DB_File (hash of array) problem
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From: James on 21 Jun 2010 20:15 I am trying to write to a database a hash of array (as seen by __DATA__ in the code). But somehow the first element of the array is missing. Any idea why? The second time, it is working correctly. $ cat run.pl use DB_File; use vars qw($db $x %h $k $v $i $key $val); ($db) = @ARGV; %h = (); $x = tie %h, "DB_File", $db, O_RDWR|O_CREAT, 0640, $DB_HASH; write_db(); read_db(); untie %h; undef $x; sub write_db { print "=== write $db ===\n"; for (<DATA>) { ($k, @v) = split; print "$k -> @v\n"; $r = \@v; for $i (0..$#v) { $h{$k}->[$i] = $r->[$i]; } } } sub read_db { print "=== read $db ===\n"; for ( $status = $x->seq($key, $val, R_FIRST); $status == 0; $status = $x->seq($key, $val, R_NEXT) ) { print "$key -> @{$val}\n"; } } __DATA__ aa 1 2 3 cc 678 99 zz foo fee fuu fun $ ./run.pl mydb === write mydb === aa -> 1 2 3 cc -> 678 99 zz -> foo fee fuu fun === read mydb === aa -> 2 3 cc -> 99 zz -> fee fuu fun TIA JL
From: Bart Lateur on 21 Jun 2010 22:26 James wrote: >I am trying to write to a database a hash of array (as seen by >__DATA__ in the code). >But somehow the first element of the array is missing. Any idea why? > for $i (0..$#v) { > $h{$k}->[$i] = $r->[$i]; > } Try a 1-based array for the DB. for $i (0..$#v) { $h{$k}->[$i+1] = $r->[$i]; } >The second time, it is working correctly. Now that is just plain weird. -- Bart.
From: Uri Guttman on 21 Jun 2010 23:44 >>>>> "J" == James <hslee911(a)yahoo.com> writes: J> I am trying to write to a database a hash of array (as seen by J> __DATA__ in the code). J> But somehow the first element of the array is missing. Any idea why? J> The second time, it is working correctly. as bart says, that is wierd. but your code is wierd too. and what do you mean by first element of which array? you have several instances of @v. all of them miss the first element? is this before or after you write the hash? you need to be more specific about errors like this. J> $ cat run.pl J> use DB_File; J> use vars qw($db $x %h $k $v $i $key $val); use vars is very old and mostly obsolete. lexicals are used now and you should declare them when first used. J> ($db) = @ARGV; and what if @ARGV is empty? check for this. also your names are very short and not informative. also below i remove several unneeded vars as well. my $db_name = shift @ARGV or die "must pass in a db name" ; J> %h = (); no need to assign () as hashes are always empty when declared or first used. J> $x = tie %h, "DB_File", $db, O_RDWR|O_CREAT, 0640, $DB_HASH; you don't check that for an error either. J> write_db(); you are using globals all over. sure, this is a short program but it is a bad habit to get into. J> read_db(); J> untie %h; J> undef $x; no need for those as they will be cleared upon program exit J> sub write_db { J> print "=== write $db ===\n"; ever heard of indenting code? J> for (<DATA>) it is poor style to use $_ as much as you do here. named vars are better and safer too ($_ is a global and can be modified elsewhere). J> { J> ($k, @v) = split; you know indenting. just oddly done. my( $key, @values ) = split; J> print "$k -> @v\n"; J> $r = \@v; J> for $i (0..$#v) { J> $h{$k}->[$i] = $r->[$i]; J> } no need for any of that code: $h{$k] = [@v] ; that is all you did there. if @v were declare in the loop with my, then you could just do: $h{$k] = \@values ; J> } J> } J> sub read_db { J> print "=== read $db ===\n"; J> for ( $status = $x->seq($key, $val, R_FIRST); $status == 0; $status = J> $x->seq($key, $val, R_NEXT) ) you tied the hash so why not use the hash interface, keys, values, each. no one uses the db interface as it is noisy while( my( $key, $val ) = each( %h ) { isn't that a bit easier to read? tie is nice in allowing a cleaner hash api vs some clunky db api. also it might eliminate your bug. i don't know the dbfile api so i can't tell why you have an off by one error only on the first run. J> { J> print "$key -> @{$val}\n"; J> } J> $ ./run.pl mydb J> === write mydb === J> aa -> 1 2 3 J> cc -> 678 99 J> zz -> foo fee fuu fun J> === read mydb === J> aa -> 2 3 J> cc -> 99 J> zz -> fee fuu fun and the second run works fine? show that output. does it still work if you delete the dbfile between runs? uri -- Uri Guttman ------ uri(a)stemsystems.com -------- http://www.sysarch.com -- ----- Perl Code Review , Architecture, Development, Training, Support ------ --------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com ---------
