[erlang-questions] List comprehension puzzler

Antonio SJ Musumeci <>
Tue Sep 20 17:04:14 CEST 2016


While in C... length(ISBN) would still be O(n) and possibly unbounded. I
don't know if there is any optimization in the guard to cause the second
length() to be skipped but that still seems expensive.

Probably best to keep count while you check or maybe something ugly like

isbn_format_valid([A,B,C,D,E,F,G,H,I,J|Tail]) ->
   check_A_to_J andalso isbn_sub_format_valid(Tail).
isbn_format_valid(_) ->
   false.

isbn_sub_format_valid([]) ->
   true;
isbn_sub_format_valid([A,B,C]) ->
   check_A_to_C.


On Tue, Sep 20, 2016 at 10:17 AM, Pagayon, Ruel <> wrote:

> Hello Lloyd,
>
> One thing to help optimise your code: Guards
>
> isbn_format_valid(ISBN) when length(ISBN) == 10 orelse length(ISBN) == 13
> ->
>   [I || I <- ISBN, I < $0 orelse I > $9] == [];
>
> isbn_format_valid(_ISBN) ->
>   false.
>
>
> This makes your "is the number equal or between 0 and 9" only be executed
> if the length is 10 or 13. Reason for this, is if user inputs a very large
> list, you won't have to compare every character only to find later that
> it's not the right length (your machine will take the toll).
>
> Also, you may have noticed we replaced *or* with *orelse*, this makes the
> second expression be evaluated if the ISBN length isn't 10.
>
> Cheers,
>
>
> *Ruel Pagayon* - 
> Platform Engineer
> Networking and Communications
>
> On Tue, Sep 20, 2016 at 9:44 PM, <> wrote:
>
>> 'Lo again Hernando,
>>
>> Shortly after I responded to your post I realized that you might be
>> interested in the problem that led to my list comprehension puzzler
>> question:
>>
>> I'm working on a web application that requires users to enter
>> International Standard Book Numbers (ISBNs). Thus I need a function to
>> validate the format of user input.
>>
>> ISBNS are either 10 or 13-digits. The standard specifies a check-sum
>> algorithm, but that seemed too complicated for my purposes. So, after
>> struggling with the list comprehension puzzler and Dan Gudmundsson's
>> helpful input, I came up with this:
>>
>> isbn_format_valid(ISBN) ->
>>    D = fun(I) -> (I >= $0) and (I =< 9) end,
>>    NotIntegers = [I || I <- ISBN, D(I) == true],
>>    Flag1 = NotIntegers == [],
>>    Flag2 = (length(ISBN) == 13) or (length(ISBN) == 10),
>>    Flag1 and Flag2.
>>
>> As you can see, our minds ran down the same track.
>>
>> There may be a faster, more elegant, way to solve the problem, but this
>> seems to do the job. Now if I can only figure out how to use the Library of
>> Congress API to retrieve book data...
>>
>> Thanks again for your post,
>>
>> Lloyd
>>
>>
>>
>>
>> -----Original Message-----
>> From: "Lloyd R. Prentice" <>
>> Sent: Tuesday, September 20, 2016 12:41am
>> To: "Hernando Gisinger" <>
>> Cc: "" <>
>> Subject: Re: [erlang-questions] List comprehension puzzler
>>
>> _______________________________________________
>> erlang-questions mailing list
>> 
>> http://erlang.org/mailman/listinfo/erlang-questions
>> Hi Hernando,
>>
>> I came up with almost the exact same code after Dan Gudmunsson pointed
>> out the error  of my ways.
>>
>> Many thanks for the solution.
>>
>> Best wishes,
>>
>> Lloyd
>>
>> Sent from my iPad
>>
>> > On Sep 19, 2016, at 11:01 PM, Hernando Gisinger <>
>> wrote:
>> >
>> > Hello
>> >
>> > 1> IsDigit = fun(C) -> C >= $0 andalso C =< $9 end.
>> > #Fun<erl_eval.6.52032458>
>> > 2> S = "123a456".
>> > "123a456"
>> > 3> [IsDigit(I) || I <- S].
>> > [true,true,true,false,true,true,true]
>> >
>> >
>> > 2016-09-18 13:56 GMT-03:00 <>:
>> >> Hello,
>> >>
>> >> Now this I would not expect:
>> >>
>> >> 4> S = "123a456".
>> >> "123a456"
>> >>
>> >> 5> is_integer(S).
>> >> false
>> >>
>> >> 6> [is_integer(I) || I <- S].
>> >> [true,true,true,true,true,true,true]
>> >>
>> >> Please tell me what I don't understand.
>> >>
>> >> Many thanks,
>> >>
>> >> LRP
>> >>
>> >>
>> >> *********************************************
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>> >>
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>> >> http://thegospelofashes.com
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>> >> save the lives of millions?
>> >>
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