| Author |
Message |
< Erlang ~ Fighting hard to understand Binaris, Matching and QLC... |
| seancharles |
 Posted: Tue Nov 27, 2007 8:50 pm |
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User
Joined: 18 Jul 2007
Posts: 57
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There *must* be a neater and more Erlang like way to do this,
Code:
all_tags(CloudName) when is_atom(CloudName) ->
Str = atom_to_list(CloudName),
Len = length(Str),
do( qlc:q([ R || R <- mnesia:table(tagcloud),
lists:sublist(binary_to_list(R#tagcloud.key),Len) =:= Str ])).
-record(
tagcloud,
{ key,
frequency = 1,
modified = now()})
The key is a composite of the tag name and term eg, in the tag cloud for search terms, the word 'binary' would be expressed as:
<<"searchbinary">>
and what I want to get back is the list of all terms that 'start with <<"binary">>' but I can't seem to find a better way!
If I try to use something like this inside the qlc expression:
Code:
[ R || R <- mnesia:table(tagcloud),
<< Str, Term/binary >> =:= R#tagcloud.key]
I get told that Term is unbound, I guess I need more experience and flying time but some pointers would be good!
For now it does what I need but it just doesn't feel 'beautiful' like the book says to go for!
Sean |
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| Jan Henry Nystrom |
| Posted: Fri Nov 30, 2007 11:09 am |
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User
Joined: 09 Oct 2006
Posts: 28
Location: Uppsala, Sweden
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How about:
Code:
all_tags(CloudName) when is_atom(CloudName) ->
Key = atom_to_list(CloudName),
Len = length(Key),
do(qlc:q([R || R <- mnesia:table(tagcloud),
binary_to_list(R#tagcloud.key, 1, Len) =:=
Key])).
seancharles wrote: There *must* be a neater and more Erlang like way to do this,
[code]
all_tags(CloudName) when is_atom(CloudName) ->
Str = atom_to_list(CloudName),
Len = length(Str),
do( qlc:q([ R || R <- mnesia:table(tagcloud),
lists:sublist(binary_to_list(R#tagcloud.key),Len) =:= Str ])).
[snip]
For now it does what I need but it just doesn't feel 'beautiful' like the book says to go for!
Sean
/Cheers Henry |
_________________
Jan Henry Nystrom
Training & Research Manager @ Erlang Training and Consulting Ltd
http://www.erlang-consulting.com |
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| seancharles |
| Posted: Fri Nov 30, 2007 11:28 am |
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User
Joined: 18 Jul 2007
Posts: 57
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Henry,
Thanks for that. Once again I cannot believe I failed to see that form for binary_to_list!!!
That is a lot better than what I had and I thank you for opening my eyes a little more on the existing API.
However, what I was really after was something along the lines of:
<< CloudName:Len/binary, Rest/binary >>
in there somewhere!!! I thought you could use that form to split a binary, Len would be 8*size(CloudName). Have I misunderstood on that score I wonder ?
Your version is a definite improvement on mine but I wanted to understand the binary pattern bit-matching idioms a little more than I do.
Thanks again
Sean. |
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| francesco |
| Posted: Sun Dec 02, 2007 9:13 am |
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User
Joined: 07 Jul 2006
Posts: 249
Location: London
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Sean, nice to see you around. We wondered where you were I will wait for Jan - Henry to reply to your questions, but I assume that his response has put you in the right direction for the other QLC query you had?
Francesco
--
http://www.erlang-consulting.com |
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| seancharles |
| Posted: Mon Dec 03, 2007 10:14 am |
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User
Joined: 18 Jul 2007
Posts: 57
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Francesco,
Thanks! 
As for how I am progressing, very well! Thanks to the other helpful replies I've had, I have made leaps and bounds with my Mnesia issues.
I re-designed my strategy and now I automatically create the table on demand, with the first dirty_read() faulting to cause the table creation.
When I create a new table, I add it to a catalog and then when I start up, I 'wait' for the catalog to be loaded, then I 'wait' for all the tables named in it to be loaded... that answered my last post on knowing what tables there are.
I am sure Mnesia would list all the tables but I don't necessarily want to wait on all of them, just the ones that the tag-cloud module is dependent one so I opted to keep my own list instead.
If I had a blog I'd post about it but my own web-site http://objitsu.com is very simple as I don't have any time to fiddle with it and besides; does the world really need another blog about nothing much ?
Anyway, thanks everybody for your help. I have tried to answer a few posts myself now as I feel a bit more confident about what I am saying isn't complete crap now.
And dare I say it, having been on a few Lisp forums, this place is damned pleasant to be a member of.
Thanks
Sean
Merry Christmas / Hannuka / Thanks-Giving / etc / whatever don't let the politically correct idiots keep us down. |
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