = nth_rest(random:uniform(Len), List),
list(Rest, Len - 1, [Elem|Result]).
nth_rest(N, List) -> nth_rest(N, List, []).
nth_rest(1, [E|List], Prefix) -> {E, Prefix ++ List};
nth_rest(N, [E|List], Prefix) -> nth_rest(N - 1, List, [E|Prefix]).
For example, one could probably do away with the ++ operation in nth_rest/3. You don't need to seed the random algorithm in every call to random. Seed it initially when you start your program, like so: random:seed(now()). If you seed it for every call to uniform/1 your results become skewed (try with [shuffle:list([1,2,3]) || _ <- lists:seq(1, 100)]).
-module(shuffle).
-compile(export_all).
shuffle(L) ->
shuffle(list_to_tuple(L), length(L)).
shuffle(T, 0)->
tuple_to_list(T);
shuffle(T, Len)->
Rand = random:uniform(Len),
A = element(Len, T),
B = element(Rand, T),
T1 = setelement(Len, T, B),
T2 = setelement(Rand, T1, A),
shuffle(T2, Len - 1).
main()->
shuffle(lists:seq(1, 10)).
This will be a bit faster than the above solution, listed here as do2 for timing comparison.
-module(shuffle).
-export([
time/1,
time2/1,
do/1,
do2/1
]).
time(N) ->
L = lists:seq(1,N),
{Time, _} = timer:tc(shuffle, do, [L]),
Time.
time2(N) ->
L = lists:seq(1,N),
{Time, _} = timer:tc(shuffle, do2, [L]),
Time.
do2(List) ->
[X||{_,X} <- lists:sort([ {rand:uniform(), N} || N <- List])].
do(List) ->
List2 = cut(List),
AccInit = {[],[],[],[],[]},
{L1,L2,L3,L4,L5} = lists:foldl(fun(E, Acc) ->
P = rand:uniform(5),
L = element(P, Acc),
setelement(P, Acc, [E|L])
end, AccInit, List2),
lists:flatten([L1,L2,L3,L4,L5]).
cut(List) ->
Rand=rand:uniform(length(List)),
{A,B}=lists:split(Rand, List),
B++A.