From: Xho Jingleheimerschmidt on 22 Jun 2010 01:45 James wrote: > I am trying to write to a database a hash of array (as seen by > __DATA__ in the code). > But somehow the first element of the array is missing. Any idea why? I don't believe DB_File supports nested data structures. > The second time, it is working correctly. > > $ cat run.pl > use DB_File; > use vars qw($db $x %h $k $v $i $key $val); Ugg. Scope variables to the smallest scope you can. You should use strict. You seem to be accidentally using symbolic references. The inner data never got stored in DB_File in the first place, it is only stored in Perl's memory. > for $i (0..$#v) { > $h{$k}->[$i] = $r->[$i]; > } The first time through, an array is auto-vivified, and it contains $r->[0]. When a reference to this array is stuffed into $h{$k}, it gets stringified to something like 'ARRAY(0x825c3dc)' because the tied hash only accepts strings, not array references. At that point, the array and the string become disconnected from each other, and the value of that auto-vivified array, the copy of $r->[0], is lost. The second subsequent time, you are using a symbol reference to a variable with the peculiar name 'ARRAY(0x825c3dc)', into which you stuff the remaining values. > sub read_db { > print "=== read $db ===\n"; > for ( $status = $x->seq($key, $val, R_FIRST); $status == 0; $status = > $x->seq($key, $val, R_NEXT) ) > { > print "$key -> @{$val}\n"; At this point, you are pulling the values out of the peculiarly named variable using symbolic references. If you separate your program so the perl instance that reads the DB is not the same one that created it, you will find the values never got stored to the DB in the first place. Xho
From: James on 22 Jun 2010 15:43
On Jun 21, 10:45 pm, Xho Jingleheimerschmidt <xhos...(a)gmail.com> wrote: > James wrote: > > I am trying to write to a database a hash of array (as seen by > > __DATA__ in the code). > > But somehow the first element of the array is missing. Any idea why? > > I don't believe DB_File supports nested data structures. > > > The second time, it is working correctly. > > > $ cat run.pl > > use DB_File; > > use vars qw($db $x %h $k $v $i $key $val); > > Ugg. Scope variables to the smallest scope you can. > > You should use strict. > > You seem to be accidentally using symbolic references. > > The inner data never got stored in DB_File in the first place, it is > only stored in Perl's memory. > > > for $i (0..$#v) { > > $h{$k}->[$i] = $r->[$i]; > > } > > The first time through, an array is auto-vivified, and it contains > $r->[0]. When a reference to this array is stuffed into $h{$k}, it gets > stringified to something like 'ARRAY(0x825c3dc)' because the tied hash > only accepts strings, not array references. At that point, the array > and the string become disconnected from each other, and the value of > that auto-vivified array, the copy of $r->[0], is lost. > > The second subsequent time, you are using a symbol reference to a > variable with the peculiar name 'ARRAY(0x825c3dc)', into which you stuff > the remaining values. > > > sub read_db { > > print "=== read $db ===\n"; > > for ( $status = $x->seq($key, $val, R_FIRST); $status == 0; $status = > > $x->seq($key, $val, R_NEXT) ) > > { > > print "$key -> @{$val}\n"; > > At this point, you are pulling the values out of the peculiarly named > variable using symbolic references. > > If you separate your program so the perl instance that reads the DB is > not the same one that created it, you will find the values never got > stored to the DB in the first place. > > Xho Thanks for your reply. Where can I find a reference to your statement, "because the tied hash only accepts strings, not array references." Anyway, I've re-written the run.pl script and run twice, see below. The second time, it looks as though the reference to an array seems working, but I may be wrong. $ cat run.pl use strict vars; use DB_File; my ($db) = @ARGV; my %h = (); my $x = tie %h, "DB_File", $db, O_RDWR|O_CREAT, 0640, $DB_HASH; print "=== write $db ===\n"; my $k; my @v; my $r; for (<DATA>) { @v = (); ($k, @v) = split; $r = \@v; for my $i (0..$#v) { $h{$k}->[$i] = $r->[$i]; print "($k -> $i -> ", $h{$k}->[$i], ") "; } print "\n"; } print "=== read $db ===\n"; my $st; my $key; my $val; my @val; for ( $st = $x->seq($key, $val, R_FIRST); $st == 0; $st = $x- >seq($key, $val, R_NEXT) ) { @val = @{$val}; for my $i (0..$#val) { print "($key -> $i -> $val[$i]) "; } print "\n"; } untie %h; undef $x; __DATA__ aa 1 2 3 cc 678 99 zz foo fee fuu fun $ rm testdb $ ./run.pl testdb === write testdb === (aa -> 0 -> ) (aa -> 1 -> 2) (aa -> 2 -> 3) (cc -> 0 -> ) (cc -> 1 -> 99) (zz -> 0 -> ) (zz -> 1 -> fee) (zz -> 2 -> fuu) (zz -> 3 -> fun) === read testdb === (aa -> 0 -> ) (aa -> 1 -> 2) (aa -> 2 -> 3) (cc -> 0 -> ) (cc -> 1 -> 99) (zz -> 0 -> ) (zz -> 1 -> fee) (zz -> 2 -> fuu) (zz -> 3 -> fun) $ ./run.pl testdb === write testdb === (aa -> 0 -> 1) (aa -> 1 -> 2) (aa -> 2 -> 3) (cc -> 0 -> 678) (cc -> 1 -> 99) (zz -> 0 -> foo) (zz -> 1 -> fee) (zz -> 2 -> fuu) (zz -> 3 -> fun) === read testdb === (aa -> 0 -> 1) (aa -> 1 -> 2) (aa -> 2 -> 3) (cc -> 0 -> 678) (cc -> 1 -> 99) (zz -> 0 -> foo) (zz -> 1 -> fee) (zz -> 2 -> fuu) (zz -> 3 -> fun) JL |