Im using perl to read a berkley database and i need urgent help!

Comments

• Jonathan

  August 2005

  Hmmm....I hope the magenta inkjet ink I just got all over my finger will wash off...anyway...
  
  : I have a program:
  :
  : use strict ;
  Says we want Perl to be anal...uh...I mean, helpful about dubious code. :-)
  
  : use DB_File ;
  :
  This loads the DB_File handler.
  
  : my %h ;
  :
  Declare a hash named h. A hash is an associative array - it has key/value pairs. For example:-
  
  my %language_opinions = (
  perl => 'rocks',
  php => 'sucks'
  ); # Joke, joke...
  
  But when you access a single value in a hash by it's key, you're just getting one value. So for example we'd do:-
  
  print "I think perl $language_opinions{'perl'}.
  ";
  
  : sub Compare
  : {
  : my ($key1, $key2) = @_ ;
  : "L$key1" cmp "L$key2" ;
  : }
  :
  Declares a sub called compare. It takes two parameters (parameters come in the @_ array) and does a case insensitive compare (L = lowercase what follows).
  
  : # specify the Perl sub that will do the comparison
  : #$DB_BTREE->{'compare'} = &Compare ;
  :
  You don't want to comment that out. :-) $DB_BTREE is a variable that is exported by DB_File. On this line you override the standard compare routine for a B-tree (a tree format highly suited to on-disk storage) with the one you define in the sub Compare. & is the sigil for subs, makes a reference so &Compare gives you a reference to compare (e.g. you aren't calling the sub, just a reference to it so it can be called later). $DB_TREE turns out to be a reference to a hash (it may actually be an object based around a hash, but we don't really care). -> works like in PHP for calling methods on an object.
  
  : tie %h, "DB_File", "astdb", O_RDWR, 0640, $DB_BTREE
  : or die "Cannot open file 'tree': $!
  " ;
  :
  Tied variables are a very cool idea...that you won't find in many other languages. Here we "tie" the hash %h with the class DB_File, passing it some initial parameters. The class DB_File then over-rides the basic operations you can do on a hash, such as getting the keys, getting/setting values, etc. In this case it over-rides them with code that stores the values in a file. But to the programmer it feels like they're just manipulating any old hash.
  
  : # Cycle through the keys printing them in order.
  : # Note it is not necessary to sort the keys as
  : # the btree will have kept them in order automatically.
  :
  :
  : foreach (keys %h)
  That's practically all English, not Perl. :-)
  
  : { #print "$_=$h{$_}
  ";
  : }
  $_ is the default variable. So each key in %h is assigned to $_ and key by key the loop is executed. So here we're just printing (or would be if you hadn't commented it out) key=value.
  
  : untie %h ;
  :
  Here we clear up after ourselves.
  
  : First of all I would be very very grateful if someone can explain what this program does. I have a rough idea. I have very little knowledge of perl I have used php a lot.
  :
  Perl is kinda like turbo-charged PHP. Harder to learn, but a lot more expressive. Oh, and it has interesting features like tied variables.
  
  : What i want to be able to do is given a key print out the corresponding value. This program is printing all the values but i want to print specific value. For example there is a key /08845241830/8339 i want to print out its specific value. Ive tried so many things it just doesnt work. Can someone help?
  :
  Replace the loop at the end with something like:-
  
  print "Enter key: ";
  my $key = ; # Read a line from the console
  chop $key; # Get rid of the newline character we read
  print $h{$key}; # Print the value for that key
  
  Jonathan
  
  ###
  for(74,117,115,116){$::a.=chr};(($_.='qwertyui')&&
  (tr/yuiqwert/her anot/))for($::b);for($::c){$_.=$^X;
  /(p.{2}l)/;$_=$1}$::b=~/(..)$/;print("$::a$::b $::c hack$1.");
  
