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Message |
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| brett hallett |
 Posted: Tue Nov 20, 2007 10:36 am |
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User
Joined: 21 Aug 2007
Posts: 21
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I've been experimenting with mnesia's various I/O functions, and when trying a dirty_read,
the returned result list seems overly complex !
dirty_read thus :
M2 =[mnesia:dirty_read(account, Key)
|| Key <- mnesia:dirty_all_keys(account)],
results in : M2
[[{account,"joe",388.280}],[{account,"Max",999}]]
ie: each record tuple {} in a list [] of it own, then each list is inside another list !!
on the other hand if I use QLC , eg:
F = fun() ->
Q = qlc:q([E || E <- mnesia:table(account)]),
qlc:e(Q)
end,
S = mnesia:transaction(F),
{_,Z} = S,
the result is
QLC (S) : {atomic,[{account,"joe",388.280},{account,"Max",999}]}
QLC (Z) : [{account,"joe",388.280},{account,"Max",999}]
So I end up with a single list populated with tuples, a much better LIST result to process,
as now I have all the tuples in one list, as my program is trying to use a common routine
to accumulate the account balances, so I need a consistent list to pass.
How do I process thislist
[[{account,"joe",388.280}],[{account,"Max",999}]]
to achieve this list
[{account,"joe",388.280},{account,"Max",999}]
I've tried a number of the list processing functions without success.
Thanks.
====================================================
{ok,cu}
113> cu:showAccs().
20/11/2007 17:16:02
QLC (S) : {atomic,[{account,"joe",388.280},{account,"Max",999}]}
QLC (Z) : [{account,"joe",388.280},{account,"Max",999}]
print3: account joe 388.28
print3: account Max 999.00
** end accounts list 3 ***
printHdr : [{account,"joe",388.280},{account,"Max",999}]
Account : AccName: Balance:
joe 388.28
Max 999.00
Total : 1387.28
** end accounts list 2 ***
ok
116> cu:accounts().
20/11/2007 17:16:44
dirty_read (M2) : [[{account,"joe",388.280}],[{account,"Max",999}]]
print1: {account,"joe",388.280}
print1: {account,"Max",999}
** end accounts list 1 ***
printHdr : [[{account,"joe",388.280}],[{account,"Max",999}]]
Account : AccName: Balance:
=ERROR REPORT==== 20-Nov-2007::17:16:44 ===
Error in process <0.578.0> with exit value: {{badmatch,[{account,"joe",3.882800e+02}]},[{cu,print2,2},{erl_eval,do_apply,5},{shell,exprs,6},{shell,eval_loop,3}]}
** exited: {{badmatch,[{account,"joe",388.280}]},
[{cu,print2,2},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_loop,3}]} **
118> |
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| Mazen |
| Posted: Wed Nov 28, 2007 9:49 am |
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User
Joined: 20 Jul 2006
Posts: 164
Location: London
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Hi Brett!
Try changing:
Code:
M2 =[mnesia:dirty_read(account, Key) ||
Key <- mnesia:dirty_all_keys(account)]
to
Code:
M2 =[hd(mnesia:dirty_read(account, Key)) ||
Key <- mnesia:dirty_all_keys(account)]
This will only work on non-bag tables and will crash if a record is removed from the table while going through it. Also check out dirty_first and dirty_next for iterating over a table.
The reason you get every element wrapped in a list is because the return value of dirty_read is in a list even if it only gets 1 record. |
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| brett hallett |
| Posted: Sat Dec 01, 2007 3:33 am |
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Joined: 21 Aug 2007
Posts: 21
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Thanks Mazen, for the suggestion -- works a treat !
I note your comments re 'dirty' operations, I'm just trying them because they are there! |
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| wuji |
| Posted: Mon Aug 27, 2012 6:54 am |
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Joined: 10 Aug 2012
Posts: 654
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