  
• the_programmer

  August 2005

  : Hmmm....I hope the magenta inkjet ink I just got all over my finger will wash off...anyway...
  :
  : : I have a program:
  : :
  : : use strict ;
  : Says we want Perl to be anal...uh...I mean, helpful about dubious code. :-)
  :
  : : use DB_File ;
  : :
  : This loads the DB_File handler.
  :
  : : my %h ;
  : :
  : Declare a hash named h. A hash is an associative array - it has key/value pairs. For example:-
  :
  : my %language_opinions = (
  : perl => 'rocks',
  : php => 'sucks'
  : ); # Joke, joke...
  :
  : But when you access a single value in a hash by it's key, you're just getting one value. So for example we'd do:-
  :
  : print "I think perl $language_opinions{'perl'}.
  ";
  :
  : : sub Compare
  : : {
  : : my ($key1, $key2) = @_ ;
  : : "L$key1" cmp "L$key2" ;
  : : }
  : :
  : Declares a sub called compare. It takes two parameters (parameters come in the @_ array) and does a case insensitive compare (L = lowercase what follows).
  :
  : : # specify the Perl sub that will do the comparison
  : : #$DB_BTREE->{'compare'} = &Compare ;
  : :
  : You don't want to comment that out. :-) $DB_BTREE is a variable that is exported by DB_File. On this line you override the standard compare routine for a B-tree (a tree format highly suited to on-disk storage) with the one you define in the sub Compare. & is the sigil for subs, makes a reference so &Compare gives you a reference to compare (e.g. you aren't calling the sub, just a reference to it so it can be called later). $DB_TREE turns out to be a reference to a hash (it may actually be an object based around a hash, but we don't really care). -> works like in PHP for calling methods on an object.
  :
  : : tie %h, "DB_File", "astdb", O_RDWR, 0640, $DB_BTREE
  : : or die "Cannot open file 'tree': $!
  " ;
  : :
  : Tied variables are a very cool idea...that you won't find in many other languages. Here we "tie" the hash %h with the class DB_File, passing it some initial parameters. The class DB_File then over-rides the basic operations you can do on a hash, such as getting the keys, getting/setting values, etc. In this case it over-rides them with code that stores the values in a file. But to the programmer it feels like they're just manipulating any old hash.
  :
  : : # Cycle through the keys printing them in order.
  : : # Note it is not necessary to sort the keys as
  : : # the btree will have kept them in order automatically.
  : :
  : :
  : : foreach (keys %h)
  : That's practically all English, not Perl. :-)
  :
  : : { #print "$_=$h{$_}
  ";
  : : }
  : $_ is the default variable. So each key in %h is assigned to $_ and key by key the loop is executed. So here we're just printing (or would be if you hadn't commented it out) key=value.
  :
  : : untie %h ;
  : :
  : Here we clear up after ourselves.
  :
  : : First of all I would be very very grateful if someone can explain what this program does. I have a rough idea. I have very little knowledge of perl I have used php a lot.
  : :
  : Perl is kinda like turbo-charged PHP. Harder to learn, but a lot more expressive. Oh, and it has interesting features like tied variables.
  :
  : : What i want to be able to do is given a key print out the corresponding value. This program is printing all the values but i want to print specific value. For example there is a key /08845241830/8339 i want to print out its specific value. Ive tried so many things it just doesnt work. Can someone help?
  : :
  : Replace the loop at the end with something like:-
  :
  : print "Enter key: ";
  : my $key = ; # Read a line from the console
  : chop $key; # Get rid of the newline character we read
  : print $h{$key}; # Print the value for that key
  :
  : Jonathan
  :
  : ###
  : for(74,117,115,116){$::a.=chr};(($_.='qwertyui')&&
  : (tr/yuiqwert/her anot/))for($::b);for($::c){$_.=$^X;
  : /(p.{2}l)/;$_=$1}$::b=~/(..)$/;print("$::a$::b $::c hack$1.");
  :
  :
  
  Thank you very much for your reply. I found it very helpful. I actually did try something similar to your suggestion before but it didnt work. I tried your code like this:
  
  use strict ;
  use DB_File ;
  
  my %h ;
  
  sub Compare
  {
  my ($key1, $key2) = @_ ;
  "L$key1" cmp "L$key2" ;
  }
  
  # specify the Perl sub that will do the comparison
  $DB_BTREE->{'compare'} = &Compare ;
  
  tie %h, "DB_File", "astdb", O_RDWR, 0640, $DB_BTREE
  or die "Cannot open file 'tree': $!
  " ;
  
  # Add a key/value pair to the file
  #$h{''} = 'Larry' ;
  #$h{'Smith'} = 'John' ;
  #$h{'mouse'} = 'mickey' ;
  #$h{'duck'} = 'donald' ;
  
  # Delete
  #delete $h{"duck"} ;
  
  # Cycle through the keys printing them in order.
  # Note it is not necessary to sort the keys as
  # the btree will have kept them in order automatically.
  
  print "Enter key: ";
  my $key = ; # Read a line from the console
  chop $key; # Get rid of the newline character we read
  print "Value: $h{$key}";
  
  untie %h ;
  
  When prompted I entered: /08845241830/8339
  
  The output was value:
  
  In other words null or nothing. Now the program I initially posted had loads of output from the database one such output was:
  
  /08845241830/8339=0845241
  
  So as you can see from the initial program it does return a value for the key /08845241830/8339 but in the second program it returns nothing. This is the problem i faced yesterday as i tried method after method to extract the value but I kept on getting nothing.
  
  Would appreciate if you can point out where i have gone wrong on this.
  
  thanks.
  
• Jonathan

  August 2005

  : Thank you very much for your reply. I found it very helpful. I actually did try something similar to your suggestion before but it didnt work. I tried your code like this:
  :
  : use strict ;
  : use DB_File ;
  :
  : my %h ;
  :
  : sub Compare
  : {
  : my ($key1, $key2) = @_ ;
  : "L$key1" cmp "L$key2" ;
  : }
  :
  : # specify the Perl sub that will do the comparison
  : $DB_BTREE->{'compare'} = &Compare ;
  :
  : tie %h, "DB_File", "astdb", O_RDWR, 0640, $DB_BTREE
  : or die "Cannot open file 'tree': $!
  " ;
  :
  : # Add a key/value pair to the file
  : #$h{''} = 'Larry' ;
  : #$h{'Smith'} = 'John' ;
  : #$h{'mouse'} = 'mickey' ;
  : #$h{'duck'} = 'donald' ;
  :
  : # Delete
  : #delete $h{"duck"} ;
  :
  : # Cycle through the keys printing them in order.
  : # Note it is not necessary to sort the keys as
  : # the btree will have kept them in order automatically.
  :
  : print "Enter key: ";
  : my $key = ; # Read a line from the console
  : chop $key; # Get rid of the newline character we read
  : print "Value: $h{$key}";
  :
  : untie %h ;
  :
  : When prompted I entered: /08845241830/8339
  :
  : The output was value:
  :
  : In other words null or nothing. Now the program I initially posted had loads of output from the database one such output was:
  :
  : /08845241830/8339=0845241
  :
  : So as you can see from the initial program it does return a value for the key /08845241830/8339 but in the second program it returns nothing. This is the problem i faced yesterday as i tried method after method to extract the value but I kept on getting nothing.
  :
  : Would appreciate if you can point out where i have gone wrong on this.
  :
  I can't actually see anything wrong with what you've done there and would have expected it to work. Can you change:-
  
  print "Value: $h{$key}";
  
  To
  
  print "$key = $h{$key}";
  
  Just to confirm it is seeing the input of the key OK. Also add "use warnings;" below "use strict;" to see if it gives us any useful warnings.
  
  Jonathan
  
  
  ###
  for(74,117,115,116){$::a.=chr};(($_.='qwertyui')&&
  (tr/yuiqwert/her anot/))for($::b);for($::c){$_.=$^X;
  /(p.{2}l)/;$_=$1}$::b=~/(..)$/;print("$::a$::b $::c hack$1.");